Enceladus-3 (March 12, 2008) Options

[Guest_AlexBlackwell_*]

Excerpt from Cassini Significant Events for 02/16/06 - 02/22/06:

"As mentioned in previous weeks, the project has been working on adopting a new reference trajectory in order to raise the minimum Titan flyby altitude for various encounters. Today the project reached a decision to proceed with the 'optocc2' trajectory. Additional work is still to be performed before delivery of the final files. This will include minor tweaks that have been analyzed in other trajectories, adjusting orbit 68 timing, and capture of an Enceladus plume occultation on orbit 28."

For the record, the new reference trajectory will result in an even more spectacular Enceladus-3 flyby [61EN (t) E3] on March 12, 2008.

[Guest_BruceMoomaw_*]

The NEP idea -- which would orbit Saturn and fly through the Enceladan plume several times to collect a sample for return -- is what Oberg was talking about in his article. But it would obviously be expensive as hell.

The vastly cheaper idea of flying by Enceladus once and then returning to Earth is a longer range version of Chris McKay's "Europa Ice Clipper", and it might very well be worth doing even for one sample. It wouldn't even need to include the complex requirement, for Europa, of dropping an impactor on Europa and then making sure it flew through the impact-debris plume -- and it wouldn't necessarily need nuclear power; there's already been a proposal for a solar-powered Saturn entry probe flyby mission based on the "INSIDE Jupiter" design, but with 4 solar panels instead of two (and without the big JOI propulsion system). Basically, we would just be flying a duplicate of Stardust -- but with much bigger solar panels -- through the Saturn system and then back to Earth. We're definitely talking about a New Frontiers-class mission, and I have no doubt that people are already thinking about proposing it for the next New Frontiers AO. The catch might be whether Saturn's gravity would be strong enough to allow a hairpin-turn direct return to Earth -- but it might well be workable, instead, to arrange a gravity-assist flyby of Jupiter on the way back INTO the Solar System to finish bending the returning craft's orbit all the way back into the inner Solar System. (The craft would initially have been launched directly from Earth to Saturn, which our stable of ELVs could easily do.)

Parenthetically, there was still some interest at the Europa meeting in dropping an impactor onto Europa and either analyzing or actually collecting the resultant debris plume. C.A. Hibbitts gave a talk on a concept whereby Europa Orbiter would drop a large impactor onto Europa (maybe during one of its multiple orbit-trimming flybys of Europa in Jupiter orbit, before it finally brakes into orbit around Europa itself), take spectra of the resultant flash, and later photograph the fresh crater from Europa orbit -- that is, an Europan version of Deep Impact.

Afterwards I had a long and interesting talk with him about variations on this idea. His idea is to try to analyze material from below the radiation-modified layer -- but he agreed that it's open to question whether long-range IR spectra could properly analyze any organics dug up by the impact. It might be better to have the craft actually fly through the debris plume and analyze it directly with a dust-impact mass spectrometer. (In any case, his impactor would be a substitute for a small piggyback lander on Europa Orbiter -- but see more on that below.)

And it might be better still to have a separate mission that would fly through the Jupiter system without stopping, drop such a really big impactor onto Europa, fly through and collect some of the impact plume debris, and then double back to Earth with it -- that is, Europa Ice Clipper, but with a much bigger (and actively guidable) Deep Impact-type impactor. I've been thinking about this idea for some time, and it turned out that he has too. Again, this is a New Frontiers-class mission -- and if Europa Orbiter really does get hung up fiscally, it might be advisable to fly it first.

One thing we finally did agree on, though, is that one of the optional goals we'd both been thinking about for such an impactor -- using a descent camera on it to get those super-closeup views of Europan surface terrain which may be important to properly design the landing system for a big Europa Astrobiology Lander -- probably wouldn't be practical. In order to have time to return the last few really close descent images just before the craft hits the surface, you need a lander that can survive landing and play back the images after landing -- a crash-lander probably wouldn't be able to return any images of Europa's surface better than the final ones from the Ranger missions, whose resolution wasn't any better than the images we can get from orbit anyway using a HiRISE-type camera of the type that's already very likely to be carried on Europa Orbiter.