Washington Post reporter Ben Guarino will be online Thursday, September 14 at 11:30 a.m. with NASA scientist Conor Nixon to discuss what will happen as the Cassini spacecraft plunges into Saturn's atmosphere.

Please send in your questions about the mission below.

Ben Guarino

Hi! We're about to get started, so here's some of our previous Cassini coverage for background: Wondering how NASA steers a spacecraft into a planet? Math, mostly. For a recap of one of Cassini's most awesome projects, here's the time it sent the Huygens probe to Saturn's moon Titan. And my Post colleague Sarah Kaplan wrote about a researcher who spent her professional career with Cassini, and now she must say goodbye.

Q: Last photo

So what will be the last photo we get back from Cassini? Or have we already gotten it?

A: Conor Nixon

Great question! Cassini is taking its last pictures in the next few hours, these will be of Saturn and its rings. The last images will arrive tonight before the cameras are turned off for the final plunge tomorrow.

The last signal will come tomorrow -- Cassini will enter into Saturn's atmosphere at 6:32 a.m. eastern. At 7:55 a.m. is when Earth will lose Cassini's signal.

Q: Cassini

Will Hubble be oriented toward Saturn for the impact? Will it be visible to Hubble?

Hubble unfortunately is just out of view of Saturn. Members of the Cassini team did check that, in case there is a chance of seeing the 'fireball' created from the plunge. Hubble orbits the Earth, and sadly will be just over the horizon at the time Cassini enters.

Q: Cassini Saturn

why did Cassini last 9 years longer than expected and what will be the outcome lifespan for Huygens currently sitting on Titan? Is data still coming in from Huygens?

A: Ben Guarino

Thanks for asking! Huygens wasn't strong enough to send data back to Earth alone -- it needed to communicate with Cassini, who beamed the data home. But that meant it only had a window of a few hours before Cassini flew beyond Titan's horizon. And then Huygens ran out of batteries. So we've gotten all of the data we'll get, and almost all of it has been analyzed. But Huygens' legacy isn't over: What scientists have learned from Huygens will inform the next landers.

Q: Return On Investment

How valuable is the extra data Cassini collected compared to original expectations?

Cassini (and Huygens) have really exceeded expectations. We knew there would be new discoveries, but the mission has lasted longer and given us much more than we expected. The discoveries at Enceladus really changed the whole mission, and we redesigned after those to keep going back to that moon for more data. The final orbits have also been incredible, like a whole new mission telling us details about the rings and small moons close to the planet.

Q: Measurement units

After the 1999 Mars probe debacle, did you all decide to use English units or metric units for all measurements?

NASA has certainly been very careful about its units of measurement. Some types of hardware are still specified in English units, which are used in engineering design. However interfaces with spacecraft trajectories in metric units are double and triple checked.

Q: Burning Up In Atmosphere

Do the satellites in earth's orbit fall out of orbit and burn up in our atmosphere without fanfare? Is there any evidence aliens have sent their own versions of Cassinis our way, maybe we've seen them burn up but mis-identified them as meteors?

Satellites do burn up in our atmosphere, mostly over oceans. Other times decommissioned satellites are sent out of orbit to a space junkyard. But the bigger ones can generate a bit of fanfare: Tiangong 1, China’s first space laboratory, is expected to plunge back to Earth near the end of this year. As far as I know it's still unclear where that will happen. (Hence the fanfare.)
And -- as much as I want to believe -- no, there's no evidence of extraterrestrial intelligence (including no alien satellites sent our way).

Q: Most Interesting Cassini Moment

Hi Conor! What's your favorite piece of knowledge or insight that Cassini's trip around Saturn has unveiled?

Hi! I am huge fan of Titan, the only moon with a significant atmosphere. This is really telling us a lot about the complex chemistry that could have occurred on the early Earth, and maybe even the origins of life. So my favorite discovery is that chemical complexity of Titan, that is giving us great reasons to go back with another mission and different types of instruments.

