Europa Orbiter, Speculation, updates and discussion Options

[Redstone]

There has been lots of discussion of a mission to Europa in the excellent thread on the Juno mission. I thought that since a Europa mission seems to be once again becoming a possibility, it deserved its own thread for news, updates and discussion. I thought I'd kick things off with a summary of past efforts on a Europa mission, and on where things stand now. If I make a mistake, please correct me!

In the course of its prime and extended missions, Galileo found evidence of liquid water under the icy surface of the planet. Planning began on a Europa Orbiter mission, with a projected arrival date of 2008, to confirm the presence of the ocean, characterize the thickness of the icy crust and identify places for a future landing. One thing to note about these earlier plans: they included a direct trajectory to Jupiter, presumably to minimize mission duration and qualms about RTGs re-entering Earth atmosphere after some (highly unlikely) targeting mishap. But NASA lacked a nice category of missions to place the Europa Orbiter in. Eventually it got lumped together with Pluto Express and Solar Probe in a Outer Solar System program labelled "Fire and Ice", a term which also got applied to the Galileo Europa Mission extension. Without a solid program to support it, (like Mars Exploration, Great Observatories, or Discovery) the mission looked like an orphan.

As Bruce Moomaw has well documented, attempts to kill off the Pluto mission led to a tug of war between NASA, the planetary scientists and the public, resulting in Congressional directives to NASA. Pluto Express became the Pluto/Kuiper Belt Explorer and then New Horizons and New Frontiers 1. (New Frontiers 2 is of course Juno.) But the cost for the Europa mission continued to rise, and the launch date recede, as the difficulty of radiation shielding and the large delta-v requirements hit home, and the mission's public profile fell. The launch date moved to 2010 and the costs moved over a $1b. Then along came Sean O'Keefe and JIMO, a justification for the Prometheus program through developing nuclear electric propulsion, not with RTGs, but with an in-space fission reactor. Launch got moved to 2011, then 2012, while the cost went even further through the roof.

With the arrival of Mike Griffin, JIMO was cancelled. As Griffin said to Congress, "It was not a mission, in my judgment, that was well-formed." But interest in a Europa mission remained and remains strong. In 2003 the National Academy of Science's Decadal Survey flatly stated that a Europa Orbiter was the top priority for the next Large scale (aka Flagship) mission. (See page 196 of the report.) NASA's current Solar System Exploration Roadmap reaffirmed a Europa orbiter as the next flagship mission. The question as always is money. As Administrator Griffin said, "The Science Mission directorate wants to do a Europa mission, the National Academy of Sciences wants to do a Europa mission, I want to do a Europa mission. When we can afford it in the budget, we'll do it."

Evidence of that support beyond rhetoric and reports trickled out with a letter from Andy Danzler, NASA's Solar System chief, to the Outer Planets Assessment Group (OPAG). He reported that he had "funded a team to take a quick look at the boundary conditions of a mission to Europa, that is, how much power, mass, travel time, etc. for various realistic scenarios. For planning purposes, this group is looking at launch dates in the 2012-2015 range, although the later dates are more likely in terms of funding." For funding details however, we have to wait for the FY 2007 budget.

OK, now the good stuff.

The latest meeting of OPAG included reports on a Reference Design for the mission. A kind of first draft which establishes a baseline which can be tweaked and modified to extract the best science return.

There are many things to like about this draft design:
* The mission is now permitted to use Earth flybys, and uses a proven trajectory, the same as used by Galileo (Venus-Earth-Earth Gravity Assist). This allows a BIG increase in the available mass.
* The orbiter uses RTGs, but not super advanced ones that require further years of development.
* The orbiter is similar to Cassini in appearance, with 2 engines, a cylindrical tank structure, RTGs at the base, the magetometer boom at the top, and space for a lander bolted to side. The similarities may make it easier to convince Congress that this is something NASA knows how to do. The most obvious configuration change is with science payload and HGA having switched places, and the addition of a radar array. And there looks like a camera the size of MRO's HiRISE!
* The mission is definitely Flagship in scope with a launch mass of over 7000 kg on a heavy lift launch vehicle. For comparison Cassini was 5712 kg at launch on a Titan IV, and Galileo was 2223 kg when launched using the Shuttle and an Inertial Upper Stage.
* There is a good opportunity for ESA participation with the lander and science instruments. NASA/ESA co-operation is on the agenda for the next OPAG meeting.
* The mission does not assume big upgrades to the Deep Space Network. If the Next Generation DSN does come along, that's just gravy.
* Despite the Europa focus, the mission appears to give at least part of a Galilleo II style tour with multiple flybys of the outer Galileans over 18 months. Only Io will have to wait.

The OPAG Europa working group is also expected to present further work at the next meeting in October. More details will emerge then. I think there is room for cautious optimism on this mission. While we won't be seeing a mission launch for at least another 7 years, the combined weight of the planetary science community does tend to get it's way in the long run. I think the momentum is finally starting to build.

[Guest_BruceMoomaw_*]

I've been thinking for some time about a modified version of Ice Clipper, in which the spacecraft would be based not on Stardust (with a small impactor) but on Deep Impact (with a much bigger Impactor kicking up a much larger amount of debris, and kicking up almost all of it from depths far below the radiation-modified upper layer). The Impactor's camera could also get extremely high-res final photos which could provide additional valuable information on small-scale surface ruggedness for the purposes of lander design.

