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.

[Jeff7]

Someone mentioned an impactor - how about using a small stream of impactors to blast successive craters, and at the end of the stream would be the instrument-laden lander itself? It might still need to drill, but not nearly as much.

[Guest_BruceMoomaw_*]

Uh-uh -- you'd need a huge weight in impactors to blast a hole of any significant depth, whereas you could achieve much greater penetration for tremendously less weight just by making the probe a melt probe (or giving the surface lander a longer drill). To say nothing of the gargantuan targeting difficulties...

[ljk4-1]

Back in 1998 I initiated a discussion list for landing a probe on Europa to explore its subsurface global ocean. Named Icepick, the discussion lasted until just a few months ago.

http://www.klx.com/europa/

You can read the discussions here. I think we hit on many if not most of the scenarios for making this mission plan a reality.

http://www.mail-archive.com/europa%40klx.com/

If someone wants to revive the Icepick list and get discussions going again, I would be most grateful. Jeff Foust ran the intial list and Web site.

[Guest_BruceMoomaw_*]

Yep, that's the site where I got my start as a space commentator -- and where Simon first ran into me. Sad to see that it's finally disappeared. Maybe I should have hung around there, but I've been juggling several plates at one time for the last few years and just never got around to dropping back in. It starts to look as though the discussion site for Europa exploration may migrate over here.

[Redstone]

Apologies if this has already been discussed, but on the OPAG site, there's a fascinating report on Europa Surface Science options. It runs to 86 pages and covers radiation issues as well as landing methods. It was based on the JIMO as the mothership, but much of the discussion is still relevant, I think. I haven't had the chance to go through it in detail, but one point caught my eye: for 375 kg, you can soft land 167 kg on the surface using powered descent. For comparison, the Huygens probe had a mass of 320 kg.

[Jeff7]

Uh-uh -- you'd need a huge weight in impactors to blast a hole of any significant depth, whereas you could achieve much greater penetration for tremendously less weight just by making the probe a melt probe (or giving the surface lander a longer drill).  To say nothing of the gargantuan targeting difficulties...

Ok, a melt probe. RTG powered I assume? Just did a quick search.....one page says this of Cassini's RTG's:
"The alpha particles naturally heat the pellets to 572 degrees Fahrenheit (300 degrees Celsius)."
Not too bad at all, more than I expected actually. That'd definitely make a hole....though I'm just thinking now, it'd encase itself beneath the ice. The water above would likely refreeze fairly quickly, even with a toasty robot beneath it. So the little meltbot would be sealed under the ice rather quicly. What would it use for communication? A fiber optic line would be risky (might get tangled), and would add weight. And I don't know how well radio waves penetrate ice.

[hendric]

So the little meltbot would be sealed under the ice rather quicly. What would it use for communication? A fiber optic line would be risky (might get tangled), and would add weight. And I don't know how well radio waves penetrate ice.

A fiber optic cable is probably the best bet, using a floating transmitter/receiver at the end of the line to keep it above the meltwater until it refreezes. Radio would require repeaters to go through the ice, which is possible, but you'd have to power them somehow, and keep them from melting down (or up!) when they activate.

[Guest_BruceMoomaw_*]

Jeff: I saw that OPAG report -- and the two more recent papers att the OPAG site that I mentioned previously elaborate on it somewhat.

Hendric: The idea of a fiber-optic line for communications -- originally the favored idea -- got the boot several years ago, both because of weight problems and becuase the slow but steady ductile sliding of Europa's ice layers would almost certainly snap it. The current plan is to have the probe carry a stack of tiny disk-shaped radio repeater packages powered by tiny RTGs, and release one every kilometer or so that it descends -- so that they're close enough to pick up each other's radio signals through the ice and thus chain-link the signal from the melt probe all the way to its surface carrier.

[Guest_BruceMoomaw_*]

I should add that the heat from the extremely tiny RTG that each such package would require would not be nearly enough to melt the surounding ice and make it sink deeper.

[deglr6328]

Wouldn't it? Let's say you want a 10W transmitter. You will need at least, I don't know, ~15W total for electronics and losses and such..? The abysmal efficiency of RTGs meas you will need at least a ~100W heat source to power the thing......ice is a very good insulator.....

I would very much like to see a plot of EM wave attenuation vs frequency for ice so that any "windows" could be identified and the necessary transmitter power could be constrained with higher confidence.

Hmm this looks interesting...

[Guest_BruceMoomaw_*]

There are two relevant JPL Technical Reports on this design. Unfortunately, JPL's technical-report server seems to be offline for now, so I've attached both reports.

Actually, each transceiver would use a mere 0.12 W power source, hooked up to a capacitor to allow periodic bursts of 1.3 W transmission power. So that's why there's no RTG ice-melting problem.

Attached File(s)

 CDAR____Europa____JPL_TR_01_2122____Bryant.pdf ( 477.38K ) Number of downloads: 1958

 

[Guest_BruceMoomaw_*]

And here's the other JPL report.

Attached File(s)

 CDAR____Europa____JPL_TR_99_2051____Zimmerman.pdf ( 832.18K ) Number of downloads: 446

 

[deglr6328]

hmmm! very interesting thank you!

[Roly]

Was anyone at the recent (October) OPAG meeting where the new Europa Orbiter was due to be discussed?

The October report and documents aren't up yet, but surely it can't be too long now. Wonder how the talks with ESA went? Hope there was some more support for the 2013 opportunity, given the extra dry mass that could be delivered (probably enough for the soft lander studied by Balint, Nov. 2004).

Roly

[Guest_BruceMoomaw_*]

I wasn't able to make it to OPAG, and have been monitoring their site for news on the presentations and final report from the October meeting. They haven't turned up yet, but I expect them soon.


http://www.lpi.usra.edu/opag/meetings.html
http://www.lpi.usra.edu/opag/reports.html