Q: Space exploration

Sorry for the ignorance (and curiosity!), but is space exploration fully funded by NASA/the U.S. govt, or is there also private investment?

No need to apologize for curiosity! The Cassini-Huygens mission was a joint mission involving NASA (and, yes, U.S. funds) but also the European Space Agency -- which was in charge of the Titan lander Huygens -- as well as the Italian Space Agency. In the years since 1997, when Cassini-Huygens launched, we've seen a boom in the commercial space industry, with major players like SpaceX (here's its recently-published blooper reel of rockets failing to land) and Blue Origin (Jeffrey P. Bezos, who owns The Post, also owns Blue Origin).

Q: Ring debris collision

The diagram of Cassini's flight path on the JPL/NASA website shows orbits that took Cassini both on the outer edge of Saturn's rings (although that could still be quite distant in practice) and between the rings and the planet*. Were there real concerns that stray ring debris (rocks, dust, ice, misplaced monoliths) could collide with Cassini? Or is there a large amount of empty space between the planet and the inner rings where the odds of collision are very slim, and Cassini kept well away from the outer rings? *bonus question: is Saturn the gas ball + rings or just the gas ball, and the rings are considered moon-like objects?

There were definitely concerns, which is why we waited until late in the mission to venture into this risky territory. However the risks were very well calculated and considered, and we never sent the spacecraft into the thickest part of the rings were it would really be in danger. The science we have returned from the ring-grazing and atmosphere-grazing orbits has truly been worth it.

Q: Cassini Carrer

My son would love a professional career on a "Cassini" like mission. He just graduated from UVA with a double major in Mechanical/Aerospace engineering. Who should he contact at NASA to discuss a career?

It sounds like he is really interested and enthusiastic, those are the graduates we love to have. My best advice is to continue to graduate school first, and do some hands-on work in aerospace at a University that is actively partnering with NASA on future missions, such as small Cubesats, which have great potential for the future.

Q: Gas Giant

If Saturn is a gas giant how will Cassini crash in to it?

Good question! Cassini will not exactly 'crash' but rather burn up and explode in the atmosphere like a meteor on Earth.

Q: NASA videos

Hi Conor. You did a very nice job on the Cassini video posted to NASA's youtube channel 2-days ago ("Cassini's Infrared Saturn"). How does NASA figure out who will be featured in these PR pieces, considering the teams are so large and full of passionate scientists and engineers?

Thank you! Many of Cassini's scientists and engineers are involved in outreach activities this week. I was one of several scientists at NASA's Goddard Space Flight Center who were asked to participate in a video about the instrument - CIRS - that was built at Goddard, before being added to the overall payload.

Q: Huygens probe

For how long did the Huygens probe send data?

Huygens sent data for 2.5 hours during its descent to Titan's surface, and then another 90 minutes after landing on the surface before the batteries ran down.

Q: Hexagonal Poles

Saturn's northern pole seems to change color in the visible spectrum depending on the Season (full summer now - more yellow) compared to blue green from winter a few years back. Are there any theories as to why?

This is a really good question: we don't know for sure, but we have some theories. It's definitely a chemical transformation/reaction, however whether it is organic (carbon chain) or inorganic chemistry (such as sulfur/ammonia molecules) we can't be sure yet. It's certainly going to stimulate a lot of follow-on lab research to try to replicate those changes in a lab environment.

Q: Discovery

What would you say is the most interesting, personally, to you, thing to come out of the entire Cassini mission? Or is it to come on Friday?

For me, it's that Cassini-Huygens taught us so much about Saturn's moons. Both of the moons Titan and Enceladus are considered to have "prebiotic" conditions -- which is to say that, in theory, they have biological potential.

We can't say anything definitive, though. In fact, these discoveries were so surprising that Cassini's designers didn't anticipate them. The probe didn't have tools to test for more complex carbon chemistry that give clearer hints at life. So the it's the most interesting, in my view, because it raises more questions: Namely, what's going on within the ethane seas on Titan and the hydrothermal geysers on Enceladus?

