OPAG, Day 1: Getting to Europa
Posted by Emily Lakdawalla
05-05-2006 19:37 CDT
Next up at the Outer Planets Assessment Group meeting was an overview of the plans for future Europa missions. All of this was being done in an environment where, three times, NASA has committed to at least studying a future Europa mission, and then reneged on that. It's not clear why getting a Europa mission started has been so hard. There has been consensus among scientists (well, with a few objectors, but they're scientists, there will always be objectors) since the end of Galileo's primary mission that Europa is the prime target for the next "flagship" mission that NASA should launch. There's clearly a lot of frustration in the outer planets community that the start has not happened, despite the fact that Congress explicitly directed NASA to start it. But there was a lot of constructive discussion over the two days of this meeting directed at what the outer planets community can do to get it started.
First, Ron Greeley gave an overview of the Europa Focus Group, sponsored by the Astrobiology Institute, which holds workshops on all aspects of Europa. The most interesting thing from his presentation is that the astrobiology community has the same broad view of how Europa should be explored as the outer planets community does: send an orbiter first to do thorough reconnaissance, and follow that later with a lander with a specialized package that can study Europa's astrobiological potential (as well as other things).
Bob Pappalardo gave a presentation on the refined science objectives for a future Europa mission. Whatever architecture or constraints are imposed on the mission to Europa, these are the community's agreed-upon list of the questions that should motivate the instrumentation and investigations performed by the mission; I already summarized them from the talk Bob gave to the outer planets community at LPSC in March. Here's the list:
- Characterize the ocean through its effects on potential fields and its dynamic relationship with the ice shell.
- Characterize processes operating within the ice shell, and the nature of ice-ocean exchange.
- Determine surface compositions and chemistry, especially as related to habitability.
- Understand the formation of surface features, including sites of recent or current activity, and identify candidate sites for in situ exploration.
- Characterize the magnetic environment and moon-particle interactions.
- Determine how the components of the Jovian system operate and interact, leading to potentially habitable environments in icy moons.
You can read the full document discussing these objectives at the OPAG website (PDF format).
After Bob presented these, Louise Prockter asked: "What do we do with this document now?" Greeley responded: "Once a document or words like this is crafted, it can be used in a lot of different places, like in any of these planning groups. But some of this, in relation to working on the Hill, may not be the kind of wording we want to use in working with a staffer. So we also want to take these and put them in terms that might be more appropriate for someone like the Planetary Society and [congressional] staffers."
Next up was Karla Clark from JPL, presenting the results of the latest study performed by JPL of a possible Europa mission called Europa Explorer. Before she gave the presentation, there was a funny but awkward episode. Curt Niebur, who is the NASA HQ representative to (from?) OPAG, had to ask all foreign nationals without Green Cards to exit the room, because Clark's PowerPoint presentation had not been vetted by the office at JPL whose responsibility it is to make sure that none of the materials used in public presentations run afoul of ITAR, that International Trade in Arms Regulations law that has been so stifling to international cooperation. About a dozen people ruefully left the room including the two ESA representatives, Peter Falkner and Gerhard Schwehm, who were there seeking ESA-NASA cooperation on a future Europa mission! ITAR frequently has very silly and annoying implications for getting work done in space exploration. The silliest aspect of the whole thing was that just half an hour earlier I'd been asked by Curt to switch my badge for one that said "PRESS PRESS PRESS PRESS" on it in great big red capital letters on a yellow background -- it might equally have said "DANGER DANGER DANGER" -- yet it was okay for me to stay in the room! I feel sorry for the people who have to enforce these silly rules. I suppose that by reporting on Karla Clark's presentation I could run afoul of ITAR. But there didn't seem to be anything shocking in her presentation; and my experience from working with the rovers and ITAR issues leads me to believe that the pictures are generally the things that are considered more sensitive than the words, so hopefully I will not get into any trouble by summarizing Clark's talk.
The JPL study was of a Europa orbiter, specifically a Europa Geophysical Explorer. "The study said: what if we used only existing technology?" Clark began. Existing technology removes roadblocks to a launch soon. Clark said there were two caveats: they planned to have a new radioisotope power supply that is being developed for Mars Science Lab, and they allotted mass for a lander but did not study what that lander would look like, how it would land, or how it would be instrumented.
