Michael Perryman has interviewed some of the scientists and project leaders in the Hipparcos and Gaia missions the interviews with photos of the persons are given at his site httpswww.michaelperryman.co.uk . Michael has also written essays  84 to date   about results from the Gaia mission and they are placed at his site. Three of the interviews are with me and transcriptions coauthored with Michael are provided below with the titles 1. An interview about astronomy and astrometry up to 1980. 2. An interview about the revival of astrometry after 1980. 3. The billionstar astrometry after 1990. The third interview begins in 1990 when I had the first ideas for a Hipparcos successor. In 1992 I made a detailed design with direct imaging on CCD detectors in a satellite proposal called Roemer. In 1993 a supposedly better option was proposed with the acronym GAIA where the capital I stood for Interferometer. In 1998 however interferometry was shown to be unsuited for the purpose and we returned to the original idea from 1992 for the further development. The name was later changed to Gaia  for the sake of continuity.
['Astrophysics - Instrumentation and Methods for Astrophysics', 'Physics - History and Philosophy of Physics']
Interviews about modern astrometry
{'Interviews about modern astrometry': 'by Erik Høg -Niels Bohr Institute - Copenhagen  \nMichael Perryman has interviewed some of the scientists and project leaders in the Hipparcos and Gaia missions, the interviews with photos of the persons are given at his site: https://www.michaelperryman.co.uk .  \nMichael has also written essays - 85 to date ! - about results from the Gaia mission and they are placed at his site.  \nThree of the interviews are with me and transcriptions, co-authored with Michael, are provided below with the titles:  \n- 1. An interview about astronomy and astrometry up to 1980\n- 2. An interview about the revival of astrometry after 1980\n- 3. The billion-star astrometry after 1990  \nThe third interview begins in 1990 when I had the first ideas for a Hipparcos successor . In  1992  I  made  a  detailed design  with  direct  imaging  on  CCD  detectors  in  a  satellite proposal called Roemer. In 1993 a supposedly better option was proposed with the acronym  GAIA  where  the  capital  "I"  stood  for  Interferometer.  In  1998,  however, interferometry  was  shown  to  be  unsuited  for  the  purpose  and  we  returned  to  the original idea from 1992 for the further development. The name was later changed to Gaia - for the sake of continuity.  \n62 pages = 1+22+19+20, including abstracts and references.', 'An interview about astronomy and astrometry up to 1980': "by Michael Perryman and Erik Høg  \nABSTRACT: Michael Perryman  invited Erik Høg to this interview in July 2021. Most surprising for a young astronomer is perhaps that space astrometry nearly failed, but is so famous today from the great impact on astronomy  by  the  Gaia  satellite  measuring  two  billion  stars.  But  the approval  in  1980 1 of  the  first  astrometry  satellite  Hipparcos  was  in  fact reached with a very narrow margin. The lay reader may enjoy the first few pages about a schoolboy in a Danish village building his own telescopes in the 1940s in order to observe variable stars in a wonderfully dark sky. The technology may then stop her/him from further reading so that the wise words would be missed which were spoken at a meeting in October 1975, just  one  sentence  that  opened  the  door  for  the  ensuing  revolution  of astronomy.  In  the  interview,  Erik  Høg  again  emphasizes  the  crucial contributions to space astrometry by Pierre Lacroute, Jean Kovalevsky and Lennart  Lindegren  especially  in  the  1970s:  There  were  simply  nobody around who could have played their roles, without them there would have been  no  Hipparcos  approved  in  1980,  and  probably  never.  This  report contains the questions by the interviewer, the answers by Erik Høg, a link to the audio recording and references.  \nHere  follows  a  script  of  the  recording.  It  quotes  Erik  closely  but Michael not so closely, with slanted text. The audio is available at this link: http://www.astro.ku.dk/~erik/xx/erik-hoeg-final.mp3 ,  \nbut you should rather open it on Michael's website with photos and other interviews: https://www.michaelperryman.co.uk/gaia-project-interviews  \nThe  minutes  and  seconds  are  tagged  at  tm=....  Total  duration  48 minutes, speaking takes 1 minute per 130 words.", 'Introduction': "Michael Perryman:  \nGaia is a scientific satellite of the European Space Agency launched in 2013. Today it is still thoroughly scanning the dark sky from a deep space orbit more than a million km from Earth. It is in the process of constructing a remarkable three-dimensional survey of more than two billion stars throughout our galaxy. I'm Michael Perryman, and in a series of conversations I am talking to scientists involved in different aspects of the mission.  \nJoining me today - and for a third time - is: Erik Høg, Professor Emeritus of the Niels Bohr Institute, University of Copenhagen.  \nErik was one of the people responsible for getting the Hipparcos astrometry mission adopted in the 1970s he was one of the scientific leaders of Hipparcos in the 1980s and 1990s and he was central in the developments of Gaia in the 1990s and 2000s.  \nHis career in astrometry has extended over a remarkable 70 years!  \nIn our first interview, we talked about the period leading to the adoption of Hipparcos  by  ESA  in  1980.  In  the  second,  we  focused  on  Hipparcos  itself,  from  its acceptance in 1980 to its completion in 1997. Today, we're talking about his work on Gaia, and on another new astrometry mission for the future.  \nSo…  What  were  the  steps  taken  in  advancing  space  astrometry  after Hipparcos ? What were the challenges in optimizing some of the details of the Gaia payload?  And  what  sort  of  future  does  space  astrometry  have  in  the  coming decades?  \nLet's find out!  \nErik Høg: a very warm welcome again.  \nThank  you  Michael  for  inviting  me  and  all  the  others  to  these  interviews  on  the different  aspects  of  space  astrometry,  and  also  for  writing  the  fine  essays  and placing all this on your website.", 'Background': 'Erik:  you  were born in 1932, and you have recently celebrated your 89th birthday.  What set you off on your long career as an astronomer?  \nWell, as a boy, I read a lot of books about physics, mathematics and astronomy. One morning I saw Venus in the telescope the teacher had set up at the school.  The teacher lent me a small telescope for a month. But after  that,  in  1946  [1]  when  I  was  14  I  realized  I  had  to  built  my  own telescopes. I first built one from two spectacle lenses, then I  ground and polished the mirrors for two telescopes and observed variable stars. The telescope mounting was built by the village blacksmith.  \nMy  teachers  now  and  later,  helped  and  encouraged  me  in  my interests.  I  must  say  that  most  people  I  ever  met  in  life  were  nice  and helpful. I could read and write before I started with six years in the village school. That was in Frejlev on the island Lolland. On the first day in school, the teacher asked how far we could count. I answered that I could go on counting because I could always say the number that followed: so in effect I knew there are infinitely many numbers. I should add that where I lived there was a completely dark sky in the country side. So I could see the stars and that was wonderful.  \ntm=04:00  \nYou  were  a  student  at  Copenhagen  University  between  1950-1956. What were your undergraduate studies?  \nIn 1950 I moved to Copenhagen which is 100 km away from where I had lived with my family where nobody had any further education than technical school. I wanted to become a teacher because my teacher in the gymnasium  had  said  I  would  not  be  able  to  live  from  astronomy. Therefore  I  studied  mathematics,  chemistry,  physics  and  astronomy  for three years and then specialized in astronomy. I was well received at the Copenhagen Observatory where we were only two or three students. My tutor  was  Peter  Naur  just  four  years  my  senior  and  very  bright  in  both astrometry  and  astrophysics.  He  was  very  engaged  in  computer development, I learnt much about computers and electronics from him.  \nIn 1953 a new meridian circle was set up at a new site at the village Brorfelde 50 km from Copenhagen and I was set to measure its stability [1]. I could buy a small motorbike to go there for observing. Sometimes I slept  in  a  haystack  when  clouds  came  because  there  were  no  houses where  I  could  stay.  But  the  fine  instrument  fascinated  me  and  I  found astrometry very interesting and very important for astronomy. Astrometry gives  astronomers  the  positions  and  motions  of  stars  and  also  the distances.', 'What was really driving you at this time?': 'Just to do what I had been asked to do, to test the stability. And then I  discovered this fantastic instrument, the mechanics interested me, and then the thoughts about the importance of astrometry.  \nWhat drives me in life is to do something useful, something needed, especially if I see that nobody else will do it, then I have to do if I possibly can!  The  telescopes  I  built  as  a  school  boy  were  needed  for  my observations so I had to build them myself. That was probably my tacit natural logic.  \ntm=06:27', 'What came after your undergraduate studies?': 'After completing the studies I served as a conscript soldier for a year. I  worked  in  a  laboratory  where  we  used  electronic  counters  [1  Sect.4]. Then I worked with Peter Naur near the meridian circle at Brorfelde where houses had now been built. But in 1957 Naur said I should go abroad to another observatory, but I said that I was not ready, I had to learn more before going. Then he said you have to go now, it is out there that you will learn!', "You've also talked about the influence of  Bengt Strömgren?": 'Yes, Strömgren was director of the Copenhagen Observatory but he spent  most  of  the  time  in  the  USA  where  he  was  director  of  two  big  \nobservatories. So I only saw him in the summer time in Copenhagen, but I corresponded with him about my work with the meridian circle.  \nStrömgren  was  one  of  the  great  astronomers  of  the  20th  century, mainly  an  astrophysicist,  but  very  keen  on  promoting  astrometry.  He pioneered  photoelectric  astrometry  by  an  experiment  in  1925  which triggered  my  invention  of  photon  counting  astrometry  in  1960.  He ordered  a  new  meridian  circle  in  1940  which  should  become  the  main instrument  of  the  new  observatory,  and  he  supported  very  actively  the approval of Hipparcos in 1980.  \ntm=08:14  \nYou  then  moved  on  to  spend  15  years,  from  1958-1973,  at  the Hamburg  Observatory  in  Northern  Germany.  What  took  you  from Copenhagen to Germany?  \nOne of the reasons for choosing the Hamburg Observatory was that its  director  Otto  Heckmann had visited Copenhagen in 1955 and he had initiated several meetings [11] of astronomers from Germany, Denmark, Sweden and Norway.  \nI came  to  Hamburg  on  a  10  month  fellowship  from  Deutsche Akademische  Austauschdient.  This  was  extended  by  a  NATO  Science Fellowship  helped  by  a  friend  from  Copenhagen  who  worked  on  the programme  in  Paris.  I  had  no  superior  in  Hamburg.  I  once  asked  Otto Heckmann for  permission  to  travel,  he  answered  that  I  was  completely free, could do what I wanted. Of course we always spoke German and I am still fluent.  \nThey appreciated my work and I became civil servant, staying on for a total of 15 years and I am now enjoying a monthly pension from Germany.  \nWhen  I  had  worked  there  for  about  three  years,  I  remember  a colleague telling that they were trying to get a permanent position for me. I  did  not  believe  him.  In  fact  I  was  never  thinking  of  a  career  but  only about doing my important work - and also about finding a good wife to live with for the rest of my life. And I succeeded by finding her in Denmark  \nin  1961,  but  that  is  another  story.  Our  three  children  were  born  in Germany.  \nI never thought of becoming a professor, a professor was somebody like Strömgren, and I could never match him.  \ntm=10:18', 'How large was the Hamburg Observatory at that time?': 'Hamburg  was  perhaps  the  largest  observatory  in  Germany  at  that time. There was perhaps a staff of 50 people including the technical staff and about ten scientists and ten research students.  \nWhat were the big questions in astronomy that were being discussed and funded at this time?  \nOne of the biggest questions in astronomy in the 1960s was about the  size  of  the  universe,  since  Walter  Baade  had  just  found  that  all distances to galaxies should be doubled. Baade had worked in Hamburg before moving to California.  \nPeople in Hamburg measured photometry of many open clusters on photographic plates taken with the big Schmidt telescope.  \nThere was no internet, nor indeed computers as we know them today. Presumably you read about advances in journals, but only through paper publications and preprints?  \nYes,  and  there  were  weekly  seminars  on  the  Saturday  where everyone was expected to attend in the beautiful library.', 'There was, presumably, little international travel?': 'I had visited the big international conferences in 1958 in Moscow and in  1961  in  California,  financed  from  Denmark,  much  helped  by  Einar Hertzsprung who lived in Denmark. We from Hamburg visited e.g. Jena, Tübingen and Göttingen for common meetings.  \nYou  mentioned  Einar  Hertzsprung,  a  very  big  name  in  astronomy someone who was active around a hundred years ago. Did you meet him?  \nYes, I have met him many times and visited him many times where he lived which was near the railway station of Tølløse where you went if you should go to the new observatory site at Brorfelde. He invited me to come there and paid my ticket and accompanied me back to the train. We talked and sometimes I measured a plate on his machine and he always ended the talks by inviting me to the other room where we would have a dessert served by his daughter.  \nWhat excited you in science: observing, or instrumentation, or finding out how the Universe worked?  \nI  often  visited  the  people  working  in  the  big  Schmidt.  