I know there will be data from Cassini itself but is something/someone watching what happens as well?

Yes! Telescopes on the Earth will be watching, in the chance that the 'fireball' of the entry can be seen from Earth. It will still be a very small flash compared to the size and brightness of Saturn, however with a large telescope there is a possibility. The Earth's hemisphere will be Australia so we have some observers there lined up!

Q: Future missions

Are there future planetary missions approved or planned with comparable discovery potential to Cassini?

NASA does have one large mission to the outer planets approved right now: the Europa Clipper. This will launch around 2022 and voyage to Jupiter, then make many flybys of the moon Europa to investigate its subsurface ocean, and pave the way for a lander to follow.

Q: Preventing contamination on future trips to Saturn's moons

Hello, thanks for taking our questions! I understand Cassini is being ditched to prevent any possible contamination on moons possibly hospitable to life. How would you prevent any contamination on future visits to these moons?

As we have explored the solar system, we have truly gotten an appreciation of many possible habitats for life are out there. So future missions will need to be very careful about "planetary protection." We can do this in several ways: like Cassini by avoiding any direct contact. But for landers, we need to really sterilize the spacecraft before takeoff, e.g. by heating to high temperature to kill any earthly microbes.

Q: Is Cassini a He or She?

Obviously, there's a lot of sentiment around this machine, do the engineers refer to it as "him" or "her," like we do with always-female ships?

There is a debate about that! Some people do want to say 'her', but in general scientists say 'it'. I think it is similar to your car, do you tend to say I taking 'him' or 'her' or 'it' to the repair shop? It depends on the person :)

Q: New Spacecrafts

What are the plans for future solar system exploration spacecrafts under the Trump administration?

NASA currently has an acting administrator, Robert Lightfoot. Once the president has proposed and Congress has approved a new permanent administrator, we will be excited to see our future direction evolve at that time.

Q: cassini's materials

are all the elements in the materials which make up Cassini likely already present in Saturn in some amounts, or might some be new introductions?

Saturn will indeed have trace amounts of all naturally-occurring elements already, so Cassini will just be adding a tiny amount more, a drop in a very big bucket!

Q: Data

What kind of data do you hope to gather with this plunge?

We will be measuring Saturn's gravity field and even its atmosphere close up right to the end. This will be the closest information we have about the planet, so it will be uniquely valuable. We will learn a lot about Saturn's interior in the final minutes, that will give us some comparison to the information now being gathering by the Juno mission at Jupiter.

Q: Data download during final plunge

Even though there will be no pictures during the final plunge, will other data be transmitted during the plunge?

Absolutely. We will be receiving real-time information from 8 of Cassini's 12 instruments, so the data will be really unique and valuable. This will include information about Saturn's close-in particles and fields (electrical/magnetic) environments, and also its gravity and atmospheric density. We are really looking forward to this unique 'look' at Saturn!

Will coverage of Cassini's loss of signal tomorrow morning be broadcast on NASA TV?

NASA is broadcasting the "Cassini Finale" -- including the final loss of signal -- via YouTube and Ustream tomorrow at 7 a.m. eastern.

Q: Re: last picture

Why turn off the cameras at all? Obviously the data after some point can't get transmitted anyway, but it isn't as if you need to save the battery power for later. I guess the question is: Why is one state more appropriate than another state for the spacecraft in order to commit suicide?

The cameras actually use up a lot of data rate, or bandwidth, compared to the other instruments. So it's a trade-off between getting some partial images, or continuous data from eight other instruments. Also, those final pictures would be very blurred and not so informative than the pictures we already have.

Q: Sand blasted?

Is Cassini at all "sandblasted" from passing through the rings? Obviously we have no pictures of the spacecraft, but if you were to speculate...

Truly, we don't know if there are small pits and scars from ring particles. However the passage through the rings has been mostly pretty clear sailing, we know that from the instruments that were turned on, and detected the amounts of particles there. Our estimates of a mostly clear zone between the innermost rings and the planet were quite accurate.

