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.

[Roly]

I know this has come up before but is it possible to do a Europa fly-by within a Discovery cost-cap?

Given that the Europa Orbiter is going to be 2012 at the earliest, surely there is scope for someone to propose something for Europa at the next Discovery AO.

I don't know that much about the design of orbiters, and this may
demonstrate (and probably does) some fundamental misconceptions, but I was
curious about a return to fly-by missions instead of orbiters like Galileo,
Cassini, EO etc. From what I have gathered, for many missions, the largest
problem is the energy required to brake into science orbit, particularly
where there is no potential for aerobraking, meaning a huge percentage of
the mass budget must be spent on a powerful engine and the requisite
propellant. I was under the impression that these problems were vastly
reduced in fly-by trajectories (?) With VEEGA trajectory perhaps this would be possible on a very economical LV (though it seems everything going forward is flying on Delta IV and Atlas now, not Delta II?) Okay so not a Taurus but there must be something in between Taurus and Delta/Atlas EELVs.

So instead of spending mass on an large engine with enough fuel to get into
a science orbit, couldn't the spacecraft instead focus on acquiring a huge
amount of data over a very short period of time in a low-altitude fly-by?
It would then sample a small area of the target at high-resolution.

It could carry (and I'm only guessing, the exposure/integration
times/pointing accuracy might make it truly impossible) stereo cameras and a
high
resolution imaging spectrometer on a motion compensation
scan-platform. All would operate
simultaneously, with power from advanced Solar arrays (Rosetta heritage?) lithium-ion batteries,
which would be charged during the cruise phase by the array to make up for any
power deficit during the very short and high-demand encounter phase. They could even be augmented with lithium primary batteries if required. The
spacecraft would be equipped with extremely wide and fast data busses,
caching the acquired data to memory and writing it out to solid-state disks
for later playback. No-RPS, easier admin. burden, compatibility with Discovery AO.

It would record this to high-capacity solid-state data recorders (multi-100s
GB), and then replay it once it had left its target, potentially over a
period of months or even years, depending on how much power was available,
local limits on the downlink and so on. This eases DSN management perhaps? Offline compression would be helpful, especially given that there might be issues with heavy compression in the Jovian environment (I'm suprised ICT did so amazingly well).

As an added bonus some of the
radiation hardening requirements would be reduced, as the spacecraft would
not be remaining in the most dangerous areas for a prolonged period,
repartitioning mass away from shielding and to the science payload. Lithium polymer batteries might be decent for providing some incidental shielding in any case.

I suspect I am heavily overestimating the ability of the scan platform to
provide adequate motion compensation, and the amount of time needed for the
instruments to acquire data - but I was curious about the idea because it
leverages some of the things that have become much better and cheaper over
time, like bus bandwidth, memory, disk write performance, processing power,
detector resolution/sensitivity - to replace things that have not, like
putting large amounts of propellant and engine mass into orbit.

I read a bit about Firebird recently, which seemed to utilize a similar approach. Perhaps there is geometry which would make possible a "Fire and Ice bird" (like the old JPL 'Fire and Ice' theme) ?

I've wondered out loud about this on other forums but no-one ever seems to have any ideas (other than Orbiters are better, which of course they are...)

Roly

[Guest_Richard Trigaux_*]

I don't know that much about the design of orbiters, ... the largest
problem is the energy required to brake into science orbit, particularly
where there is no potential for aerobraking, meaning a huge percentage of
the mass budget must be spent on a powerful engine and the requisite
propellant.

Would not an economic trajectory achieved with a ship in orbit around Jupiter, using the moons as a gravitational assistance, until getting a nearby circular orbit at the level of Europa, which provides the most economic way to brake and land?

The only inconvenience of this is that it would require some months, and also staying into radiation belts for a long time. So what is gained in fuel may be lost in hardening the electronics.