The mission outline would go like this: "It has 1.5 years of Jupiter system science because we would spend a lot of time getting our velocity down to where we can get into orbit around Europa. This is followed by a 90-day prime mission around Europa." However, if they can build a spacecraft that performs as well as Galileo did beyond its design lifetime, "we're talking upwards of a year in orbit around Europa," Clark said. "For science instruments, we're looking at 180 kilograms, 10 instruments, including a high-res imager and a sounding radar. And we had 340 kilograms of unallocated mass, which could be used for system robustness, additional radiation shielding for a longer lifetime, or a lander."
Someone asked: "Is that enough of a mass margin to go to a smaller launch vehicle?"
Clark: "No." The proposed mission would launch on a Delta IV Heavy, using a Venus-Earth gravity assist trajectory. Clark explained that they had looked at using a smaller launch vehicle -- an Atlas V -- but that it was just cutting the mass margin too close. "We took this mission and tried to put it on an Atlas and it did not work. You might be able to do a different mission on an Atlas, but not this mission."
"One of the issues we dealt with is getting data back from Europa. Radiation-hard memories are difficult. We came up with a system that returns gigabits per day, 250 times as much as Galileo." Someone asked whether that was all through the Deep Space Network, and she said yes, and that they were assuming 24-hour-a-day access to DSN resources during the 90-day nominal mission.
Clark explained that previous Europa mission studies had been hampered by the requirement of a direct-to-Jupiter trajectory (no gravity assist, hence more restricted mass) and a lack of understanding of the exact difficulties imposed by the radiation environment at Jupiter. "With a direct trajectory you come up with a 1-ton spacecraft; with an Earth gravity assist -- 2 tons; with VEGA [Venus-Earth gravity assist] you get about a 3-ton spacecraft. The cost of increased mass is increased flight time. Direct is about 3 years; EGA is about 5; VEGA is between 6 and 8 years.
"We've spent several years taking Galileo data and incorporating that into a model of Jupiter's radiation environment, so we have a big advancement in the radiation-hard components and subsystems. There have been developments in radioisotope power systems (RPS) and what we've come up with is an orbiting mission that is feasible with current technology."
Clark showed a graph comparing the proposed Europa Explorer to the proposed Europa Orbiter mission that was canceled in 2001. I suspect that this comparison had a little bit of salesmanship in it, with some likely optimistic numbers, but one of the contrasts really stuck out. For the Europa Orbiter, the mission had a nominal length of 30 days and it could not be extended for planetary protection reasons. Clark explained why. "All orbits around Europa are unstable" because of the influence of Jupiter and the lack of understanding of Europa's gravity field. "This is something we didn't really understand in the original Europa Orbiter. The eccentricity grows very rapidly, and uncontrolled orbits impact Europa on the order of one month. That is dependent on which orbit you're in. There are some orbits that take longer to impact, but it's very, very dependent on what gravity field is and we don't know what that is. We can find them once we are there."
With the new proposal they would go into orbit, figure out the "J3 values" (a parameter describing the difference in the shape between Europa's northern and southern hemispheres), and then make adjustments. "Once you know what the J3 values are, you know what orbital eccentricity gives you an orbit that is 'frozen,' then you would have to move the orbit." The fact that they can keep their orbit stable means two things: their nominal mission can be longer -- minimum 90 days -- and they can extend the mission as long as they can be sure that they can control the spacecraft.
Another big challenge has been Jupiter's radiation environment. Radiation is bad all around for a spacecraft but one of its worst effects is on memory -- that is, on the data that the mission will be returning. The JPL proposal solves this problem by keeping data in memory for as brief a time as possible. "We assumed that for a short period of time, we could get continuous coverage by the Deep Space Network. This has been done before; we felt 90 days was pushing it. We'd have 300 Megabits of storage on board for science data; we ended up with 21 Gigabits per day. It's a realtime mission -- you use onboard memory as a buffer, and you send the data down." Someone asked whether that required continuous coverage by the largest, 70-meter dishes; Clark said yes.
Talking about the possibility of using the 340-kilogram margin for a lander, Clark showed a few concepts that other working groups had developed, and where they weighed in. An impactor required around 100 kilograms; airbag landers ("Europa Pathfinder") came in at a bit over 200; soft landers came in between 300 and 400, with one outlier at 800. So, she argued, a soft lander is probably not possible, but an airbag assisted lander could be. However, there was a bunch of muttering among the people sitting around me to the effect that a lot of the 340 kilograms would probably be eaten up in the 20% mass margin imposed as a risk-mitigation strategy, and there were questions about whether it could be better used for other things -- like, for instance, a scan platform for the optical remote sensing instruments. However, Clark pointed out that their proposed design included a gimbaled main antenna in order to keep up that continuous data transmission to Earth; that would make a scan platform much less necessary. (Cassini lacks either a scan platform or a gimbaled antenna, so it can either take pictures or communicate but not both, something that has made planning Cassini's mission very complicated.)
