I'm back from the Europa Focus Group meeting... Options

[Guest_BruceMoomaw_*]

...which I decided to attend literally at the last possible minute, which is why I didn't alert you guys in advance. Very interesting -- both the discussions about the likely design of the mission (and how to retrieve it from cancellation), and many of the actual science presentations (which aren't on the Web yet, although they probably soon will be). I'll give you some more information tomorrow -- although I can't resist telling Alex that Tom Spilker's subgroup took my ideas about a Europa penetrator, and the printed information I gave them on the subject, seriously enough to recommend making further inquiries to NASA HQ on it. (And without my browbeating them, either. Nyaah.) The case for it, however, is still extremely far from certain.

As I say, more tomorrow.

[Guest_BruceMoomaw_*]

What we're talking about is similar to Paul Lucey's design for his "Polar Night" lunar penetrator mission. (It's no accident that he shares my enthusiasm for the possibility of a Europa penetrator.) To wit: you put the penetrator into an orbit with a periapse only 2 km or so above Europa's surface, use a liquid-fueled retropackage to cancel out ALL of its orbital velocity at that point (about 1.8 km/second for both Luna and Europa), use gas jets to point the penetrator nose-down and spin it up for stabilization, then detach the propulsion module and simply let the penetrator drop, to hit the surface at about 75-100 meters/second. (The simpler "Jovian Moon Impactor", aka the "bowling ball" -- which seems to be the Penetrator's main rival now as a piggyback lander concept for Europa Orbiter -- would do exactly the same thing, except that it wouldn't bother with attitude control after the deorbit engine shut off.) They refer to the technique as "stop-and-drop".

Clearly you need both a lot of deorbit fuel (ALL vehicle concepts that descend and land from Europa orbit are about 50% fuel, with full-fledged soft-landers requiring only slightly more), and a good guidance and control system. The latter may be the catch -- you need to cancel out the penetrator's forward orbital speed very accurately, AND point it nose-down with high accuracy. Lucey thinks it's doable for acceptably low cost -- and the Discovery review board that appraised his concept reached a similar conclusion -- but obviously this is still open to debate. The sources I've read say that a 5 degree error in angle of attack (e.g., the penetrator's nose is slewed up to 5 degrees from straight down) is acceptable -- but Europa's surface may be much harder than the regolith that lunar and Martian penetrators must cope with. William Moore said he'd heard that "1 or 2 degrees" was a more accurate estimate of the acceptable pointing error -- and of course you must also make sure you totally cancel out the penetrator's forward orbital velocity as precisely as possible.

Even if your penetrator is going straight down, you also have the problem of hitting a sloping surface ("incidence angle"), which as I say may also be a good deal more likely on Europa than for the Moon and Mars. This problem is indeed lesser, although it is by no means trivial. But I discovered just yesterday that the Deep Space 2 penetrators were explicitly designed to deal with these problems, and the fact that they were cruddily constructed and tested does not mean that their solution is unworkable. You use a wide, flat surface contact plate with a hole in its middle, and the penetrator perched in a cylindrical sleeve above that hole. If the vehicle hits the surface at a moderate angle, one side of the plate will hit the surface first, and the plate will then of course instantly tip so that it's lying flat against the surface -- and only then does the plate come to a stop (with the penetrator being pointed normal to the surface), and the penetrator's own inertia then carries it through the hole and down into the surface. There's a picture of the concept at http://www.nasatech.com/Briefs/Oct98/NPO20295.html . Depending on which Web document you read, the DS-2 penetrators were intended to deal with surface slopes up to 20, 27 or 30 degrees, and angle of attack errors of 9, 10 or 12 degrees -- at which point we're in the range needed for a Europa penetrator.

There is another problem: the possible sheer hardness of Europa's surface. I hadn't appreciated until the meeting just how shallow Europa's regolith layer is thought to be, since the satellite apparently resurfaced itself only about 60 million years ago and has only been re-accumulating impact regolith since then. The regolith is likely to be no more than a meter or so deep anywhere on the surface, and in the freshest parts of the surface -- which are just where we want to land -- it will be a good deal shallower than that. Such areas may even be virtually bare, solid ice -- and ice at Europa's cryogenic temperatures is as hard as concrete.

The DS-2 penetrators were designed to survive crashing into permafrost, but not into solid rock. However, they were also deliberately blunt-tipped -- presumably to minimize their burial distance -- and a sharper-nosed penetrator could be a good deal more durable. One 1979 test in which penetrators were crashed onto solid rocks found that "Only large single rocks greater than 10 times the penetrator diameter caused deflections appreciably greater than 10 degrees...No catastrophic failure of the penetrator occurred during these tests." And a 2004 Sandia Labs document says that "Penetrators have recently been successfully tested into concrete targets at striking velocities approaching the rigid body limit of approximately 4000 ft/sec."