I  saw  they noted the photometer reading on paper and later punched the numbers on cards to be processed in the IBM 650 computer in the city of Hamburg which was 20 km away. I said the reading could be punched directly on cards. This was considered to be a good idea, so I designed and built the digital electronics.  \nIn  those  years  I  had  to  build,  e.g.,  even  the  power  supply  from  basic components. I also digitized a spectrum scanner in order to measure lowdispersion spectra taken with the Schmidt.  \nSo besides of digitizing I was working on an astrophysical project, but in 1960 I got a very good idea for astrometry, an idea that would lead to the satellite Hipparcos.', 'Contributions before Hipparcos': 'tm=14:16  \nYour time at Hamburg was before there was any discussions of a space mission.  I looked up your very first paper, published in 1960 in the Annals  of  the  Hamburg  Observatory:    "Proposal  for  a  Photoelectric Meridian Circle\'. Your central idea was that starlight from the telescope would pass through a fixed grating and be detected by a photomultiplier  \ntube.  Indeed  the  first  figure  in  that  paper  looks  rather  rather  like  the principles that would be used two decades later for Hipparcos . Why was this such a useful advance?  \nYes, you describe my idea quite nicely. The idea came to me on the 22 July 1960  [2].  My  notes  say  that  I  worked  up  to  12  hours  per  day  on  that subject during the following time. I set out to study all aspects of the idea: optimal design, technical feasibility, theoretical basis and limitations e.g. due  to  the  Earth  atmosphere,  the  expected  accuracy  and  I  published  a number  of  papers  during  the  following  years.  I  presented  it  at  a conference in the autumn of 1960 in Jena.  \nI have had the luck often in life to get good ideas at the right place at the right  moment  of  time.  In  this  case  the  idea  was  not  born  out  of  any immediate  local  need,  but  because  the  technical  possibilities  for  digital astrometry  had  come  together  in  my  mind.  The  idea  was  born  from Strömgren\'s  experiments  with  slits  in  1925,  from  Naur\'s  ideas  about computers, and from what I had seen at the big technical fair in Hannover.  \nBut  Otto  Heckmann  immediately  saw  the  potential  for  the  planned expedition  to  Perth  in  Western  Australia  with  the  Hamburg  meridian circle.  This  expedition  was  part  of  international  undertaking  by  ten observatories. They should observe 20,000 stars of SRS, it was a Southern Reference Star catalogue.  \nThey had planned to use the old visual micrometers as they had just used to observe reference stars in the northern sky. But with my digital method all observations could be punched on tape and be treated in a dedicated computer soon after.  \nBut Heckmann  must have asked himself: can I trust that the young Dane, Erik Høg will stay on for the long development?   He asked me in a nice way  and  I  answered:  "this  idea  is  worth  several  years  of  my  life".  It became a success, and Heckmann was very soon able to get the funding in place.  \nThis  instrument  idea  was  used  on  the  Hamburg  Meridian  Circle between 1960-1967. Was it a success?  \nIt  became  a  success  after  a  long  struggle.  In  fact  it  was  NOT  USED  in Hamburg in the time you mentioned. It was only tested there because the development itself took six years, much longer than expected. It became a success  during  the  five  years  in  Perth  from  1967-72,  thanks  to  a  very dedicated and capable team led by the experienced Dr. Johann von der Heide and the young technical assistant Gerhard Holst.  \nThere  were  up  to  10  observers.  The  instrument  was  really  only  a prototype and met with many problems.  \nI  read  in  your  1960  paper  that  the  results  were  recorded  on punched  tape  and  in  fact  I  remember  punched  tape  as  a  recording mechanism from my own PhD in radio astronomy in the late 1970s. Tell us a little more how the observations were actually made in practice.  \nObservation at the instrument was done by two people, usually a married couple.  The  wife  was  sitting  with  the  star  list  in  a  small  cabin  at  the instrument.  She  set  the  star  number  on  toggle  switches  and  told  her husband  the  declination  of  the  star  to  be  observed.  He  turned  the telescope as needed and started the punching on tapes when he saw the star entering the field of view. So the instrument was in fact the first semiautomatic meridian circle, this was a revolution. It produced about 10 big rolls of tape every night.  \nThey were processed on the dedicated computer by the same observing team the next day they were on duty. This computer was a GIER [3] of Danish  manufacture.  GIER  was  one of  the  first  transistorized  computers and it was crucial for the project because it was ten times faster than the cheapest IBM computer we could have afforded, but of course any laptop today is a million times faster than GIER. I had  written all the programs in ALGOL  60,  which  was  the  new  language  to  programming  in  which  my former  tutor  Peter  Naur  had  a  leading  role.  He  won  the  2005  Turingaward, also known as the "Nobel-prize of computing science."  \nAnother big chapter in your early work was the Perth 70 Catalogue of about 25,000 stars.  Tell us more about that.  \nYes,  I  visited  several  times  for  a  couple  of  weeks  to  help  solve  the problems. The result was the Perth 70 Catalogue of about 25,000 stars, published in 1976, in the Annals of the Hamburg Observatory. Much later I published  several  long  historical  reports  [2]  about  the  work,  mostly  in Nuncius  Hamburgensis,  a  series  of  books  edited  by  Dr.  Gudrun Wolfschmidt. I am very grateful for her frequent encouragements to write and  to  speak  at  her  meetings  on  the  history  of  astronomy  during  the recent ten years.  \nI  think  you  formed  strong  and  long-lasting  relations  with  the German  astronomical  community  at  that  time,  which  went  on  to  be important  in  your  work  on  Hipparcos  and  Gaia.  Who  were  your  main contacts then and in later years in Germany?  \nI  knew practically all astronomers in Germany, Walter Fricke was one of them,  Hans  Siedentopf  in  Tübingen  another,  the  list  is  too  long  to continue,  but  I  enjoyed  the  time  in  Germany  and  I  still  have  contact  to many.', 'The Hipparcos years': "You moved back from Hamburg to Copenhagen to take up a post at  the  Copenhagen University Observatory in 1973, where you remained until your retirement in 2002, when you moved to the Niels Bohr Institute as guest professor. Everything we've talked about so far took place before the ideas of a space mission came about. But let's turn to that. The first ideas  for  a  space  mission  were  put  forward  by  Strasbourg  astronomer Pierre Lacroute at the thirteenth General Assembly of the IAU in Prague in 1967.  When did you hear about these ideas for the first time?  \nLacroute presented his ideas of space astrometry [6] [10] at a meeting in Bordeaux  in  July  1965  [4].  This  was  the  first  time  that  such  type  of astronomy was proposed for a space mission. The potential  advantages were clear: no atmosphere and no gravity, and perhaps thermal stability if that would be technically feasible.  \nHow did you react when you saw the ideas of Lacroute set out on paper? Did you think that it was feasible?  \nI attended the presentation by Lacroute in Prague in 1967, but to me and most others the technical problems seemed utterly underestimated. Also because he said it could be done for 10 million French franc.  \nWho  else  in  Europe  was  getting  excited  by  the  idea  of  a  space mission for astrometry? Was it mainly astronomers in France?  \nYes,  the  proposal  did  not  start  any  activity  outside  France,  people  were interested,  but  it  was  fortunately  shared  by  other  French  astronomers. This was really Professor Pierre Lacroute's great vision.  \nWas there some specific moment that CNES abandoned this as a purely French - or French-led - mission?  \nYes,  I  know  that  in  1970  CNES  decided  not  to  pursue  any  purely  French space missions, but only European programs. Especially Jean Kovalevsky was  able  to  convert  space  astrometry  from  being  a  national  project  to become European, through ESA.  \nHow did the idea of a space mission become something considered by ESA?  Who else was pushing this idea?  \nJean Kovalevsky, he was director of a new astrometric observatory in the Southern France and he was really bringing the idea to ESA. Would such a mission  interest  a  sufficiently  important  scientific  community?  That  was the question asked, and for this reason Kovalevsky convened a Symposium on Space Astrometry in Frascati. It was held in October 1974 and gathered 41 participants from Europe and the United States.  \nIn  the  two  days  Lacroute  presented  his  ideas.  There  were  many  other presentations,  one  of  them  by  me  where  I  predicted  that  automatic meridian  circles  could  compete  with  space  astrometry  as  predicted  by Lacroute.  \nAre you saying that, at this time, you didn't really believe in space astrometry?  \nNo, I was not at all in favour of space astrometry at that time. I could not have fostered the idea of space astrometry since my ideas for astrometry went in a different direction. I must say that without Pierre Lacroute there would have been no Hipparcos mission approved in 1980.  \nThe same is true for Jean Kovalevsky: So Lacroute was the father of space astrometry  and  Kovalevsky  was  the  architect,  I  would  say.  I  knew  the scene of astrometry of that time, I knew all the relevant people and I am sure there were nobody who could have played their roles.  \nThis is a remarkable little part of the story Erik, that someone who would go on to be such a strong and leading advocate of space astrometry should start being so sceptical!  \ntm=26:48  \nWho were the ESA staff that your first came across, and in what capacity?  \nI  got  a  call  from  ESA  in  Paris  where  I  was  asked  if  I  would  join  a  study group on space astrometry for a meeting. My answer was YES. I did not believe it was a good idea, but if ESA wanted my advice I had to go and give it.", 'What happened at that meeting?': 'At the first meeting of the group in October 1975, Lacroute presented his two ideas called  Spacelab  option  and  TD  satellite.  I  asked  the  chairman Vittorio Manno which of the two ideas he wanted us to discuss first. He answered very wisely: "Just forget what is on the table, only think of how you can use space technology for your science."  \nThat  advice  had  a  great  effect  on  me,  Vittorio  Manno  spoke  the  right words to turn me towards Hipparcos. I could suddenly think freely. I had never  been  interested  in  space  technology,  but  I  could  go  back  to  \nDenmark where I was able to make a quite new design of an astrometry satellite which I called TYCHO [7], much simpler and more accurate than Lacoute\'s options. After six weeks I sent the proposal to ESA where it was well received.  \nSoon after, I was invited to visit ESTEC for a couple of days. I was tutored by  Maurice  Schuyer  on  e.g.  low  earth  orbits.  I  explained  the  revolving scanning  required  for  my  design  and  I  remember  that  they  could  not understand what I meant, and they said that such a scanning motion had never been tried before.  \nBut when Hipparcos had been approved they soon understood everything very well these engineers.', "What were the key features of Lacroute's original design?": "Lacroute's  original  design  had  three  key  features  which  were  kept  in Hipparcos: The satellite should rotate and systematically scan the entire sky. The satellite should have a split mirror in front of a telescope so that two fields on the sky one after the other would come to the focal plane. The  stars  should  cross  over  slits  in  the  focal  plane  and  be  detected  by photon counting.  \nBut other features in his design were too complicated and unrealistic in my view.  \nWhat  were  the  biggest  problems  that  you  foresaw  in  this  early concept?  \nIt  was  too  inefficient  to  use  only  slits  and  photomultipliers,  I  therefore introduced  a  grid  and  an  image  dissector  tube.  This  made  the  light collection 100 times more efficient, so that a smaller telescope would be sufficient.    My  design  with  a  star  mapper  was  able  to  use  an  input catalogue so that the observing program could be limited to 100,000 stars chosen for their scientific interest.  \nAnother important feature was that measurement should be only in one direction,  only  along  the  scan,  no  measurement  in  the  cross  direction should  be  made.  Therefore  the  split  mirror  could  be  in  only  two  parts, much simpler to manufacture, and the data reduction would be simpler.  \nAlso a new scanning was proposed: revolving scanning where the spin axis is pointed at a fixed angle to the sun. This required active attitude control.  \nAll these features were new. Lacroute quickly adopted an image dissector tube and only this feature in a new design [7] of a scanning satellite. His design was called Option B in the following years while the TYCHO design was called Option A.", "What happened to Lacroute's design?": 'In 1977 our team at ESA decided that an input catalogue should be used as  needed  in  my  Option  A,  which  was  now  often  called  Astrometry Satellite.  This  could  be  decided  after  alternative  options  had  been excluded.  \nSo, Lacroute\'s Option B was not much discussed. He considered it to be simpler than Option A, but it was not. Lacroute was not a good instrument maker, but he had the immense courage to try during ten years because he had a great vision for science. And he learnt English at an advanced age for that reason.  \nWe  have  been  talking  about  some  of  the  technology  design elements. But a key aspect of getting a mission like this adopted is to build the  support  for  Hipparcos  throughout  the  scientific  community.      You played  a  big  part  in  building  up  this  support  for  Hipparcos  throughout Europe - please tell us about that.  \nBack in Denmark I started an "inquiry on projects" [6 A2.1] after the use of an input catalogue had been decided. I visited a number of astronomical institutes  especially  in  Germany  to  give  talks  about  the  project.  