Someone asked if this study included a cost analysis. Clark said it did not, and that there were a lot of factors that made cost hard to pin down; she finally said "I like to say it's 2 plus or minus 1 billion. It depends upon a lot of things that are outside of our control: launch vehicle, instrument cost, whether or not there is a lander, and whether the new radioisotope power supply is available."
It seemed that this presentation was regarded as interesting and a good start -- but not official enough; it lacks the detail to really determine whether it is feasible. Still, Bill McKinnon remarked, "Overall, I think this is much more comprehensive and realistic than the previous studies."
With Clark's presentation over, the foreign nationals were allowed to return to the room, which was good because Gerhard Schwehm was next on the agenda, talking about ESA's plans for a Europa mission. He explained that while ESA wants to go to Europa, the mandate of the mission had to be much broader than that. "The three themes: characterize Europa as a planetary object and a potential habitat; study the origin, formation, and evolution of Jovian satellite system; and Jupiter system science: atmosphere, magnetosphere, and nebula. This group is not only focused on Europa but on Jupiter system science. In Europe you have to have much broader focus in order to get the support of the scientific community. If we get one mission to Europa it will be many decades before the next one." He said that the appeal would be broader because of the presence of Juno. "This is complementary to Juno, because Juno focuses on Jupiter; this mission would focus on satellites. The two address Jupiter's magnetosphere and atmosphere in an ideally complementary way."
The architecture that ESA is looking at is very interesting. "The themes can be addressed by the combination of a Europa orbiter and a Jupiter orbiter relay satellite. A lander element was considered but is not currently foreseen as feasible within the budgetary envelope. We would like to have a lander, but in the group we are realistic that we stay within a budget limit."
The mission can be proposed to ESA in a process beginning in the middle of this month, Schwehm said. "Responses will be due mid-October, and we would like to complete down-selection by early next year. Out of the ESA-NASA Europa working group, we hope we can have a team that proposes a mission to Europa and the Jovian system and we hope that the participants in the ESA-NASA Europa orking group will participate in the preparation of mission proposals. We hope that if we pass the first round -- that is, if the Europa mission proposal is selected for assessment study -- that NASA will be able to commit to a joint ESA-NASA study team. We have to see how under present ITAR regulations we can work. We have demonstrated in the past we can work. If we do not do that, the next 10-15 years will be a problem. The community at ESA believes that Juno and the newly proposed ESA-led Europa mission will form two ideally complementary components of an international Jupiter System Exploration Program; close communication between the two mission teams will maximize synergies and science return. I think that with the budget situation on both sides of the Atlantic, we have to do our best to get the most science out of our decreasing budgets."
Schwehm handed over the floor to Peter Falkner, who talked more specifics about the proposed ESA mission design. "The configuration is one Europa Orbiter and one Jovian relay spacecraft. We know from the radiation environment around Europa that orbiter lifetime is very limited. We came to the conclusion it was better to have an orbiter to relay data more slowly to Earth, even after the Europa orbiter is already dead. We don't bring to Europa what we don't need there. And with additional relay satellite it is easier to address other Jovian system science topics."
The mission architecture would be about a 1-ton spacecraft, launched on a Soyuz-Fregat into a Venus-Earth-Earth gravity assist trajectory, taking 6 years to reach Jupiter if launched in 2017 or 2023. ESA is considering either chemical thrusters or solar electric propulsion. The relatively small mass, split between two spacecraft, gives a very small payload of 34 kilograms on the Europa orbiter and 16 on the relay satellite. "Because of the mass challenge, we are looking into a highly integrated payload suite. We marry all instruments together as much as possible, reduce harness, and reduce multiplication of subsystems."
This proposed mission sounds pretty cool but it will be competing against many other proposals; and unlike NASA, ESA doesn't have all these different divisions between the Mars, outer planets, astronomy, and heliophysics groups; all of these branches compete together. So it's unclear what the chances are that a Europa mission can make it through this process at ESA; but it's clear that ESA thinks Europa exploration is a priority, and they want to work together with NASA on it. I hope NASA can make things work better with regards to all that ITAR stuff to allow this cooperation to happen.
My eyes are crossing so that's going to be it for today. I've gotten through about half my notes from yesterday; a day and a half worth of notes remain, and I'm off to join my colleagues at the International Space Development Conference tomorrow!