Another problem may be the penetrator's afterbody, containing its relay antenna, which you want to leave on the surface, connected to the penetrator by a cable. In the case of the general DS-2 type design, the afterbody would actually be the cylindrical sleeve through which the penetrator would slide to go into the ground -- probably a short sleeve, wrapping only the penetrator's front end. But if you hit a hard enough surface, the afterbody's going to bounce seriously. However, it might be possible to give it a bottom lip, protruding through the contact plate and equipped with a sharp lip and maybe even short spikes; and make the contact plate itself brittle (like the aeroshells on the DS-2 probes) -- or even make it a radiating array of breakable outrigger rods -- so that the contact plate would shatter on impact and the afterbody would tend to anchor itself into even a hard surface.

Clearly there's a huge swarm of questions and uncertainties about the concept, and it may well turn out to be unworkable. But if the impact problem can be solved, it has very definite benefits. For one thing, it's much lighter than any other possible lander except the bowling ball. (Lucey's lunar penetrators, even including their deorbit and attitude packages, weighed only 135 kg apiece -- and survived crashing into a 3-foot layer of plywood with an onboard mass spectrometer and neutron detector working fine afterwards.) Its biggest selling point, though, is one of the conclusions reached by the Group meeting: it's the only possible concept for a small piggyback Europa Orbiter lander that has any chance of doing meaningful biological studies. Europa's regolith layer is virtually certain to have had any biological remains that it held fried beyond recognition by Jupiter's radiation, and also contaminated by incoming carbonaceous meteoroids. You need to get about a meter below the surface to get good biological samples, and any soft lander that could drill or carry a little short-range cryobot to do that is absolutely certain to weigh far more than the 340 kg which is all the extra payload margin the current design for Europa Orbiter can carry. (Even the frequently mentioned "Europa Pathfinder" concept -- a hockey-puck type rough lander using airbags -- has now been firmly rejected; its airbags, even if their weight alone is raised to 225 kg, are probably too fragile to work on Europa's rough and supercold surface.)

Apparently the only possible alternative to a penetrator for a lander on Europa Orbiter is the "bowling-ball" type lander -- which has viewports for cameras, and even a bunch of little sharp-edge sampling cups mounted on it that could allow it to pick up ice samples on impact for non-biological chemical analyses. But since biology is the raison d'etre for Europa landings, is such a lander even worth the trouble? Or should we instead pour that 240 kg of added orbiting payload into more orbital experiments, a faster data rate, and above all more radiation shielding to allow a longer lifetime for the Orbiter? (The current concept for the Orbiter gives it a minimum of 3 months in Europa orbit -- but for each month longer that you want it to work, about 100 kg more shielding is required.)

Interestingly, there was a growing feeling expressed at the meeting that a small piggyback lander may be worth doing, not so much for science as for engineering data on Europa's surface that will be crucial to design the later big Astrobiology Lander that will be the follow-up to the Europa Orbiter. You want to make damn sure that something that expensive and long-term survives landing. Even given a very high-powered HiRISE-type camera on the Orbiter, that means that you want very close-range descent images (which must be played back after landing) and/or surface images, to gauge the small-scale roughness and feature types of this moon's strange surface. (Lucey's lunar penetrators carried descent cameras.)

You may want (as Ed Strick suggests) a short-lived preliminary seismometer just to get the data on Europa's seismic behavior that will be needed to design a later, better one. You may want to measure the acidity of the ice if it's really high, to make sure that cryobots and ice-analysis gear can survive it. And (as Tom Spilker told me) you may also want to measure the debris content of the ice, to see if there are so many meteorites and other rocks, pebbles and grit in it that a Cryobot would have trouble melting its way through without a grinding head as well. (It looks as though by far the best surface sampler for an Astrobiology Lander will be a short-range cryobot that melts its way 10-100 meters through the surface, which can melt and filter two orders of magnitude more ice than a drill can.) Spilker suggests sonar on a little preliminary lander -- or at least a speed-of-sound experiment, like that on Huygens, to measure the overall rock content of the immediate surface ice that the piggyback lander is sitting on. (You may also want to measure the ice's salt content -- if it's really concentrated, an ice-melting cryobot might eventually build up a lump of concentrated salt ahead of it that would also stop it without a grinding head.) These are the sorts of things that even a tiny hard lander of the Bowling Ball type could get.

Anyway, this is how the Europa Focus Group's overall discussions of a Europa lander seemed to be tilting. (And at least I gave them a little useful input, since I was the one who brought up the penetrator concept to Spilker's subgroup that was discussing the engineering design of a lander, and they were genuinely interested in it and recommended some further inquiries to NASA and JPL about its possible feasibility. They also want to get in touch with Lucey -- although Torrance Johnson remains highly skeptical about the whole idea. One guy named Paul Holland seems to have turned into a flat-out fan of the idea.) Coming up next: my summary of the actual scientific talks at the meeting -- which were a good deal more interesting overall than I'd figured they would be.

[stevesliva]

The latter may be the catch -- you need to cancel out the penetrator's forward orbital speed very accurately, AND point it nose-down with high accuracy.

If at your penetrators' 4km periapse it's connected to a 4km tether with a counterweight that smacks into Europa first, will the penetrator fall straight down? I guess if the tether breaks, you fall up, though. Doh.