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.

[Redstone]

I think you have to balance the size and capability of the spacecraft against the permissible frequency of visits. Because of Europa's distance, and the large delta-v required to go into orbit, a sequential program like that for Mars is not going to be feasible. So there is more demand for the number of question-answer cycles to be kept to a minimum, even if that means more capable, and hence more expensive missions.

When it comes to any form of lander, I think anything complex will send mission risk and cost too high. But penetrometers may be vulnerable to being axed once the squeeze begins for spacecraft resources and funding, even if we are looking at 7 tonnes for the mission. The Decadal Survey *did* identify a Europa orbiter and lander as separate missions, after all. If the lander is an international contribution, that would make it more secure.

The HiRISE style camera is interesting. Certainly the 30 day prime mission is way too short a time to return the amount of data involved in mapping at that kind of resolution. Since the mission will have a wide angle camera for the global mapping, the question would be where to aim the big mirror. One aspect of the mission that would help is the many flybys and steady final approach to Europa before orbit insertion, which would give lots of opportunities for preliminary surveys. Also, if the mission carried a lot of onboard memory, then once in orbit thumnails could be sent, and then selected detail returned. But at that point the mission team would have to make up its mind *fast* (i.e. on a daily basis) on what was to come back in high resolution.

[JRehling]

I think you have to balance the size and capability of the spacecraft against the permissible frequency of visits. Because of Europa's distance, and the large delta-v required to go into orbit, a sequential program like that for Mars is not going to be feasible. So there is more demand for the number of question-answer cycles to be kept to a minimum, even if that means more capable, and hence more expensive missions.

An alternative is to be resigned to the fact that Europa exploration is going to take a lot longer than Mars exploration. The trade-off, simply put, is: Do we want to get the most bang for our buck but have it take more time to fly all the missions we want, or do we want to get the science sooner and risk some missions/instruments that end up missing the point due to some yet-unknown characteristic(s) of Europa?

Don't kick me out of the enthusiast club, but I can't fabricate a case for urgency here. If it takes 8 billion-dollar missions to reach a certain level of understanding, vs a quartet of 3-billion-dollar missions completed in half the time, how do you explain (to the public??) that getting the answers sooner is worth the extra $4 billion? Assuming a fixed budget for exploration, this also means the rest of the solar system gets gyped out of many missions. There is opportunity cost.

When it comes to any form of lander, I think anything complex will send mission risk and cost too high. But penetrometers may be vulnerable to being axed once the squeeze begins for spacecraft resources and funding, even if we are looking at 7 tonnes for the mission. The Decadal Survey *did* identify a Europa orbiter and lander as separate missions, after all. If the lander is an international contribution, that would make it more secure.

I think a smash-and-grab mission that uses an impactor to blast some ice up to a collector that is on a free-return trajectory to Earth has to be considered.

As I see it, the lander concept comes down to two main investigations: What is the composition of the non-H2O stuff? Is there a seismic/thermal/magnetic indication of the structure/activity of the crust and subcrust?

An orbiter can start to speak to the magnetic and probably thermal (by scanning the nightside and eclipsed-dayside in IR) issues. Smash-and-grab would give us a point sample of composition.

I think a very strong candidate plan for the first two missions would be an orbiter that performs detailed surfacing mapping, including scrutiny of whether or not the non-ice component is the same compositionally everywhere. This mission would screen for the best possible locations for any future surface mission, whether it be smash-and-grab, a Pathfinder-style lander, or a penetrator-lander. It is certainly risky to launch a lander of any style without having that basic reconnaisance completed.

The case for the second mission being a lander seems elusive to me. The magnetic and thermal questions will be addressed in part by the orbiter (of course, note: the conditional nature of that statement is already evidence that the second mission should be designed around the results of the first). A smash-and-grab mission would not provide the seismic data of a lander, but would provide infinitely better analysis, in earthly labs, of surface samples -- for far less delta-v.

The combo strategy I have mentioned before for the first landed mission would be to have a lander with seismic capabilities touch down (or penetrate) first, then have an impactor (with its own camera, of course) strike the surface nearby shortly thereafter, providing a known seismic event that would probe the crust fantastically. That same impactor could be the one spraying particles up to the catcher's mitt on the free return trajectory. In all, three elements involving the surface, designed according to the results of the orbiter mission, with a broad wealth of returned data: seismic, magnetic, and thermal data from the landed probe at one location, closeup imagery of a second location, precise seismic data which would give excellent data on one location in the crust, and samples for earthly labs!

Seen this way, the great upside is not to link Mission 1 and Mission 2 to the same launch, but Mission 2 and Mission 3.

The HiRISE style camera is interesting. Certainly the 30 day prime mission is way too short a time to return the amount of data involved in mapping at that kind of resolution. Since the mission will have a wide angle camera for the global mapping, the question would be where to aim the big mirror. One aspect of the mission that would help is the many flybys and steady final approach to Europa before orbit insertion, which would give lots of opportunities for preliminary surveys. Also, if the mission carried a lot of onboard memory, then once in orbit thumnails could be sent, and then selected detail returned. But at that point the mission team would have to make up its mind *fast* (i.e. on a daily basis) on what was to come back in high resolution.

I agree that sophisticated regimes for selecting imagery returns are called for. I don't see why such a mission could not have truly massive memory (cmon, that's light), and the ground crew would have the entire duration of the mission to request imagery for downlink -- imagery taken the first day should still be available for downlink on the last day. Store everything, or at least a heck of a lot. It's a nice thought that the orbiter could have a great set of high resolution imaging in its memory, and the ground crew could peruse the low resolution map, and then request detailed observations in terms of a downlink, as opposed to in terms of a new, future observation.

[ljk4-1]

I think a smash-and-grab mission that uses an impactor to blast some ice up to a collector that is on a free-return trajectory to Earth has to be considered.

Such a mission was developed called Europa Ice Clipper. A 50-pound ball would be slammed into Europa by a flyby probe, which would fly through the debris cloud, grab some samples of Europa, and return them to Earth.

http://www.astrobiology.com/europa/ice.clipper.html