I've even wondered if it might be advisable to launch such a mission BEFORE the Orbiter; a high-res camera and near-IR camera on the main craft, coupled to a very high-capacity and high-speed data recorder, could get high-resolution terrain and compositional data on quite a respectable part of Europa's surface just from a flyby (like the "Firebird" Io flyby once proposed as a Discovery mission). One possible motivation for such a mission flying first has disappeared, though: Janus Eluszkiewicz's argument that Europa's upper layers might be riddled with large cavities that would seriously interfere with the depth penetration of a radar sounder -- making it advisable to test the effectiveness of Europan radar sounding from a flyby first -- has come under very serious fire on the grounds that he simply assumed that such cavities could exist when the physical evidence is against it: http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2346.pdf .

And the big problem with a smash-and-grab mission remains: given the very small amount of surface material that it would collect, could even supersensitive Earth-based labs properly inspect the sample for biological evidence? (Especially since it's quite possible that the heating the samples would inevitably undergo as they plowed through the aerogel collector layer would break down organic compounds.) if so, it might be preferable to initially analyze Europa's ice using in-situ instruments, even given their greatly reduced sensitivity and flexibility, simply because they could analyze much bigger amounts of material. JPL's own design study for an initial lightweight Europa soft lander ( http://www.lpi.usra.edu/opag/jun_05_meetin...ssion_Study.pdf ; http://www.lpi.usra.edu/opag/jun_05_meetin..._Trace_OPAG.pdf ) calls for such an organic-isotopic analysis -- using a combined liquid chromatograph and mass spectrometer -- as one of the two top priority instruments for a Europa lander, the other being a seismometer for data on ice-layer total thickness. I myself would regard organic analysis as even more important.

The problem is collecting a big enough sample for such analysis on a small lander -- and collecting it from a fair depth, below the radiation-scrambled surface layer, using a lightweight sample-collection system. The JPL study (which focuses on a surface lander, just because that's the one design it was contracted to examine) expresses concern about this, but doesn't mention specific solutions. A penetrator would seem to be the logical solution. The "Polar Night" Discovery mission proposed to analyze lunar polar ices -- which could well end up as the second in the new series of US lunar exploration probes -- called for three penetrators, each weighing only 30 kg, surviving a crash into the surface at 75 meters/sec and burying themselves 1-2 meters deep ( http://www.nrl.navy.mil/techtransfer/exhib.../PolarNight.pdf ; http://www.mae.usu.edu/faculty/tmosher/Gen...edia/Mosher.pdf ). They would each carry a neutron spectrometer (not necessary for Europa) and a mass spectrometer, and impact tests in which these instruments were fired into a 2-meter layer of plywood and exposed to 1200 Gs (four times their planned load) showed them surviving just fine.

Again, though, if interesting compounds are seriously diluted in the Europan ice, the problem is acquiring enough of them to analyze -- which might require a heated probe to melt its way down through several dozen meters of ice and filter diluted compounds out of the resulting large amount of meltwater ( http://lasp.colorado.edu/icymoons/europacl...ps_EurAbode.pdf ). But such a probe would almost certainly be too big to carry as a piggyback on Europa Orbiter.

If a small penetrator COULD have a chance of analyzing enough material to be worthwhile, however, it would seem vastly preferable to a surface lander as a piggyback on Europa Orbiter in almost every way. It would easily dig below the radiation-modified surface layer (unlikely to be more than a meter or so deep); it would be much lighter than a surface lander; it could land on virtually any terrain, no matter how rugged; it would bury itself and thus provide its own shielding from Jupiter's radiation (which is otherwise a major problem for a moderately long-lived lander); and it would couple its seismometer to Europa far more rigidly than a surface lander. It would probably be unable to obtain post-landing terrain photos, but it could record descent photos during the last few seconds before impact and play them back later for almost equally good imaging data.

[Guest_vjkane2000_*]

If a small penetrator COULD have a chance of analyzing enough material to be worthwhile, however, it would seem vastly preferable to a surface lander as a piggyback on Europa Orbiter in almost every way.  It would easily dig below the radiation-modified surface layer (unlikely to be more than a meter or so deep); it would be much lighter than a surface lander; it could land on virtually any terrain, no matter how rugged; it would bury itself and thus provide its own shielding from Jupiter's radiation (which is otherwise a major problem for a moderately long-lived lander); and it would couple its seismometer to Europa far more rigidly than a surface lander.  It would probably be unable to obtain post-landing terrain photos, but it could record descent photos during the last few seconds before impact and play them back later for almost equally good imaging data.

Penetrators have been proposed for many missions, but they always suffer from the same problems: very limited room for sophisticated instruments (big difference between a spectrometer that can detect water and one that can unambiguously classify organic molecules) and the need for entry into the surface to be near vertical. At the same time, there are some nice features to penetrators for Europa: they did beneath the surface ice (good for sampling) and have a meter or two of ice shielding them from the radiation (good for a longer life). A potential issue: if the Europeans do the lander, I don't think they have much experience with penetrators, but could be quite wrong on this.