I  also distributed to astronomers mostly in ESA countries a questionnaire asking for their favourite scientific projects and how many stars they would like  \nto have observed by the astrometry satellite. Catherine Turon did similarly in  France.  These  answers  were  basis  for  the  Hipparcos  Input  Catalogue later compiled by the Input Catalogue Consortium.  \nYou also recognised that the processing of the data on the ground was going to be a big challenge - given the state of computers. How did you and others start to address this?  \nEven with my new design the data reduction was a formidable task with the  computers  available  in  1976:  it  was  to  derive  positions,  proper motions  and  parallaxes  for  100  000  stars  from  10  million  angular measures  in  the  course  of  three  years.  But  fortunately,  I  already  knew Lennart Lindegren [5] since 1973 when he was a 23 year old student at Lund Observatory, how I met him there is story of its own.  \nIn September 1976 I introduced him to space astrometry and my design. After  four  weeks  he  sent  me  a  nicely  typed  report  with  all  the mathematical formulae for the method which was later used during the mission, the \'three-step method\'. Two weeks later came a report with the first simulations and expected accuracies.  \nWithout  his unfailing genius I would  say  in all mathematical, computational and optical matters the project would not have been ripe for approval in 1980, and probably never.  \nThe  data  reduction  was  a  possible  line  of  attack  for  the  other competing projects during the decision process in 1980?  \nOh yes, and without Lindegren we would have lost.  \nSo, I agreed completely when a colleague said to me: "The best you have ever done for astronomy was to find Lennart." But it was not at all easy to have  Lindegren  included  in  the  Science  Team  selected  for  the  Phase  A study. The members of this team were selected at a meeting in late 1976 of the ESA Astronomy Working Group. At the end they found that there would  be  too  many  members  if  Lennart  were  included  and  it  was proposed that I could still collaborate with him since he lived not far from  \nCopenhagen. I said then that it was more important that Lennart became a member and that I could stay outside. We both became members.  \ntm=35:15  \nIndeed, Lennart Lindegren played a crucial part in the planning and execution of Hipparcos in the 1970s, 1980s, and 1990s, as well as in the planning  and  execution  of  Gaia,  in  which  he  continues  to  be  deeply involved today. This is not the time to go into Lennart\'s contributions, but can  you  tell  us  about  how  your  paths  first  crossed,  and  how  you  went about encouraging him to contribute more to space astrometry?  \nIn September 1973 I heard from a Swedish astronomer that there was a young student in Lund who was working on the old meridian circle. Soon after  I  visited  Lund  to  give  a  seminar  and  there  I  was  introduced  to  the young  Lennart  Lindegren.  He  showed  me  modestly  the  very  nice improvements he had made on the old instrument. I said I was impressed, but there was still a long way to an operating instrument. He said he knew that and that he had done the work only because he was impressed by the fine mechanics.  \nI  offered  him  two  subjects  for  a  PhD.  Soon  after  he  began  work  on meridian circle observations of the major planets from Perth which he did with excellence.  \nIn the ESA system then, the first step in mission selection was called a Phase A study. When was this carried out.  \nThere were two Phase A studies. The final one was completed in March 1978.  \nWho were the ESA people  that  you  came  across  during  the  preselection period, i.e. in the late 1970s?  \nRoger Pacault chaired our meetings and sometimes Henk Olthoff.  \nThere  was  a  lengthy  political  debate  conducted  in  the  scientific advisory committees of ESA during the late 1970s about the relative merits of Hipparcos, and its main competitor at that time, Giotto - a mission to  \nComet  Halley.  There  was  a  strong  division  at  this  time,  almost  along national lines: Germany was favouring Giotto, France favouring Hipparcos. How did you see this competition?  \nThere  was  a  tight  competition  around  Hipparcos.  I  doubted  much  that Hipparcos could win. I once said to a prominent astrophysicist, that there was only this one chance to get space astrometry. If Hipparcos was not selected this time there would not be a second opportunity. I personally would leave the project. He gave me the wise advice, not to use this as an argument, but only to use scientific arguments, and I followed his advice.  \nSo  around  1980,  how  was  Hipparcos  perceived  within  the  wider scientific  community  at  this  time?  Were  other  astronomers  generally interested, supportive, or excited?  \nAstrometrists were generally supportive and in effect many worked later on the realization of Hipparcos.', 'But it was still an uphill battle in getting it accepted?': "Oh  yes,  it  was  very  uphill  [8].  Only  few  astrophysicists  declared  their support and more were against Hipparcos. This is best illustrated by the crucial  role  of  Edward  van  den  Heuvel  in  the  final  decision  of  ESA's Astronomy  Working  Group  in  January  1980.  He  was  a  famous  X-ray astrophysicist but he was a strong advocate of Hipparcos for its scientific value, when he was asked to present Hipparcos in the group. Before the meeting he had been able to convince two of the astrophysicists to vote for Hipparcos which then got the majority. That was a great surprise.  \nWithout  Ed  van  den  Heuvel,  Hipparcos  would  have  lost  to  the  EXUV mission  (EXtreme  UltraViolet)  and  nothing  could  have  changed  that decision. I have written a report [8] with documentation of these events. I did  not  hear  about  these  events  until  much  later  when  I  met  van  den Heuvel and he told me.  \nAround  the  1980s,  astrometry  played  an  important  part  in American  science,  in  part  because  of  its  relevance  to  solar  system  \nnavigation. One of the big players in US astrometry at the time was the US Naval Observatory based in Washington. How did they react to Hipparcos?  \nYes, they were interested and I always gave a talk there when I visited the US.  \nIn your own mind, did you see Europe as being in competition with the US at that time?  \nNo,  there  was  no  competition  from  US  because  astrophysics  was  so dominant there.  \nYou've  said  in  your  more  recent  writings  that  a  mission  like Hipparcos  would  never  have  been  undertaken  in  the  US.  What  did  you have in mind when you said that?  \nCould NASA have realised a Hipparcos-like mission? NO! For two reasons: the  American  astrometric  community  had  much  less  resources  of competence to draw from than were available in Europe, and secondly, as an American colleague said to me in those years: 'You can convince a US Congressman that it is important to find life on other planets, but not that it is important to measure a hundred thousand stars.'  \nWhich  other  nations  were  interested  or  supportive  of  space astrometry at that time - UK, Russia, other Scandinavian countries?  \nThere was much interest for Hipparcos in Russia, or USSR as it was. For example they published the first translation of an article on Hipparcos by me.  \nYou have also said in your own notes and memoirs that if Europe had not accepted Hipparcos in 1981, it would have been a lost opportunity that might never have re-appeared. What did you mean by that?  \nIt appears that the approval by ESA could well have failed, in which case I am sure Hipparcos would never have been realised. This proposition has been  countered  by  a  colleague:  'You  can  never  know  that,  something  \ncould have happened.' But please consider the situation of astrometry at that  time  [10].  For  decades  up  to  1980  the  astrometry  community  was becoming ever weaker, the older generation retired and very few young scientists  entered  the  field.  I  myself  would  have  lost  the  faith  that  the astrophysicists would ever let such a space mission through, and others would also have left the field of space astrometry.", 'Do you think that a revival later would have been possible ?': 'If  someone  would  have  tried  a  revival  of  the  idea  one  or  two  decades later, the available astrometric competence would have been weaker, and where  should  the  faith  in  space  astrometry  have  come  from?  When Hipparcos became a European project in 1975 and the hopes were high for a realization, the competence from many European countries gathered and eventually  was able  to  carry  the  mission.  This  could  not  have  been repeated after a rejection of the mission.', 'Politics and influence': "tm=43:37  \nYou  have  received  some  significant  international  recognition  for your work over the years. In 1999, you were awarded the European Space Agency's  Director  of  Science  Medal  for  your  leading  contributions  to Hipparcos.  In  2005,  the  International  Astronomical  Union  named  an asteroid  after  you:  Erikhoeg.  In  2009,  you  received  the  Russian  Pulkovo Observatory's Struve Medal for your contributions to astrometry. In 2014, you received an Honorary Doctorate from the Ukrainian National Academy of Sciences in Kiev.  And in 2019, you received the Instrumentation Prize of the  German  Astronomical  Society,  for  your  lifelong  work  in  astrometry. How  have  your  contributions  been  viewed  in  your  home  country  of Denmark?  \nIn Denmark there was strong support for astrometry and that put me on the  path  in  science.  Danish  funds  supported  my  work  on  space astrometry,  but  all  resources  for  astrometry  of  the  Copenhagen Observatory went into the automatic meridian circle in  Brorfelde  which  \nwas later moved to La Palma. That was OK and a great success in its own, but  with  the  result  that  the  observatory  had  no  resources  for  space astrometry, except that I was of course allowed to do this work.  \nWas  Tycho  Brahe  someone  whose  achievements  inspired  you  in any serious way? I recall that we had a wonderful early meeting of your data analysis team on Tycho's island of Hven.  \nI have often read about Tycho Brahe and a few years ago I wrote a paper [12 and 13] about his fruitful interaction with the Landgrave in Kassel who was  a  bit  older  than  he.  It  was  surprising  for  me  to  see  that  the observations in Kassel were more accurate than those on Hven, but they were of fewer stars and they were published many years later too late to have any impact on the progress of astronomy.  \ntm=45:55  \nCan you point to some specific strength or personal quality that's helped you on your scientific journey?  \nI  think  of  myself  as  a  kind  person  interested  also  in  the  well-being  of others. Sometimes I was too kind and too patient and then realized I was being misused. But sometimes my patience was so stressed that I lost my temper which I regret.  \nThat final comment surprises me. I have known you for forty years and worked very closely with you for more than thirty and I do not think I ever  recall  you  losing  your  temper.  You  always  spoke  with  passion  with honesty and I think as a scientist one cannot do more than that!  \nDid  you  look  back  to  anyone  in  particular  who  inspired  you, encouraged you, or influenced you early on in your career?  \nI would mention my teachers in Copenhagen as a student: Peter Naur and Bengt Strömgren as I have already explained.  \nIs  this  something  that  you  think  about  today  -  inspiring  or encouraging young people setting out on their own paths in science ?  \nLook around and be kind to yourself and to others.", 'Closing off': "This has only touched on a few aspects of the Gaia mission and beyond, in particular…  \n….  some  of  the  main  points  seen  through  the  eyes  of  one  of  space astrometry's leading scientists  \nand it's been interesting to hear some of your thoughts as you look back over this important period", 'References': '[1] Høg E. 2011, Astrometry history: Roemer and Gaia  \n26pp at: http://arxiv.org/abs/1105.0879  \n[2] Høg E. 2014, Interferometry from Space: A Great Dream  \nIn: Asian Journal of Physics Vol. 23, Nos 1 & 2  (2014),  \nSpecial Issue on History of Physics & Astronomy, Guest Editor:  Virginia Trimble.  \nhttp://arxiv.org/abs/1408.4668  \n[3] Høg E. 2021, The Gaia Successor in 2021.  \n3 pp at: http://arxiv.org/abs/2107.07177 [astro-ph.IM]  \nhttp://www.astro.ku.dk/~erik/xx/GaiaSuccessor2021.pdf  \n[4] ESA 2022, Refereed publications from ESA missions.', 'An interview about the revival of astrometry after 1980': "by Michael Perryman and Erik Høg  \nABSTRACT: Michael Perryman invited Erik Høg to this interview in October 2021. The  first  space  astrometry  satellite  Hipparcos  was  approved  by  ESA  in  1980  and launched  to  a  three  year  mission  in  1989.  This  interview  is  about  Hipparcos,  and how it was developed to measure 100 000 stars more accurately than could be done from the ground, measuring positions, motions and distances. Erik Høg was one of the scientific leaders of Hipparcos in the 1980s and 1990s, active in the design and data analysis. He introduced an add-on to the satellite in 1981 called Tycho which made the mission even more successful. This resulted in the Tycho-2 Catalogue in 2000 of the 2.5 million brightest stars in the sky with accurate positions and motions and including two-colour photometry. Hipparcos was followed by the launch in 2013 of  its  follower  Gaia  which  is  presently  revolutionizing  astronomy.  This  report contains  the  questions  by  the  interviewer,  the  answers  by  Erik  Høg,  a  link  to  the audio recording and references.  \nHere follows a script of the recording. Erik in red is quoted closely but Michael not so closely, with slanted text.  \nThe audio is here: http://www.astro.ku.dk/~erik/xx/hoeg2-final.mp3 , but you should rather open it on Michael's website with photos and other interviews: https://www.michaelperryman.co.uk/gaia-project-interviews The minutes and seconds are tagged at tm=... Total duration 45 minutes.", 'The early steps': "After several years of scientific preparation, which we discussed in our first conversation, the  Hipparcos space astrometry mission was eventually accepted by ESA's advisory committees and confirmed by ESA's Science Programme Committee in Paris in September 1980.  \nDo you remember where you were when you heard this news, and how you felt?  \nNo I do not remember where I heard this great news, perhaps because in most of September  that  year  I  was  in  China,  invited  by  Mrs  Yeh  Shi  Hua  and  Dr.  Hu Ningsheng. It was the first of many visits and China became part of my life, but that is another story.  Hipparcos stayed as my top priority for over twenty years.  \nBut  how  did  you  react  to  the  news  when  you  heard  of  its  acceptance  excitement, relief, or what?!  \nI do not exactly remember, but I am sure I was happy - and perhaps a bit surprised as I have already explained.  \nYes because it was a long struggle to get the mission  accepted.  \nBefore we talk more about the work of you and your scientific teams, let me summarise briefly  for  our  listeners,  how  these  sorts  of  space  science  missions  are organised.  \nESA appoints a Project Manager (in this case Franco Emiliani) responsible for the  technical  development  and  a  Project  Scientist  responsible  for  the  scientific aspects, and ensuring that the science goals are achieved obviously these systems have to work closely together, respecting each other's constraints and goals.  \nESA eventually selects an Industrial Consortium responsible for building the satellite. And as Project Scientist for Hipparcos, I set up a scientific advisory team to guide all aspects from start to finish. At the same time, ESA approved the setting up of three scientific consortia across Europe, one to compile the catalogue that would be  the  starting  point  of  the  satellite  observations  and  two  independent  teams  to process the satellite data and create the final catalogue.  \nSo, in 1981, we were sitting with a pile of technical notes which resulted from several years of theoretical studies about the mission's feasibility. What do you recall of the first steps that ESA took in moving from these studies to a real mission?  \nWhat first  comes to my mind is  the invitation to a meeting  in 1981 in  Paris with engineers  from  MATRA,  one  of  the  a  companies  competing  to  build  the  satellite. They wanted to gather an advisory team of scientists, which they did.  We often met in Toulouse over the next few years. This was under a contract where we promised  \nconfidence  because  there  was  a  competing  company  also  hoping  to  build  the satellite. ESA did not much like such advisory teams, but they were tolerated.  \nBack  to  the  first  meeting  with  MATRA  where  I  was  the  only  scientist  yet.  They impressed  me  by  showing  a  number  of  designs  of  the  optical  telescope  system. Three of the designs were definitely better than the one we had considered in our Phase A studies: I now understood how clever industry can be in optical design.  \nAnd in the end MATRA obtained the contract for Hipparcos.  \nYes,  and  we  benefitted  from  their  expertise  and  experience  in  many  other areas as the project progressed…", 'The Hipparcos Science Team and pre-launch': "From 1981 to the end of mission in 1997, the Science Team numbered about 12  people  from  across  Europe  covering  different  scientific,  astronomical,  and technical competences we met in person every 4-6 months, reviewing progress, and assigning  tasks  and  priorities.  You  were  one  of  ESA's  (my)  scientific  advisers throughout this time and you also led one of the two data processing teams, which you called NDAC.  \nLet's  talk  first  about  the  Hipparcos  Science  Team  -  which  I  chaired  for  17 years. Tell us about these meetings from your perspective. What sort of topics were handled by the Science Team and what sort of preparation did this demand from your side?  \nI remember that a great deal of my time those years was dedicated to this work. My wife believes it was never out of my mind. We never took more than one or two weeks summer vacation. Not until 1992 did I take as much as three weeks, enough to bring us by train and ship as far north as Tromsø at the polar circle in Norway.  \nIn  preparation  of  the  meetings  I  read  a  lot  of  documents  on  technical  or organisational  matters,  and  we  prepared  documents  for  the  Science  Team  to promote our ideas, and to present the mission at various meetings. I enjoyed the  \nmeetings and I was always fully alert, while I sometimes saw a dreaming engineer when science was discussed.  \nI  regret that I was unable to take good notes while a few of my colleagues wrote with  a  perfect  hand  in  solidly  bound  books.  I  took  notes  but  only  on  a  piece  of paper.  I  have  sometimes  asked  Lennart  Lindegren  what  happened  at  a  certain meeting and he just looked it up in his book. The chairman, you Michael, always had a carefully planned agenda which was followed, and tables were always booked in a nice restaurant for the evening. I enjoyed these social occasions very much. I should add that we all shared the bill for the evening, but ESA paid our travel expenses.  \nOur Science Team meetings sometimes took place at the home institute of one of the  team  members.  Such  a  meeting  in  Copenhagen  had  a  session  at  the  location where Ole Rømer put up the meridian circle he had constructed in 1704. This place was 20 km outside Copenhagen because Rømers official observatory on top of the famous Round Tower was too unstable for his delicate observations of positions.  \ntm=9:00  \nMost  of  our  meetings  were  at  ESA's  Research  &  Technology  Centre  at Noordwijk,  in  The  Netherlands.  There  was  always  a  lot  of  technical  information provided by the project team and guidance requested from our scientific advisors… Did you see a big change in the speed of progress starting in the early 1980s?  \nI  do  not  remember  anything else than a nice speed of progress all the time after 1981.  \nThere is a big gulf between early ideas for a space science mission and the detailed design, the technical development, the integration and testing and of course the launch and operation. After the Phase A studies in 1980, which led to a baseline mission  concept  were  there  any  major  new  technical  developments  made  to  the basic design?  \nThe baseline was maintained, but it was now implemented with reality, e.g. with a good optical design as I mentioned. Gas jets were introduced for attitude control instead of the reaction wheels that I had imagined.  \nDuring  the  Phase  A  Studies  you  already  made  a  number  of  innovative contributions to the instrument and satellite design. But in the period we are talking of now were  there  aspects  of  the  instrument,  or  the  satellite  technology,  that concerned you most,  or where you felt you had the most influence in the years up to launch?  \nI only remember that I always felt I was doing useful work on a great astronomical project.  \nWe want to focus here on some of the satellite data analysis aspects. Before launch,  from  1981-1989,  your  consortium  NDAC  (for  Northern  Data  Analysis Consortium) was focused on preparing for the satellite data…. first, why was it called NDAC?  \nThere  were  two  data  analysis  teams  who  developed  the  software  independently, and would process the satellite data independently   -  the  other  was  the  FAST Consortium,  led  by  Jean  Kovalevsky.  We  called  ours  the  northern  consortium because the FAST consortium was more in the southern part of Europe.  \nWhat did these preparations for the data processing software involve, and how was your team actually structured?  \nFrom 1981 we had to build a consortium as required by ESA. It was clear from the beginning that it should involve the Royal Greenwich Observatory where I had long had  contact  with  Andrew  Murray  -  whom  I  had  brought  into  ESA's  Astronomy Working Group (AWG) to succeed me. Also Mike Cruise from Mullard Space Science Laboratory  was  supporting  the  project.  I  had  always  seen  Murray  as  the  natural consortium leader, but he insisted that it should be me when we began forming the consortium  called  NDAC.    Of  course  Lund  Observatory  with  Lennart  Lindegren should  come  in.  In  Copenhagen  I  had  support  from  the  Danish  Space  Research Institute, and from the Danish Geodetic Institute. In both cases we could get access to  computing  facilities  and  to  methods  of  treating  large  systems  of  equations, especially through Knud Poder from the Geodetic Institute.  \nWhen you were building up the data processing software for the treatment of the real data there were many studies and simulations that were undertaken and the  results  of  these  simulations  and  tests  was  also  fed  back  to  the  design  of  the satellite…  \nWhat sort of results from these early studies was fed back into the satellite design?  \nWe undertook the first simulations in Copenhagen in 1980 which were crucial for the  design  of  angular  one-dimensional  observations  with  an  astrometry  satellite with revolving scanning. We could show that the subsequent reconstitution of the celestial  sphere  could  give  (1)  relative  positions  and  (2)  absolute  trigonometric parallaxes,  for  a  network  of  stars  covering  the  entire  sky,  after  only  one  year  of observation.   Many  other  simulations  were  made  to  develop  the  data  reduction chain of NDAC.  \nYes,  and  these  studies  led  to  the  choice  of  basic  angle,  piloting  of  the detector, gas jet firings for attitude, etc.  \nLet's mention the main task leaders of your consortium NDAC: these people stayed with the project for its entire duration!  \nMany people were involved in the consortium: all 19 are listed by name in the final catalogue. Let me mention here those working on the data reduction during most of their  time:  Carsten  Skovmand  Petersen  and  myself  from  Denmark,  Lennart Lindegren (Consortia Leader from 1990) and Staffan Söderhjelm from Sweden, with Dafydd  Wyn  Evans,  Floor  van  Leeuwen  and  Andrew  Murray  from  the  Royal Greenwich Observatory, United Kingdom.  \nI  always aimed at a collaboration of very few institutes: at RGO, Copenhagen and Lund for the sake of efficiency and ease of organisation, but efficiency was also a hard necessity with the very limited resources I could expect to have.  \nYour  approach  to  the  data  analysis  problem  was  quite  different  to  the structure of the other data analysis team, called FAST?  \nTwo consortia were working in parallel on the Hipparcos observations in order to compare the results from two independent groups for verification of these unique space observations. Jean Kovalevsky led the other consortium (FAST) and he created a  much  bigger  organisation  than  I  did  for  NDAC.  He  wanted  to  involve  as  many countries in Europe as possible to make Hipparcos a real European mission, a very laudable  goal  indeed,  and  he  succeeded  with  a  consortium  of  82  persons  in  five countries: France, Germany, Italy, The Netherlands and (a few from) the US.  In this way,  Kovalevsky  paved  the  way  for  further  European  space  astrometry  after Hipparcos.  \nThe limitations that existed in computers and computer networks at this time meant  that  the  successive  steps  of  the  data  analysis  were  made  by  physically sending the data from one institute to the next, tell us about this.  \nIn  NDAC  the  satellite  raw  data  were  first  treated  at  the  Royal  Greenwich Observatory  (UK),  and  the  output  was  sent  on  magnetic  tapes  to  Copenhagen, where the abscissae of the stars along great circles were derived. They were used to derive  the  five  astrometric  parameters  for  all  stars.  At  Lund,  in  Sweden,  the processes were defined mathematically, and the double stars treatment also took place there, led by Staffan Sõderhjelm and Lennart Lindegren.  \ntm=17:00", 'Tycho': 'When  Hipparcos  was  accepted  by  ESA  in  1980,  it  targeted  measuring 100,000 stars to a typical accuracy of 2 milliarcsec, 2 thousandths of an arcsec we ended  up  doing  a  bit  better  than  that  in  practice:  almost  120,000  stars  to  an accuracy of 1 milliarcsec.  \nBut a big development came in 1981, with your ideas for an add-on to the satellite, which you called Tycho. Let me read from my own account of the mission which gives the background:  \nIn  April  1981,  already  well  into  the  satellite  design  phase when significant modifications would normally have been rejected out of hand for reasons of risk, and for the increased cost that they could incur, Erik Høg pointed out that a rich seam of satellite data was lying untapped. He realised that the signals from the satellite attitude detectors contained a staggering quantity of star  positions  that  were,  quite  simply,  not  being  sent  to  the  ground.  But  once  pointed  out  the  \nharvest  was  obvious:  positions  for  the  million  or  more  stars  not  being  observed  by  its  main detectors.  Working  quickly  to  optimise  the  system,  and  with  a  little  urgent  lobbying,  the  small amount of additional funding needed was made available. A filter and additional detectors were added, an extra telemetry channel set up to send the data to ground, and a new data processing team put in place to handle the data stream.  \nSo Erik, in addition to the two existing processing teams you set up another the Tycho Data Analysis Consortium which you then led through to completion. How did  this  idea  of  using  the  satellite\'s  star  mappers  to  get  this  extra  star  catalogue come to you?  \nThe  idea  came  in  March  1981  when  I  wanted  to  define  suitable  meridian  circle observations of reference stars for the Hipparcos attitude determination. I realized that the photon counts from the star mappers could be used to detect many more stars  than  the  few  thousands  required  for  the  attitude.  My  estimate  was  that positions and magnitudes could be obtained for at least 400 000 stars, and perhaps many more. But this  required  that  all  the  data  had  to  be  transmitted  to  ground, about  100  Gbytes,  the  same  amount  as  for  the  main  Hipparcos  mission.  I immediately wrote three short notes to ESA about this Tycho project as I called it.  \nThis  was  really  a  very  interesting  example  of  something  which  was  quite obvious  to  everyone  as  soon  as  you  pointed  it  out,  but  had  escaped  everyones attention  except  your  own.  But  I  think  that  everyone  on  the  Science  Team immediately understood its importance?  \nThe  proposal  was  well  received  by  you  Michael,  and  in  the  Science  Team.  I remember especially seeing the faces of Michael Grewing and Jean Kovalevsky at the meeting. The name Tycho was accepted without discussion, and this was a great relief  for  me  since  I  remembered  that  Kovalevsky  had  introduced  the  name Hipparcos for the satellite instead of the name Tycho that I had wanted.  \nOf course, convincing ESA to find and approve the funding for the changes was another matter?!  \nLater  in  1981,  ESA  approved  the  implementation  and  the  extra  cost  as  just mentioned by you, Michael. From Vol.4 XI I read: " Had the Tycho proposal come a  \ncouple of months later, the satellite design would have been frozen, and the idea of the Tycho project would have been a lost opportunity. This wonderful idea would have  been  difficult  to  forget  about,  even  though  we  were  immersed  in  the fascination of the main mission, and in all the work it gave us.\'  \nThe  data  reduction  for  the  Tycho  experiment  still  had  to  be  set  up  and organised. So late on in the mission, and with a such a big data volume, that was not an easy challenge, Erik?  \nNo,  that  was  not  easy  at  all.  I  recall  a  moment  at  the  beginning  when  I  was overwhelmed having to do that and said so to Lennart Lindegren, when we were waiting for the bus somewhere. He just smiled and said he was sure we could do it, I then even believed he would do it in Lund!  Lennart was always a great support. I never hesitated Michael, when I had the good ideas in those years, and luckily I did not know how big the tasks would be.  \nMichael  Grewing  was  the  first  to  tell  his  interest  by  letter  when  I  proposed  the Tycho  project,  and  as  director  of  the  astronomical  institute  in  Tübingen  he maintained  an  excellent  young  team  there  over  the  years.  Carlos  Jaschek  was director of the institute in Strasbourg, and a team there also did a wonderful job. But these two teams, together with Copenhagen and Lund, were still not enough for the task ahead.  \nDerek McNally in the UK made an effort to form a team, but continued support for a team failed. So we had a big problem. But then Roland Wielen as director of the Astronomisches Rechen-Institute in Heidelberg (who had recently taken over from Walter Fricke) he offered to establish a team and of course I gladly accepted.  \nUli Bastian of that team told me later that at first he did not want to work on Tycho because he was always an astrophysicist. But Wielen insisted: then he had to do it and he did it wholeheartedly throughout the years.  \nAll this led to the Tycho Catalog of a million stars, published at the same time as the main catalogue in 1997. Tell us about the scientific team that you organized to process this additional data.  \nThe observations consisted of photon counts from the satellite\'s star mapper slits, continuously obtained during the 37 months of mission. Detection of a star in this data stream - or rather of a signal above the noise which could therefore be a star crossing  the  slits  -  that  was  recorded  as  a  function  of  time.  Stars  were  later confirmed and identified with real stars in the sky using two sources of information: 1) accurate knowledge of the pointing of the satellite telescope for every moment of time,  the  so-called  satellite  attitude,  and  2)  a  list  of  3  million  stars  with  positions known from ground based observations.  \nThis  may  seem fairly  simple  in  principle,  but  it  was  quite  complicated  in  practice, because  the  work  had  to  be  done  within  a  tight  time  schedule  and  it  had  to  be divided  between  three  institutes  due  to  the  limited  computer  capacity  and  manpower. It  is  fair  to  say  that  no  astrometric  research  before  had  been  done  under such constraints as the Hipparcos and Tycho data reductions. Therefore, we owe a cordial thank you to all those working in the consortia for their dedication to our great projects during many years.  \nIn addition to new people that you brought in to the Tycho Consortium, many individuals already working in the NDAC and FAST teams also contributed expertise and data?  \nYes, indeed. The Tycho Input Catalogue of 3 million stars was based on early access to  the  Guide  Star  Catalog  for  the  Hubble  Space  Telescope  as  provided  by  Brian McLean  and  Jane  Russell.  The  satellite  attitude  was  provided  -  from  the  main satellite data processing - by Franceso Donati and Pier Luigi Bernacca in Italy (from the  FAST  Consortium)  and  by  Floor  van  Leeuwen  in  the  UK  (from  the  NDAC Consortium).  \nMany people were involved in the Tycho consortium: all 37 are listed by name in Vol. 1 of the final catalogue. Let me mention those working on the data reduction  \nduring most of the time: Claus Fabricius, myself, Valeri Makarov, Holger Pedersen, and Carsten Skovmand Petersen from Denmark, Daniel Egret, Jean Louis Halbwachs, Jean Kovalevsky, Catherine Turon and Pierre Didelon from France. Further members were:  ten  from  Germany,  two  from  Italy,  two  from  The  Netherlands,  one  from Sweden, one from Switzerland, four from United Kingdom, and two from the USA.  \nAmong the many Tycho Consortium meetings that you held over the years, do any stand out in particularly in your memory?  \nWe  had  many  meetings  of  the  consortium  during  the  1980s  at  the  participating institutes,  and  one  of  the  meetings  took  place  in  1984  on  the  island  Hven,  the historic site near Copenhagen where Tycho Brahe with his many collaborators made his famous observations of one thousand stars during twenty years from 1576-96. At the meetings we discussed the progress of work and defined how to proceed and we had a nice and productive time together. I stayed a year as guest professor at the institute in Tübingen and two months in Strasbourg.  \ntm=28:00  \nWe mentioned that the Tycho Catalogue was published in 1997, at the same time as the main Hipparcos catalogue. But  this was all followed by the improved Tycho-2 Catalogue in 2000. What was the difference between Tycho-1 and Tycho-2, and what was needed to construct it?  \nOne  big  difference  for  our  work  on  Tycho-2  beginning  about  1997  was  that  we collected all star mapper detections from the whole mission close to any star in the input catalogue. From these fifty or more detections we could decide whether we had a real star  with much better statistical certainty than from the single detections used  for  Tycho-1  and  this  resulted  in  accurate  positions  for  2.5  million  stars,  a fantastic number for that time.  \nThis possibility had been clear to me all the time, but it had only become possible with the greater computing power we could now buy. We were also able to get the needed private funding for the computer and for Claus Fabricius and Valeri Makarov to  do  the  work  in  Copenhagen.  Andreas  Wicenec  supplied  the  satellite  data  from Tübingen on a new kind of tapes. Uli Bastian and Peter Schwekendiek in Heidelberg  \nsupported with computations, and the final satellite attitude was provided by Floor van Leeuwen at the Royal Greenwich Observatory.  \nAnd then, very importantly, there are good proper motions for every star in Tycho-2?  \nYes!  The other big advantage over Tycho-1 is that Tycho-2 indeed contains accurate proper  motions.  They  were  based  on  the  Tycho-2  positions,  which  were  then combined with a new analysis of the positions observed during the past 100 years and  given  in  144  ground-based  astrometric  catalogues  provided  by  the  US  Naval Observatory in Washington DC by Sean Urban.  \nI  recall  meeting Sean at a conference in 1997 in Gotha and realizing this fantastic possibility.  They  had  not  yet  completed  the  new  analysis  but  we  received  all  the positions just in time to derive the proper motions for our publication in 2000. Had they come later our funding would have ended before we could do so. Tycho-2 has been the most cited astrometric catalogue ever since 2000 apart from the Hipparcos Catalogue.  \nClose double and multiple stars could also be treated much better in the Tycho-2 processing. Claus Fabricius and Valeri Makarov published 2-colour photometry for almost 10 000 components in systems with separations above 0.3 arcsec.  \ntm=32:00  \nAnd the Tycho-2 Catalogue took on a renewed importance with the advent of  Gaia,  because  the  positions  from  Tycho-2  from  the  early  1990s  could  be combined with the early Gaia positions to give the TGAS (or Tycho-Gaia) Catalogue with almost 25 years of proper motions and which was also important for the early Gaia Astrometric Iterative Solution.  \nDid you have any involvement in this latest TGAS Catalogue?  \nNo I did not, except sharing the joy over TGAS. The rate of citation for Tycho-2 has stayed constant even after publication of Gaia results beginning in 2017.', 'Launch': "Hipparcos was launched by Ariane 4 from French Guyana in 1989. As one of the key scientists, and one of the consortium leaders, you attended the launch. Tell us about your experiences, seeing the launch, and witnessing this new chapter in astrometry .  \nThe Hipparcos Science Team and many others witnessed the launch in Kourou, but some of us had the special honor to fly in the super-sonic Concorde. From Hipparcos we were the consortia leaders as well as  Professor  Pierre  Lacroute,  the  father  of space astrometry. We flew from Paris to Dakar and then on to Kourou, far above the highest  clouds,  and  we  could  see  the  setting  sun,  all  the  time  at  the  same  angle above the horizon while we crossed the Atlantic. I had a window seat and noticed that the wall became warm during flight, due to air friction at the super-sonic speed.  \nWe saw the impressive launch facilities in Kourou, had our luxurious breakfast at the beach, and we saw the launch from a spot very close to the rocket. We saw the fire and smoke and we saw the big rocket slowly lifting off. But we heard nothing, not until many seconds after. We were happy seeing the glowing fire from the rocket moving away up there, and we toasted in champagne.  \nAfter a spectacular and flawless launch, we had big problems -  the apogee boost  motor  intended  to  circularize  the  satellite  orbit  failed  to  operate  and  the satellite  was  stuck  in  an  elliptical  10-hour  orbit  rather  than  the  planned geostationary.  \nI  don't  want  to  dwell  on  the  enormous  technical  problems  that  followed from this but instead let's concentrate more on the treatment of the satellite data. Do  you  remember  the  moment  when  the  first  'reference  great  circles'  were constructed  and  when  the  first  sphere  solutions  or  first  Tycho  results  started  to appear?  \nI do not recall precisely these great moments. But I remember a conference in 1989 of the Pulkovo Observatory near Leningrad (now St Petersburg) where I had been asked to present Hipparcos on behalf of the Science Team. It was during the months after the post-launch apogee boost motor failure, and everybody asked about the  \nlatest news from the great Hipparcos mission - which had a very uncertain future after the boost motor failure.  \nIn the presentation I had shown a picture of the first Tycho observation. It was a plot with the very first recording of transits over the four slits in the star mapper during satellite commissioning. I had made a number of prints for distribution and I can still see all the eager hands reaching to get such a print of the first Tycho observation.  \nA  distinguished  person  approached  me  during  a  break.  Away  from  the  crowd  he presented himself as coming from the Russian space agency and offered to launch a second Hipparcos in case ESA could not.  \nAnd this is where you first met Valeri Makarov, who played a big part in the Tycho processing?  \ntm=36:00  \nYes, Viktor K. Abalakin, director of the Pulkovo Observatory, introduced me to his young people: Mark Chubey and Valeri Makarov. I had before heard of Russian plans to  launch  a  Hipparcos  successor  ten  years  later  in  order  to  derive  very  accurate proper  motions  from  positions  at  the  two  epochs.  Now  I  met  these  people  with whom I had very fruitful discussions during the following years. Valeri Makarov soon became my very bright collaborator in Copenhagen as I've already mentioned.  \nA year later I was again invited to Pulkovo to tell about Hipparcos progress. They took me on a visit to Moscow to the famous space control centre, and further on to their observatory in the Caucasus. I enquired about their plan for a successor. After a day trying to understand their design, I realized that I could not understand how it would  work,  and  that  instead  I  was  in  the  process  of  designing  a  better  more accurate  Hipparcos  satellite.  During  the  further  travel  I  continued  the  design  and discussed it with them. This led to further visits in Pulkovo, Moscow, Copenhagen and Lund for discussion of our ideas. Finally in 1992 I made a design with Lennart Lindegren of a mission with CCD detectors which we called Roemer and which later became Gaia [1].  \nI  also recall a conference in Cambridge probably in 1990 when the first analysis of some Tycho observations had been made. I could then see that we would be able to make a catalogue of one million stars, not only 400 000. That was great news. The reasons  were  a  slightly  higher  quantum  efficiency  of  the  photo-multipliers  and  a more  accurate  background  determined  with  a  method  invented  by  Andreas Wicenec using the median value of the photon counts during an interval of time.  \nI  think  it's  worth  emphasising  that  the  final  catalogue  accuracies,  both  for the main Hipparcos Catalogue, and for the Tycho Catalogue were very close to the theoretical predictions that you and your colleagues had made much earlier on in the mission design - for such a complex instrument, this was a great achievement?  \nI  would  rather  say  that  the  predicted  performance  was  significantly  exceeded  in several respects. The final accuracy of Hipparcos reached 1 milli-arcsecond instead of  the  predicted  2  milli-arcsecond,  and  this  is  quite  significant  in  the  view  of  an astrometrist. And the Tycho-1 Catalogue contained one million stars - compared to the 400 000 in my original proposal. The Tycho-2 has even 2.5 million stars and also accurate  proper  motions.  So  Hipparcos  performed  much  better  than  expected  in spite of the bad orbit. We really must admire the responsible operations team for their technical expertise.  \ntm=40:00", 'Finalising the catalogue': "The eight years from 1989, when the satellite was launched, to 1997 when the final results were published were very hectic, all of the teams working to a tight schedule,  to  get  the  results  out  to  the  wider  astronomical  community,  and  when research based on the data actually started.  \nYou  had  handed  over  leadership  of  the  NDAC  Consortium  to  Lennart Lindegren  soon  after  launch  to  focus  your  efforts  on  the  Tycho  data,  and  you presented  the  Tycho  Catalogue  for  the  first  time,  at  a  major  international conference in Venice in 1997.  \nWhat  was  it  like  Erik  to  be  in  this  position,  and  how  did  the  wider astronomical community react to it?  \nThe Venice conference was a great event in every respect, the setting itself in Italy where I knew that you and Pier Luigi Bernacca had done the perfect preparations. The results we could present were very well received.  \nI  know you gave many lectures, and many interviews with the press, after the catalogue publication. Do any particular events stand out in your memory?  \nI think first of the big International Astronomical Union conference in 1997 in Kyoto, Japan, where we could present our results  - and we could again see they were very well received.  \nLooking back now Erik, In one lifetime you have helped take astronomy from the confines of the Earth out into space with two highly successful space missions.  \nYou could never have imagined such progress when you started out in the field 60 years ago?  \nNo,  I  never  imagined  anything  like  that.  I  began  with  astrometry  at  the  new meridian circle in Denmark in 1953 when I was just 21 years old. In Copenhagen I saw that astrometry was central for astronomy. But soon after I  understood that astrometry  was  not  so  highly  rated  everywhere.  From  1958  at  the  Hamburg Observatory at first on a 10 months fellowship I wanted to do astrophysics because I clearly saw that real astronomy happens in that area while astrometry was rather old-fashioned.  \nBut in July 1960 something happened. I had a revelation you might say, the vision of photon  counting  astrometry  with  meridian  circles.  It  could  be  done  using computers, punched tapes and all that. That brought me back to astrometry. It has turned out after all, that I have done more for astrophysics than I could ever have done as an astrophysicist.  \nThe idea was well received in Hamburg-Bergedorf, the director Otto Heckmann saw that it should be used for the meridian circle expedition to Australia which was in preparation. And Heckmann trusted me, he trusted that I had the talent and that I had the stamina to hold on to the project for years. I had the luck to have the right  \nidea at the right place and right time and I had the luck to meet the right people. The  same  luck  I  had  later  with  the  design  of  Hipparcos  in  1975,  the  Tycho  star mapper project, and the Roemer mission with CCDs which led to Gaia. It seems that the luck was there again with the proposal in 2013 of a Gaia successor in twenty years, a project now having a high ESA priority and a quite probable launch in 2045 [2].  \n- As  I  mentioned  in  our  first  interview.  You  received  some  important international recognition for your work over the years:\n- In  1999,  you  were  awarded  ESA's  Director  of  Science  Medal  for  your  leading contributions to Hipparcos.\n- In  2005,  the  International  Astronomical  Union  named  an  asteroid  after  you: ErikHøg.\n- In  2009,  you  received  the  Russian  Pulkovo  Observatory's  Struve  Medal  for  your contributions to astrometry.\n- In  2014,  you  received  an  Honorary  Doctorate  from  the  Ukrainian  National Academy of Sciences in Kiev.  \nAnd in 2019, you received the Instrumentation Prize of the German Astronomical Society, for your lifelong work in astrometry.  \nYou  have  played  a  leading  role  in  the  move  of  astrometry  to  space,  and inspired many others, but there is still far to go for the generations that follow?  \nYes there is far to go and the people are there and generations will come, all well prepared.", 'The billion-star astrometry after 1990': "by Michael Perryman and Erik Høg  \nABSTRACT: Michael Perryman invited Erik Høg to this interview in February 2022. The  second  space  astrometry  satellite  Gaia  was  approved  by  ESA  in  2000  and launched to a five year mission in 2013. This interview is about Gaia, and how it was developed to measure millions of stars much more accurately than had been done with  the  first  astrometry  satellite  Hipparcos,  measuring  positions,  motions  and distances.  Gaia  is  presently  revolutionizing  astronomy.  Erik  Høg  was  one  of  the scientific leaders of Hipparcos and Gaia, active in the design and data analysis. His work with astrometry began in 1953, 69 years ago, at a meridian circle. In 2013 he proposed  a  successor  to  Gaia  which  has  become  a  candidate  for  launch  by  ESA about 2045. The report contains the questions by the interviewer, the answers by Erik Høg, a link to the audio recording, and references.  \nHere follows a script of the recording. Erik in red is quoted closely but Michael not so closely, with slanted text.  \nThe audio is available here: http://www.astro.ku.dk/~erik/xx/hoeg3-final.mp3 , but  you  should  rather  open  it  on  Michael's  website  with  photos  and  other interviews: https://www.michaelperryman.co.uk/gaia-project-interviews The minutes and seconds are tagged at tm=.... Total duration 52 minutes.", "Ah!   You've found them interesting?": 'Oh yes! For example, I was fascinated the other day listening to the interview with Bruno Sicardy about solar system occultations. He told how amateur astronomers with their 20-40 cm telescopes and CCD cameras observe light curves when a star is occulted by a solar system body. From these light curves Sicardy and his group in Paris have studied the atmospheres of Pluto, of asteroids or satellites of Saturn, and they  have  even  discovered  a  ring  system  around  an  asteroid.  This  has  become possible with the very accurate positions of stars from Gaia because the event can now  be  predicted  to  within  seconds  and  also  precisely  where  on  Earth  it  will happen. This has activated many amateurs in Europe and around the world. It is just one of the many remarkable results coming from Gaia, and it reminds me of my own visual  photometric  observations  in  1949  as  a  school  boy  of  an  eruption  of  the  \nvariable star SS Cygni with my homemade (12 cm mirror) telescope - as many other amateurs did in those years.  \nThat  is  a  wonderful  connection  between  your  early  days  and  the  present results from Gaia...  \ntm=3:40', 'Early steps': "So, Gaia…  \n- … a mission which is today revolutionising almost all areas of astronomy\n- … and which is, now, generating more scientific papers each year than any  \nother ESA mission. See [4]  \nWhere shall we start ?  \nI think we should go back to the 1990s,  \nand ask you to describe the interest in new missions that Hipparcos itself had generated?  \nYes,  there  was  excitement  and  interest  around  the  world  to  extend  space astrometry to more stars and better accuracy. And there were many ideas.  \nCan you recall some of these for us - there were a lot but it would good to show how just much interest there was at the time?  \nYes, in the US there were (POINTS, USNO - NEWCOMB, MAPS, FAME, AMEX, OBSS, JMAPS,  OSI;  by  leading  astrometrists,  including  Reasenberg,  Duncombe,  Jefferys, Seidelmann, Johnston, Hemenway +)  \nand in USSR/Russia (Lomonossov, REGATTA-ASTRO, AIST, LIDA, OSIRIS;  by Chubey, Makarov, Yershov, Nesterov + )  \nbut even SIM (by Mike Shao) which did very well in the US Decadal Survey in 2000, was abandoned in 2010  \nDIVA  (in Germany; by Siegfried Röser,  Uli Bastian +)  targeted 0.2 mas to 15 mag  \nJASMINE (by Gouda, Yamada +)  is still being developed in Japan  \nin China, STEP (exoplanet detection) was proposed in 2013  \nand there were also other astrometric missions proposed to ESA: NEAT (M3) and Theia (M5).  \nA total of 19 names of ideas or projects are listed here.  \ntm=5:40  \nAlthough we'll not go into details, all of these - except JASMINE - eventually fell by the wayside…  \nleaving Europe, again, in the 'driving seat' in space astrometry. Why do you think this was?  \nThere are several reasons. The ideas needed for space astrometry started in Europe already in 1960 with photon counting astrometry, and in 1975 the basic design of a scanning satellite was on the table - as we talked about in the first interview: It was photon counting astrometry in a telescope with a beam combiner imaging two fields of view scanning stars on great circles and covering the whole sky systematically. An input catalogue with 100 000 stars selected for the astrophysical interest should be observed during 3 years to obtain positions, proper motions and parallaxes with an accuracy of 2 milliarcsecond. That was on the table.  \nA year before that, in 1974 the early ideas for space astrometry had changed from being a purely French undertaking to become a European project within ESA. That  opened  a  major  opportunity  for  the  still  existing  astrometric  community  in Europe to become active, and so they did.  \ntm=7:09  \nAnd things  then  moved  quite  quickly  from  these  early  ideas  in  Europe,  to reality?  \nYes, given the basic design and the active scientists, European industry was able to develop  the  Hipparcos  satellite  for  approval  in  1980  and  launch  in  1989.  The European  astronomical  communities  in  ESA  countries  joined  efforts  to  treat  the observations  and  produce  the  final  Hipparcos  and  Tycho  Catalogues  in  1997,  and they surpassed all expectations with respect to accuracy and number of stars. All this  gave  a  solid  foundation  in  ESA,  in  European  industry  and  in  the  scientific community for future space astrometry in Europe when new good ideas appeared.  \nIt  is  characteristic that the development in Western Europe has followed a very  straight  path:  From  meridian  circles  to  Hipparcos,  then  to  Gaia,  and  now towards GaiaNIR which will probably be launched about 2045 as a large ESA mission.  \ntm=8:15  \nYes, it is an interesting point about the common development of the scientific and industrial aspects giving this enormous basis for future ideas. We talked about Hipparcos in previous conversations and we are going to talk today focusing on Gaia.  \nWhat were the first steps along the path to a new space mission in Europe after Hipparcos ?  \nThe first step towards a new ESA mission was taken in 1990. During a visit to the Pulkovo Observatory I discussed with my USSR colleagues their existing proposals for a successor to Hipparcos. This led me within a day to begin designing a better more  accurate  Hipparcos.  This  generated  further  visits  to  Pulkovo,  Moscow, Copenhagen  and  Lund  for  discussion  of  the  ideas  with  Lennart  Lindegren,  Mark Chubey, Valeri Makarov and Valodja Yershov. Finally in 1992, I made a design with Lennart  Lindegren,  from  Lund,  of  a  mission  with  CCD  detectors  which  we  called Roemer  and  which  later  developed  into  Gaia.  Roemer  was  first  presented  at  a Symposium of the International Astronomical Union in Shanghai in September 1992.  \nIt  is  worth noting that all this was going on fairly soon after Hipparcos had been launched, well before the final Hipparcos Catalogie had been published.  \nSo  what  were  the  main  features  of  this  design,  and  how  did  it  differ  from Hipparcos?  \nThe  proposal  used  direct  imaging  on  CCDs  with  time-delayed  integration,  and  it contained many other features implemented in the final Gaia. For a 5 year mission an astrometric accuracy of 0.1 milliarcsec was predicted at 12 mag, more than 10 times better than Hipparcos. The astrometric efficiency was 100 000 times higher, because CCDs have ten times higher quantum efficiency than photomultipliers, and because  thousands  of  stars  could  be  observed  simultaneously,  and  this  was obtained with the same telescope aperture of 29 cm as Hipparcos. Astrometry and multicolour photometry for 400 million stars were also included.  \ntm=10:50  \nGoing on in ESA at this time was the long-term planning for European space science, called Horizon 2000, which had been initiated by ESA's Director of Science, Roger-Maurice Bonnet.  \nYou saw the next opportunity for space astrometry with ESA's call for a new mission in 1993?  \nThe Roemer proposal had aroused much interest in the Hipparcos Science Team. I should mention that the Hipparcos Science Team, which supervised all aspects of the  Hipparcos  mission  until  its  catalogue  publication  in  1997,  was  still  the  main  \nforum  for  top-level  discussions  amongst  the  European  teams  involved  in  space astrometry.  \nThe ideas in Roemer were adopted in a mission proposal submitted to ESA on 24 May 1993 for the Third Medium Size ESA Mission (M3). The proposers from seven  countries  were:  Kovalevsky,  Lindegren,  Halbwachs,  Makarov,  Høg,  van Leeuwen, Knude,  Bastian,  Gilmore,  Labeyrie,  Pel,  Schrijver,  Stabell,  and  Thejl.  We proposed to measure 100 million stars and to obtain an accuracy of 0.2 mas at V=13 in a 2.5-yr mission with a 34 cm telescopic aperture. At the faintest magnitudes 1517, accuracies would be about 1 or 2 milliarcsec. So, a similar accuracy to Hipparcos but much fainter.  \nThis  mission  was  not  one  aimed  at  solving  some  particular  problem,  but rather  to  explore  the  rich  complexity  of  Nature  in  order  to  find  new  patterns, anomalies  and  even  contradictions  to  existing  ideas.  This  was  the  spirit  of  the proposed ROEMER mission.  \ntm=12:50  \nSo it  was  very  broad  in  its  reach?  But  you  did  give  some  details  about  the scientific importance and objectives?  \nYes,  the  scientific  objectives  were  detailed  in  ten  pages  of  the  proposal,  covering stellar  structure  and  evolution,  stellar  kinematics,  cosmic  distance  scale,  double stars  and  unseen  companions,  solar  system  objects,  general  relativity,  and  the celestial reference frame. The proposal, compiled and edited by Lennart Lindegren, had 28 pages. And there were two annexes, which I edited,  running to more than 60 pages.  \nROEMER was also an acronym for 'Rotating Optical Observatory for Extreme Measuring  Efficiency  and  Rigour' .  A  section  called 'The  FIZEAU  option' was included 'to point out a possible improvement towards a scanning satellite with ten times  the  angular  accuracy  of  ROEMER' .  The  section  described  a  larger  scanning satellite with 'two confocal Fizeau-type (or 'wide field') interferometers whose axes form a basic angle of the order 140 deg.' , that was a description fitting very well to the  later  GAIA.  But  the  included  optical  system  underwent  major  development before  it  was  called  GAIA,  an  acronym  for  Global  Astrometric  Interferometer  for Astrophysics.  \nThe Roemer proposal was submitted to ESA in May 1993, as you said…", "and then it was reviewed by ESA's Astronomy Working Group, the AWG… What was the result of their deliberations?": "We heard to our delight that our proposal was rated the best scientifically among all astronomical  proposals  for  M3.  But  the  bad  news  was  that  it  was  considered  to come too soon after Hipparcos and it was not sufficiently ambitious with respect to accuracy.  It  was  therefore  referred  to  a  Cornerstone  Mission  study  if  10  -  20 µarcsecond accuracy could be demonstrated.  \ntm=15:10  \nSo  in  1993  Roemer  really  'lost  out'  to  the  cosmic  microwave  background mission Cobras/Samba later called Planck  \nWhat did you think of that?  \nAt that time I did not think of the competing and winning mission, only that we had lost.  \nSo  the  Astronomy  Working  Group  considered  Roemer's  science  very important for Europe,  \nbut  that  the  mission  was  not  'ambitious  enough'…  Is  that  how  we  can summarise it?  \nYes, that is also what I remember. I thought it was a severe blow to a good proposal and that a ten times better accuracy was rather unrealistic. But you and Lennart had the  courage  to  think  bigger  and  your  response  built  on  ESA's  request  for interferometry.  \nSo what came next?", 'GAIA and Gaia': "The major advance was the proposal led by Lennart Lindegren and yourself, Michael to  use  a  small-baseline  interferometer,  housed  within  the  Ariane  5  fairing,  to achieve better accuracy and bigger mirrors to get to fainter magnitudes. Although the idea of an interferometer was later dropped due to technical complexity, the advantage  was  that  interferometry  had  been  foreseen  within  the  Horizon  2000 cornerstone plans, and this really led to the mission gaining support; both because of its science goals, but also because of its technological appeal - even though other  \ninterferometry  groups,  especially  infrared  science  -  were  still  pushing  their  own ideas.  \nThe reply to the ESA call for proposals of Cornerstone studies was submitted on 12 October 1993, a proposal to study for astrometry 'a large Roemer option and an interferometric option' , GAIA. They should be studied as two concepts for an ESA Cornerstone Mission for astrometry 'without a priori excluding either' , as Lindegren wrote in the cover letter. But in those years we soon considered interferometry to be  very  good  for  our  purpose  and  the  Roemer  option  was  forgotten  for  several years.", 'And you soon set out your own big ideas?': 'In fact, I was also soon able also to "think big" and proposed in August 1994 a larger Roemer  mission,  called  Roemer  Plus,  with  larger  apertures  to  obtain  the  high accuracy. It used direct imaging and no interferometry. But it attracted no interest because everybody, including myself, had come to believe that interferometry was the right way to go. But it was NOT at all, as industry showed us in January 1998.  \nIn  1995  I  proposed  a  new  optical  design  GAIA95  with  Claus  Fabricius  and Valeri Makarov, it was a Fizeau interferometer with Gregorian telescope, providing dispersed fringes for simultaneous astrometry and spectrophotometry. This design was  immediately  adopted  for  a  minisatellite  project  DIVA  by  the  German  space agency DLR. The project was started and led by Uli Bastian and Siegfried Röser and was  joined  by  US  participation,  but  it  was  abandoned  in  2002  after  seven  years when one of the German funding partners had dropped out. Luckily so we must say, because our German colleagues could then come back and join efforts to develop Gaia.  \nTen years ago I have told the two fascinating stories of "Roemer and Gaia" and  of "Interferometry  from  Space:  A  Great  Dream" in  two  reports  [1]  and  [2] available on arXiv.  \nAs well as your technical contributions to Gaia, which we\'ll return to in just a moment,  \nyou also looked at some specific scientific objectives  \nOne was figuring out what magnitude Gaia had to reach to measure specific Galactic populations ?  \nYes, I prepared a table of "Some Galactic kinematic tracers" which was placed as the first table in the Gaia Study Report in 2000. It was elaborated during the years since the earliest discussions I had in 1994 at the IAU Assembly in the Hague. It contains selected tracers with the limiting magnitude and astrometric information necessary to  study  them, and it "demonstrates that GAIA will provide adequate precision to meet the scientific goals" . The selected tracers belonged to all Galactic populations: clusters,  disk,  halo  and  bulge,  and  the  table  was  useful  to  justify  the  18-20  mag completeness limit.  \ntm=20:40  \nA nother of your scientific ideas was your work on astrometric microlensing and what could be done with Gaia … can you describe this?  \nThe possibility to detect both photometric and astrometric effects of microlensing was discussed in the ROEMER proposal in 1993. The effect of relativistic gravitation appears when a massive foreground object, called a MACHO, passes in front of a star  further  away  when  this  star  is  observed  close  to  the  crossing,  and  given  the billions of photometric and astrometric observations by the proposed mission, the effect might actually be detectable. Such observations could potentially be used to determine the mass of the foreground object, the MACHO.  \nIn 1994 I had a collaboration with the Russian cosmologists I.D. Novikov and A.G.  Polnarev  and  I  showed  that  all  six  physical  parameters  of  a  MACHO  (mass, distance,  two  position  coordinates  and  two  proper  motion  components)  could  in principle be derived from observations of an encounter event with a star for which the position and proper motion are known.  \nThis  was  a  nice  theoretical  result,  but  the  practice  is  difficult.  However,  in fact twenty years later, the detection of events with Gaia and photometric follow-up observations from a network of telescopes on the ground has become a rich source of science. Over a thousand transient sources had already been detected with Gaia data up to October 2016.  \nIt is a wonderful example of scientific discoveries coming out of this.  \ntm=22:30  \nGoing back to the important challenge of optimising Gaia, can we talk about two of your main activities.  \nThe first of these, was your work on how to best sample the pixels of the CCD focal plane, and you contributed many technical notes on this over several years?  \nYes, I dedicated much effort to this optimisation. The problem was that not all the billion  CCD  pixels  could  be  sampled  and  sent  to  ground,  because  the  read  noise would be excessive and the telemetry rate was limited. So a clever selection of a subset of pixels was required onboard.', 'And you showed that even at 20-21 mag, much of the sky is empty of stars, so its not useful to send either?': 'Yes, much of the sky is empty, even at 20-21 mag, but there are other complications from diffraction spikes, and double and multiple stars, and the sky is not so empty inside  clusters  and  galaxies.  All  this  had  to  be  optimised  so  that  the  best  science could be extracted from the data in powerful computers on the ground by clever astronomers and technical experts. We also took care that the environment of all detected  objects  could  be  mapped  in  an  area  of  about  one  square-arcsecond  in order to detect companion stars or a nebulosity around the object which might itself be a compact galaxy or a quasar.', 'You had some other people working closely with you on this?': 'Yes, much of this work on sampling was done in collaboration with Frédéric Arenou (from  Paris)  and  Jos  de  Bruijne  (working  under  you  in  ESA),  and  the  work  was recorded in a series of technical reports. There were actually a total of 64 reports since 1997 with authors from Copenhagen about sampling, detection and imaging for Gaia. The final sampling used in the satellite operations built on our work, but was further optimised by industry and especially Jos de Bruijne at ESTEC.  \ntm=25:10  \nAnother  of  your  major  activities  in  optimising  the  Gaia  payload  was  your work on the photometry.  \nLet me introduce this by saying that, from the earliest ideas for Gaia, I was very clear that measuring a billion or more objects was made far more powerful if we could also record their colours or spectra.  \nTell us first about the scientific reasoning behind this?  \nSure, photometry was important, in fact for two different reasons. The instrument gives small but significant chromatic errors in the astrometry which can be corrected  \nif a colour index of the star is known, and we need to know that for every transit for the sake of variable stars with changing colours.  The other reason is astrophysical: the  colour  gives  its  position  in  the  Hertzsprung-Russell  diagram,  and  also  (if  the filters  are  carefully  chosen)  the  spectral  type,  metallicity,  and  reddening.  Many other  photometric  surveys  are  going  on  but  we  needed  a  reliable  source  giving prompt results: So it had to be by Gaia itself.  \nAgain,  there  were  several  key  people  helping  you  to  think  through  the objectives, and the possible solutions?  \nMy colleague in Copenhagen, Jens Knude, was my constant support and he brought his  lifelong  dedication  to  photometry,  especially  with  the  Strömgren  uvby  system with intermediate width bands suited for early type stars.  \nPeople in Lithuania were also keen to support, in fact Vytas Straizys wrote to me already in 1994. He was director of the Vilnius Observatory and had developed the  "Stromvil"  system  suited  for  classification  of  all  spectral  types,  also  in  the presence of interstellar reddening. So the experts joined our efforts very early. For photometry  and  spectroscopy,  the  first  idea  was  to  add  a  third  telescope  with  a collecting  aperture  of  0.75  x  0.70  m 2 in  addition  to  the  two  large  telescopes  for astrometry. This was the design by Matra Marconi Space at the end of the industrial study in mid 1998.  \nVarious  possibilities  were  considered,  ranging  from  multiple  filters,  to  lowresolution spectroscopy  \nHow did you advance this particular study, and what was the final solution adopted?  \nWhen  the  cornerstone  study  started,  a  Gaia  Science  Team  was  formed  in  2001, under  your  leadership  Michael,  with  various  working  groups  devoted  to  studying specific aspects. One of these was the photometry working group with Carme Jordi from  Barcelona  and  me  as  co-leaders.  We  had  many  special  meetings  on photometry and wrote many reports in those years. In the first design, photometry in  four  broad  bands  was  obtained  in  the  astrometric  telescopes  and  in  seven medium-width spectral bands in the smaller telescope. The next design, which was described in great detail in The Concept and Study Report to ESA in 2000, increased this to eleven intermediate bands. In 2006 our work ended with a paper of 25 pages  \nin the journal Monthly Notices of the RAS ( not Astronomy & Astrophysics as I said), with 35 authors. The number of bands had increased to a total of 19.  \ntm=29:30', "But that wasn't the end of this long and important story?": "NO! All this  changed dramatically. In  fact  it  changed  while  we  were  finalizing  the MNRAS ( not A&A) paper, but only a few of us knew that. The change had to be kept confidential because it was taking place during the competitive bid for the industrial contract  for  Gaia.  One  of  the  competitors,  Matra  Marconi  Space  (who  eventually became the winning contractor), was proposing to simplify the design for reasons of mass and cost. They dropped the dedicated photometric telescope and reduced the number of focal planes from four to only one.  \nThey  asked  you  Michael  whether  it  would  be  acceptable  to  replace photometric filters with two low-dispersion prisms. They reluctantly accepted that you engaged also me, and I also wanted to have Carme Jordi, Anthony Brown from Leiden,  and  Lennart  Lindegren  involved.  It  was  a  complicated  problem,  but eventually a very careful optimisation of the prism properties was done, resulting in the very satisfactory Gaia photometry that we have today.  \nOne of the points I wanted to get across in these conversations is that having an idea for a space mission is one thing…  \nbut there is usually a difficult journey in convincing all the others that need to be convinced;  \nand a longer and even more arduous journey in designing and optimising an instrument  \nand especially a space instrument, where you have just the one chance to get it right!  \nWe embarked on the ideas for a follow-on mission starting in the 1990s, in parallel with Hipparcos  \nand your ideas, and your commitment to the end goal, contributed so much to Gaia  \nBut there comes a time when our main work is done, and we hand over to others?  \nYes, after leaving the Science Team in 2007, I was no longer involved in Gaia affairs. No Danish involvement in Gaia data reduction was conceivable. I was 75 enjoying  \nmy pension, and the institute was entirely focused on astrophysics, which I find very reasonable. Claus Fabricius and I are therefore the last Danish astrometrists in the row beginning with Tycho Brahe.  \nThe Copenhagen University Observatory no longer exists. It started on top of the  famous  Round  Tower  in  Copenhagen  built  in  1642  where  Christen  Sørensen Longomontanus,  Tycho  Brahe's  assistant  on  Hven,  had  begun  his  work.  And  the Brorfelde  Observatory,  50  km  from  Copenhagen,  where  I  made  the  very  first observations with the new meridian circle in 1953, was closed for science when the whole staff moved to Copenhagen in 1996. In 2005 the 363 year old Copenhagen University  Observatory  was  absorbed  in  the  Niels  Bohr  Institute  -  without  any celebration or funeral.  \nWell,  your  formal  involvement  ceased,  and  the  hectic  workload  associated with it ceased…  \nbut  you've  maintained  a  big  interest  in  Gaia,  and  indeed  in  the  role  and future of space astrometry?  \nI am doing nearly all my work from home, but I kept an office in the institute until 2019 and often visited the cosmology group for talks. I wrote a popular article in Danish together with two cosmologists, Peter Laursen and Johan Samsing, about the beginning of the Universe, and more recently an article focused on explaining the cosmic horizon. I am happy that Johan Fynbo and his student Kasper Heintz adopted and implemented my idea of detecting quasars by their zero proper motion using Gaia results.  \ntm=34:00", 'And you are continuing to give talks?': "Yes, I have given a lot of talks around the world about my life with astrometry, and I wrote many reports about the history of astrometry. After my talk in Stuttgart in 2019, when we received the instrumentation prize from the German Astronomical Society, I spoke in Paris and in Berlin. I've given other more recent talks online, in 2020  first  to  an  audience  in  Potsdam,  then  to  Perth  in  Australia  invited  by  my colleague Andreas Wicenec from the Tycho and Tycho-2 projects when he worked in Tübingen,  and  to  South  Africa  invited  by  Mattia  Vaccari  who  was  my  student  in Copenhagen  in  the  1990s,  -  Mattia  came  from  Padua  and  studied  how  to  map galaxies with Gaia data.", 'Launch and after': "You'd flown out, at ESA's invitation as a VIP, to see the launch of Hipparcos by Ariane 4 from French Guiana in 1989.  \nWith  all  of  your  contributions  to  the  developments  underpinning  its successor, Gaia,  \n- I was  greatly  disappointed  that  ESA  did  not  recognise  your  central contributions  to  this  next  step  when  deciding  on  the  attendees  at  its  launch  from French Guiana in December 2013.  \nI suppose that there was little 'corporate memory' of the 20-year journey in making Gaia happen.  \nBut  you  did  see  the  launch  relayed  live  in  Copenhagen;  tell  us  about  your memories of this great day?  \nNo,  I  was  not  invited  to  see  the  launch  of  Gaia,  and  of  course  I  was  also disappointed.  \nBut to my great delight, however, the day of Gaia launch came to please me very  much  in  Copenhagen.  My  colleagues,  especially  Uffe  Gråe  Jørgensen,  had arranged  a  meeting  for  the  occasion  at  the  Niels  Bohr  Institute  (where  I  had  my office) with invited guests, talks and transmission of the launch. I had invited several guests, my wife Aase was there, and my admired tutor from the 1950s, Professor Peter Naur, who I talked about in my first interview with you. After a short talk at the  meeting  I  went  with  Peter  Naur  in  a  taxi  to  the  TV  studio  where  we  both appeared for interview during the launch. You cannot imagine how happy I was to see him enjoying this event with me, his student from sixty years ago!  \nThat must have been a remarkable and rather strange event with your tutor again after 60 years!!   Did he show any particular surprize or pride in seeing what you had achieved?  \nYes, I could see it and he said it is unbelievable what you have achieved Høg (Naur spoke Danish: Det er utroligt, hvad du har nået Høg ). And I should add that we had frequent contact with each other on the telephone, until he died a few years later.  \nSo, Gaia was launched in 2013, and operations started in mid-2014, after a 6month commissioning phase.  \nOur original target mission duration was for 5 years, but with consumables on-board for 10 years,  \nand thanks to the superb mission operations team in ESOC, and the science operations team in ESAC, Gaia has now acquired more than 7 years of high-quality data  \nwith three Data Releases issued so far - in 2016, 2018, 2020 and the next one has just been announced for the 13 of June.  \nErik, you are now 89, and 90 in June I think, and as we've said, you are not involved in the analysis of the data coming from Gaia  \nbut  I  know  that  you  are  watching  the  many  teams,  and  the  huge  data processing task involved.  \nTell  us  something  about  your  perspectives  on  this  enormously  successful mission and some of the scientific results that you have seen reported.  \nIt  is  overwhelming  to  see  what  has  come  out,  and  that  the  data  reduction  and everything has been able to follow up. Very good management, very good people. I just  listened  to  your  interview  with  Anthony  Brown,  leader  of  the  data  reduction consortium,  it  touched  me  deeply,  almost  to  tears.  How  lucky  I  am  to  live  long enough to see all this myself.  \ntm=39:10  \nYes, I feel very similar Erik, it is a wonderful and dedicated team, more than 400 people working hard to get these wonderful results out.  \nAs I said at the end of our second interview…  \nin  one  lifetime  you  have  helped  take  astrometry  from  the  confines  of  the Earth  \nout into space with two highly successful space missions.  \nBut your contributions haven't stopped with Gaia…  \nTell us what you've been doing to help prepare for the future?  \nIn  May  2013  I  heard  from  Claus  Fabricius  (in  Barcelona)  about  ESA's  call  for  new large  missions,  just  11  days  before  deadline.  The  call  was  a  surprise,  but  It  was immediately clear to me that it should be a Gaia successor in 20 years. When writing the  proposal  I  corresponded  with  colleagues,  some  urged  me  to  submit  it,  a  few others said I should not submit because it was not ripe and because such a proposal should be submitted by a committee, not by a single person. But I had only a few days to write and to decide. I submitted the proposal in time, which was 6 months before Gaia was launched.  \nI was sure the mission should use filter photometry with the same high angular resolution of 0.12 arcsec as the astrometry for the benefit of narrow double stars  and  dense  fields.  It  should  not  be  prism  photometry  as  in  Gaia  with  a resolution about one arcsec. The prism photometry of Gaia has its advantages, but there is no reason to repeat it. Likewise, the medium-dispersion spectrograph could be omitted since there are no strong scientific reasons to repeat these observations, and cost could be saved.", 'Please tell us about the scientific ideas behind this idea of a second Gaia ?': 'The idea was that astrometric observations of the Gaia stars after 20 years would give ten times more accurate proper motions from positions at the two epochs than from a single mission. The benefit for kinematic studies would be immense for more distant stars, and also for stars close-by due to better accuracy, and for maintaining the  celestial  reference  system.  Exoplanets  with  long  Jupiter-like  periods  could  be detected and the whole population of such systems in our neighbourhood could be discovered. During the following years I wrote a long report with all the scientific cases while corresponding with many colleagues and I presented the ideas at many meetings.  I  had  response  from  at  least  a  hundred  competent  people  in  oral discussion or correspondence.  \ntm=42:20  \nBut you eventually discarded the original idea of a second Gaia,  \nfocusing  instead  on  observations  in  the  infrared,  as  the  Japanese  JASMINE project were considering?  \nNO, NO Michael, nothing was discarded, we developed a new capability as  I  shall now explain.  \nWith Jens Knude and other colleagues in Copenhagen I began in 2014 to look at the possibility to extend the wavelength range into the near infrared, beyond the 1000 nm possible with the classical CCDs. The benefit would be the observation of redder stars i.e. cooler stars or stars imbedded in nebulosity.  \nIn 2015 a colleague at the Lund Observatory, David Hobbs, joined me with much energy as he still does and we focused on GaiaNIR, a successor in 20 years with an Near-InfraRed capability. A technical study was conducted by ESA in 2017. Especially the NIR detector is critical since we need TDI, time-delayed integration,  \nwhich was easy to do in the visual with CCDs in Gaia but is difficult in NIR.  The vast majority  of  the  stars  observed  by  Gaia  shall  be  observed  again  by  the  successor since a good sensitivity at less than 1000 nm shall be maintained. The positions at two epochs will be obtained for them and thus the highly accurate proper motions.  \nAn industrial study of NIR detection is making progress. The new mission will be able to peer into the obscured regions of the Galaxy and measure up to 10 or 12 billion  new  objects,  in  addition  to  two  billion  Gaia  stars,  and  reveal  many  new sciences in the process. ESA has now ranked the development of this mission so high that a launch about 2045 is quite probable, see [3].  \ntm=44:40  \nYou mentioned 10 billion stars? May I quote some words from a decade ago from someone who I described at the time as an undisputed authority in astrometry, this person said that \'The Gaia astrometric survey of a thousand million stars cannot be surpassed in completeness and accuracy within the next forty or fifty years.\'  Do you recognise those words!!!  \nYes Michael, of course I recognise these words, I believe I wrote them myself about 2005 and I am glad to hear them again from you. I knew how long it would take to get a new large mission approved and developed, and we can expect the first results from  GaiaNIR  in  2050  if  all  goes  well  -  which  is  rather  within  the  time  frame  I mentioned. The mission will supersede Gaia in two respects: The infrared capability and the 10 billion stars.  \nI  am happy that I have been able myself to help bringing this bright future within reach. It is like a dream, but it seems to be true!  It has happened so soon because the proposal with a realistic design was submitted in May 2013 even before Gaia had been launched and work was started on the project. Then two years later, David Hobbs became active for the Gaia successor. David has worked on Gaia since 2007 and he brought great scientific and technical insight and he has made full use of all the possibilities offered by ESA.  \nAs I mentioned before, you\'ve received some welcome recognition for your work over the years;  \nAnd  in  particular,  in  2019,  you  received  the  Instrumentation  Prize  of  the German Astronomical Society,  \nit was jointly with Lennart Lindegren and me for our contributions to Gaia, and in your case, emphasising your lifelong work in astrometry.  \nI was very happy about this recognition from the German Astronomical Society, and especially that I received it together with you, Michael and Lennart. We three have enjoyed a wonderful collaboration for so many years, with Lennart it began in 1973 almost 50 years ago, and with you in 1981.  \nLet  me  also  tell  this  Michael:  When  the  European  Astronomical  Society announced its new prize in 2008, the Tycho Brahe Medal, I thought secretly, that I might get it! But when you then finally got it in 2011 I thought: that is perfectly right , Michael deserves it for his brilliant leadership of the Hipparcos and Gaia Teams since we four consortia leaders of Hipparcos had already received the Director of Science Medal in 1999.  \ntm=48:00  \nYou  have  played  a  leading  role  in  the  move  of  astrometry  to  space,  and inspired many others,  \nBut there is still much to do for the generations that will follow?  \nLet me recall what we wrote in the proposal for the ROEMER mission 29 years ago: " The mission is not primarily aimed at solving some particular problem, but rather to explore the rich complexity of Nature in order to find new patterns, anomalies and even  contradictions  to  existing  concepts". This  is  also  the  spirit  of  the  proposed GaiaNIR mission, and this vision has become true in a spectacular way for the Gaia mission  when  we  see  [4]  that  the  number  of  scientific  papers  per  year  has  been larger since 2019 than for all the other ESA missions taken together, and only based on the first 22 (DR1 and DR2) months of Gaia observations. Imagine how it will look after 10 years=120 months! - You will see that Michael.', 'Erik Høg: thank you very much for joining me again today.': 'Thank you for inviting me, and for your perfect and very kind guidance.  \nAllow me now, Michael, to leave astronomy for a moment and speak about my  concern  for  our  civilization  which  is  shared  by  many  people.  NOT  the  planet Earth  is  in  danger  as  we  often  hear  in  discussion  of  climate  and  global  warming, because plants, animals and the human race will of course survive and adapt to the changing conditions. It is "only" human civilization that will suffer increasingly. The living conditions have been improving for a large fraction of people on Earth during the  past  centuries,  but  this  will  not  continue  for  very  long  for  many  reasons: increasing pollution of atmosphere, soil and oceans, people smuggling, international crime, and because our political structures are unable to fundamentally reverse this trend.  \nBut 500 years ago, Martin Luther spoke a good motto for everyone:  \n"Even if I know that the world will perish tomorrow, I will plant a tree today."  \nWell,  I  think  that  astronomers  -  like  us  -  can  offer  their  own  very  unique perspective on this problem,  \nbeing so conscious of the size and emptiness of our Galaxy and beyond and the fact that we do appear, so far at least, entirely alone in its vastness Erik, thank you very much again.  \ntm=51:30', '1 p at: http://www.astro.ku.dk/~erik/xx/GaiaPublications.pdf': 'Further references about the history of astronomy related to space astrometry in the early 1970s and about Erik Høg are given at the first interview. Most papers by Høg may be found on ADS by searching for "Høg, E." or "Hoeg, E.".'}