Q: Moment of Destruction

Will scientists be able to tell how quickly Cassini is being destroyed by Saturn's atmosphere? Is there anything we can learn from how quickly/slowly the burn up occurs?

Yes indeed, the exact timing of the loss of signal will tell us about the atmosphere, how dense it is.

Q: Risk of Contamination

I understand why Cassini is being steered into Saturn's atmosphere - to avoid the risk of contaminating one of the moons. But was Huygens sterilized while Cassini wasn't?

Per the Committee on Space Research, Huygens was a Category II mission, where there is "significant interest" about chemical evolution and origins of life, but at the time was judged "only a remote chance that contamination carried by a spacecraft could jeopardize future exploration." So Huygens was not sterilized. (Enceladus is a Category III world.)

Q: Images

How much higher resolution are images that are taken by Cassini versus those that can be taken by an earth based telescope or something like Hubble? I realize there is a lot more information than just images that a probe gathers.

Much better detail from Cassini! Hubble's cameras are fantastic, but Cassini is a lot, lot closer.

Q: On board systems

Obviously Cassini is running low on power... But have any of the systems onboard the spacecraft failed? Or shown signs of failing over the years? Is the lack of power the only thing preventing Cassini from continuing on?

It's actually lack of fuel, not power (i.e. electrical power). Cassini is nuclear-powered for electricity, which could last a long time into the future (e.g., the Voyager probes just turned 40!) But the fuel is running out, so we would no longer be able to "steer" our way around Saturn's systems of moons. The safest plan therefore is to safely dispose of the spacecraft before the tanks are exhausted.

Q: Enceladus

Any missions planned for Enceladus? I know there are worries about contaminating with earth-borne pathogens but I'd love to know more about this high-potential moon closeup and on the ice.

Indeed, Enceladus is a high-priority target for NASA in future. Several mission have been proposed this year for NASA's mid-size 'New Frontiers' class of missions. These would orbit Saturn and make repeated flybys of Enceladus with a new suite of instruments designed to detect life.

Q: Contamination

Is there any concern that the "burn up and [explosion] in the atmosphere" will contaminate the environment on Saturn as far as future science goes?

Saturn is really, really huge and Cassini, although the size of a small school bus, is still very tiny. Cassini will burn up high in the atmosphere and will be dispersed far and wide through Saturn's atmosphere. So no, no worries about contaminating Saturn. Enceladus was a different story!

Q: Going in low and fast

A previous article described very low periapses in the end mission as equivalent to the "Deathstar-trench attack run". Has the panning speed of the camera been adequate to get usable pictures during the low, high-speed "cloud-skimming" periapses in the near-end mission? Or did Luke turn off his tracking computer? Were you trying to reach any particular places by altering the orbit this way, or was it just gradual slowing?

It's a bit like taking pictures out of your car moving down the freeway (as a passenger, of course!) Close up objects will be blurred, but you can still get great pictures of the more distant scenery. For Cassini, the closest images to Saturn showing a lot of clouds flying by, which still may give us usable information. However the images we took of the rings are truly remarkable and very scientifically important!

Q: Definition of Saturn

What was the answer to...." *bonus question: is Saturn the gas ball + rings or just the gas ball, and the rings are considered moon-like objects?"

It could be either way: the rings are an entity in themselves, but they also interact with the planet by slowly adding water and other molecules into the atmosphere.

Is there any possibility that Cassini may survive the "plunge"? Assuming it does (just for kicks), would it be possible to continue receiving data from it's resting place?

No, that will not happen. Think of a meteor in the Earth's atmosphere. Cassini will break apart and vaporize. Although maybe a good science fiction story for you to write?

We're going to wrap up -- thank you for thoughtful questions and reading about the end of Cassini. Stay tuned tomorrow, too, for our coverage of the final signal Cassini sends back to Earth.

In This Chat Ben Guarino writes for The Washington Post’s Speaking of Science section. Conor Nixon
Conor is a space scientist working in the Planetary Systems Laboratory at NASA Goddard Space Flight Center.
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