Ceres: Keeping Well-Guarded Secrets for 215 Years
In 1801, when an astronomer pointed his telescope at a seemingly star-like point of light, he probably had no idea a robotic emissary from Earth would one day be sent there.
New Year's Day, 1801, the dawn of the 19th century, was a historic moment for astronomy, and for a space mission called Dawn more than 200 years later. That night, Giuseppe  Piazzi pointed his telescope at the sky and observed a distant object that we  now know as Ceres.Today, NASA's  Dawn mission allows us to see Ceres in exquisite detail. From the images Dawn  has taken over the past year, we know Ceres is a heavily cratered body with diverse  features on its surface that include a tall, cone-shaped mountain and more than  130 reflective patches of material that is likely salt. But on that fateful  evening in 1801, Piazzi wasn't sure what he was seeing when he noticed a small,  faint light through his telescope."When  Piazzi discovered Ceres, exploring it was beyond imagination. More than two  centuries later, NASA dispatched a machine on a cosmic journey of more than 3  billion miles to reach the distant, mysterious world he glimpsed," said  Marc Rayman, mission director and chief engineer for Dawn at NASA's Jet  Propulsion Laboratory, Pasadena, California.Piazzi was the director of thePalermo Observatory in Sicily, Italy, which has collected documents and  instruments from the astronomer's time, and published a booklet on the discovery of Ceres. According to the observatory, Piazzi had been working on a catalog of star positions on January 1, 1801, when he noticed something whose "light was a little faint and colored as Jupiter." He looked for it again on  subsequent nights and saw that its position changed slightly.What was this  object? Piazzi wrote to fellow astronomers Johann Elert Bode and Barnaba Oriani  to tell them he had discovered a comet."I have  presented this star as a comet, but owing to its lack of nebulosity, and to its  motion being so slow and rather uniform, I feel in the heart that it could be  something better than a comet, perhaps. However, I should be very careful in  passing this conjecture to the public," Piazzi wrote to Oriani.A Missing Planet?Piazzi didn't entirely keep this secret. He told the press that this object was a comet, but did not provide data from his observations, which generated criticism from  other astronomers. Piazzi then became sick for a time, and said he could not  observe the object any more.As newspapers spread the word that a comet had been found, astronomer Jerome de Lalande, based in Paris, wrote to Piazzi requesting relevant data in February. The  Italian astronomer obliged in April, after recovering from his illness. One of Lalande's students, Johann Karl Burckhardt, performed calculations that  revealed Piazzi's discovery did not have an orbit consistent with a comet's orbit. Instead, the data appeared to better fit a circular orbit.Of course, there was no email in those days, and letters that Piazzi wrote to his friends Bode and Oriani about the so-called comet were delayed due to the Napoleonic Wars. They finally reached the astronomers in March.The news was  especially interesting to Bode because he had championed theTitius-Bode hypothesis: that the positions of planets in our solar system follow a specific pattern, which predicts each planet's distance from the sun. Uranus, discovered in 1781, fit the prediction, too. But the  pattern also demanded that there be a planet, yet undiscovered, between Mars  and Jupiter.To find this  missing planet, a group of German astronomers had established a society called  the "Celestial Police" (Himmelspolizei in German), with Franz Xaver  von Zach as its secretary, in 1800. There were 24 astronomers who each scoured  a 15-degree piece of zodiacal sky for the missing object. However, Piazzi did not receive his  invitation to join this group until after he had spotted Ceres.Bode calculated  an orbit based on Piazzi's data, and he believed that the object Piazzi saw was  the missing planet that fit his formula (which was later discredited). Oriani,  meanwhile, also calculated an orbit, and on April 7 asked von Zach to publish  the news in his well-known astronomy journal, Monatliche Correspondenz, that such a planet may have been discovered.Almost a 'Lost Comet'As of spring  1801, besides Piazzi, no one had been able to observe the new celestial object because of cloudy skies and the object's position in its orbit -- it was no  longer visible at night, and the sun blocked astronomers' views. Meanwhile, Piazzi still did not publish anything on the object, while he continued to refine  his data. Several of his colleagues grew upset with Piazzi for holding back information. Without the data from his observations that concluded on Feb. 11, confirming  his discovery would be more difficult -- since February, Ceres had been lost.Why did Piazzi hesitate to make his data public? One reason might be that, though Piazzi was a skilled  observer, he didn't have a solid theoretical knowledge of astronomy, so he couldn't calculate orbits quickly. Secondly, he risked the credibility and  reputation of both himself and the observatory. But while he wavered, colleagues in Germany such as Bode firmly believed that there needed to be a planet between Mars and Jupiter. It was their conviction that helped keep the work going on this object, said Ileana Chinnici, who edited the Palermo Observatory's booklet on Ceres."Without the determination of the German astronomers, Piazzi would have been just the discoverer of a lost comet, in the  best case. They 'believed' in the existence of the planet and were driven by  the endeavor to confirm it. This shows how powerful are ideas, models, theories  -- yesterday as well as today," Chinnici said.The Search for CeresAt last, in  July 1801, Piazzi worked on calculating the object's orbit and made public his  data about his observations from earlier in the year. And while other  astronomers had already come up with their own names -- such as Juno, Hera and Piazzi (to honor the astronomer) -- Piazzi himself announced that the "new  star" was called Ceres Ferdinandea. The "Ferdinandea" part honored King Ferdinand of Sicily.Ceres, the Roman goddess of agriculture, was also the patron deity of Sicily, where Piazzi then lived and worked. Bode, who had wanted to call the object Juno, agreed on  Ceres: "You have discovered it in Taurus, and it was re-observed in Virgo, Ceres of the old times. These two constellations are the symbol of agriculture. This occurrence is quite unique."By the end of  July 1801, many astronomers believed Ceres was a planet, but they needed  additional confirmation and observations. Piazzi published his complete data  set in von Zach's journal in September and, by doing so, got the attention of a young mathematician who would become instrumental in the fate of Ceres.Twenty-four-year-old  Carl Friedrich Gauss had been experimenting with mathematical methods for which  he would later become famous. When he applied those methods to Ceres, he came up with different predictions for its position than what others had calculated.  Though some were skeptical about Gauss's results, his calculations enabled von Zach to be the first to see Ceres again, on Dec. 7, 1801, followed by other prominent astronomers of the time, and by Piazzi himself on February 23, 1802.Asteroids: A New Category of ObjectsWe credit Gauss for calculating the orbit of Ceres. But he did not resolve a fundamental question: What is Ceres?In March 1802, Heinrich Wilhelm Matthias Olbers discovered a second, similar object, which  later became known as Pallas. William Herschel, one of the most famous  astronomers in history, then wrote an essay proposing that both Ceres and  Pallas represented an entirely new class of objects: asteroids. Herschel wrote  of Ceres: "if we called it planet, it would not fill the space between  Mars and Jupiter with the dignity required by that position."Though Herschel  considered it an achievement that Piazzi had encountered the first example of  an asteroid, Piazzi was disappointed. He thought that Herschel, who had  discovered Uranus, just wanted to downplay Ceres. Piazzi wrote to Oriani:  "Be they called planetoides or cometoides then, but never asteroides. [...]  If an Asteroid Ceres must be called, so must also be called Uranus."Nonetheless, the  door had opened for many more asteroids to be observed. The discoveries of Juno  in 1804 and Vesta in 1807 (which would later become the first target of NASA's  Dawn mission) reinforced Herschel's notion that asteroids are a class of their  own. Herschel coined the term "asteroid" because of their star-like  appearance in telescopes. Today, we know there are hundreds of thousands of  asteroids in the main asteroid belt between Mars and Jupiter.Piazzi's LegacyPiazzi could  not have known that NASA's Hubble Space Telescope would one daydeliver many intriguing images of Ceres, allowing scientists to confirm that  the body is, indeed, round like Earth. He could not imagine that in 2006, long  after his death, the International Astronomical Union would upgrade Ceres from  asteroid to dwarf planet, receiving the same classification as Pluto, which had  not yet been discovered in his lifetime. He did not know that in 2007, NASA's  Dawn mission would launch from a place called Cape Canaveral in Florida to  embark on an unprecedented journey to orbit Vesta and Ceres.He likely  didn't imagine that a space observatory named after Herschel would find in 2013  that there is water vapor emanating from Ceres, following up on 1992  observations of hydroxide at Ceres from NASA's International  Ultraviolet Explorer.Nor could he  have guessed that on March 6, 2015, Dawn would be successfully captured into  Ceres' orbit, and would spend the rest of the year sending photos and other  valuable data back to Earth. He wouldn't know that scientists would use the  Hubble Space Telescope's unique capabilities in November 2015 to observe Ceres  in the ultraviolet spectrum, complementing Dawn's observations.Now, as we commemorate  the 215th anniversary of Ceres' discovery this month, Dawn is observing the  dwarf planet from its lowest orbit ever: 240 miles (385 kilometers) from the  surface. The many craters and other features that Piazzi could not see with his  telescope are being named after agricultural deities or festivals, extending  the theme that Piazzi began with the name "Ceres.""Our  knowledge, our capabilities, our reach and even our ambition all are far beyond  what Piazzi could have imagined, and yet it is because of his discovery that we can apply them to learn more, not only about Ceres itself but also about the dawn of the solar system," Rayman said.Dawn's mission is managed by the Jet Propulsion  Laboratory for NASA's Science Mission Directorate in Washington. Dawn is a  project of the directorate's Discovery Program, managed by NASA's Marshall  Space Flight Center in Huntsville, Alabama. UCLA is responsible for overall  Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built  the spacecraft. The German Aerospace Center, Max Planck Institute for Solar  System Research, Italian Space Agency and Italian National Astrophysical  Institute are international partners on the mission team. For a complete list  of mission participants, visit:http://dawn.jpl.nasa.gov/missionMore information  about Dawn is available at the following sites:http://dawn.jpl.nasa.govhttp://www.nasa.gov/dawn