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.

[dvandorn]

It's not just that orbiters are better (though that rather goes without saying, in many ways).

First, there is the issue of time-and-change-dependent observations. If one of your goals at Europa is finding places where the ice crust is thin enough to provide some form of access to the ocean below, you need to observe Europa over a matter of weeks and months and track the movements of the crust.

Second, there is the issue of coverage. No matter how you design your approach trajectory, you're going to be able to observe only a tiny fraction of the surface during a fast fly-by. And there will only be one side sunlit during that snapshot. So no matter how capable a fly-by probe might be, its choice of what it can view is severely limited.

Lastly, there is the politics of funding. We are *barely* to a point where we can think about getting Congress to fund a Europa Orbiter mission -- a mission that we really need, in order to answer fundamental questions and set up a possible landing mission (and, more ambitiously, a mission to explore the subsurface ocean). Because of the points I raised above, a fly-by mission is far less likely to provide those answers, no matter how much data it returns about a very small portion of Europa. And now, *in addition* to funding an orbiter for a billion or more dollars, we're going to ask them to fund an interim fly-by mission for another half a billion dollars? If we were to do that, we'd end up with either the less-useful fly-by and NO orbiter, or (more likely) just getting laughed off the Hill for trying to get them to fund *two* different missions to the same chunk of ice millions and millions of miles away -- a chunk of ice most of them think is worthless in the first place.

Is that a little better answer than a simple "orbiters are better" statement?

-the other Doug

[JRehling]

Second, there is the issue of coverage.  No matter how you design your approach trajectory, you're going to be able to observe only a tiny fraction of the surface during a fast fly-by.  And there will only be one side sunlit during that snapshot.  So no matter how capable a fly-by probe might be, its choice of what it can view is severely limited.-the other Doug

Something that the continued Cassini imaging of Saturn's many moons has made me aware of is the value of terminator observations, which are even more severely limited than observations intended merely to map albedo. A single flyby will show a world in 50% illumination, but only a small fraction of the surface in the low sun angles near the terminator. When Cassini's mission is done, we will be able to generate low-resolution DEMs for all of Saturn's inner icy moons, which would be very hard to achieve by performing direct measurements of altimetry for so many worlds (JIMO comes to mind).

Amateur astronomers know how dull an object the full Moon is, and Mariner maps of Mercury show how much more detail is seen at the terminator than mid-disk. An orbiter (even a Jupiter-orbiting craft) would provide not just twice the imaging of Europa -- in terms of terminator observations, it would provide perhaps 100 times as much imaging.

Any flyby of Europa better be a sample return.