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_*]

The reply to the second question is simply: we don't know. We don't know whether life could have evolved out of prebiotic molecules on Earth had the water been as acid and/or saline as Europa's appears to be; there has been at least one abstract I've read expressing doubt, but given our stupefying continuing level of ignorance about how the chemical process occurred on Earth itself, we just don't know.

As for the first question: even if we don't get all the way down to isolated "smokers" on the floor of Europa's ocean, we should be able to detect microbes (or their remnants) from such locations spread uniformly through the ocean water -- after all, that's how life on Earth gets transferred from one isolated smoker to another and so survives after the first smoker finally goes out. And if the alternative theory is true that Europan microbes may derive their nourishment instead from chemicals manufactured by radiation in Europa's upper ice layer and then gradually transferred down to the ocen by geological processes in that ice layer, the principle is even more true -- in fact, in that case the life would probably be concentrated at the TOP of the liquid-water layer.

This also leaves the question of whether we can find evidence of microbes in Europa's liquid ocean without even having to bore down through the ice layer to the ocean, by instead analyzing the surface ice itself to look for such remains transported up to the surface by those same slow geological processes in the ice layer. The consensus seems to be that this is a real possibility -- but, since there's a thin layer of brittle supercold "nonconvective" ice 1-3 km thick on top of the warmer main ice layer which slowly convects (and which even perhaps carries pockets of still-liquid brine upwards), we are going to have to be careful to choose landing sites that look likely to have had buried material erupted all the way up to the surface. (There are several types of Europan surface features that show promise of this, which is another reason why we do need a Europa orbiter first to pick out good landing sites.)

[Guest_Richard Trigaux_*]

This also leaves the question of whether we can find evidence of microbes in Europa's liquid ocean without even having to bore down through the ice layer to the ocean, by instead analyzing the surface ice itself to look for such remains transported up to the surface by those same slow geological processes in the ice layer.  The consensus seems to be that this is a real possibility -- but, since there's a thin layer of brittle supercold "nonconvective" ice 1-3 km thick on top of the warmer main ice layer which slowly convects (and which even perhaps carries pockets of still-liquid brine upwards), we are going to have to be careful to choose landing sites that look likely to have had buried material erupted all the way up to the surface.  (There are several types of Europan surface features that show promise of this, which is another reason why we do need a Europa orbiter first to pick out good landing sites.)

Yes, good approach in a first time, before trying to reach the bottom of the ocean (what I think possible but more complicated). But the very upper layer of ice (about 1m) is exposed to strong radiations, and thus sterilized, and anyway not representative of the global ice chemistry. So we need to drill from the very first landing, even if only 1-2m. For this reason penetrators were proposed (sticking themselves in ice like an arrow, which solves the problem of soft landing) or using a more classical drill, or a heat source able to melt ice. (RTGs were proposed, but RTGs are weak, a chemical source would perform better for this very purpose). A tip would be to place all the electronics into the drill, so that it is protected from radiations and it can last for much longer, very useful if it carries a seismometre.


Also a RTG was proposed to melt the ice down the ocean. But are RTGs powerfull enough for this? I would rather see a bot with a screw-shaped nose and a body with fins, like in sci-fi novels, running with a high gear rate, it would be much more power-efficient and faster than just melting ice. We have plenty of places on Earth to test this, in the Antarctic ice shelds. If there are interesting results from a surface examination, there will be a strong support for the idea of looking at the botton of the ocean.

[Ames]

The beauty of an RTG is that it creates a LOT of "waste" heat that can be put to use here for "free".
Also the electricity generation efficiency can be increased if there is an effective cold sink.

So an RTG probe is Perfect - as long as you don't hit a buried rock/meteorite.

Nick