[X] My Assistant

[Guest_BruceMoomaw_*]

http://www.nasa.gov/home/hqnews/2005/jun/H...rontiers_2.html

Yeah, I know it ain't Saturn, but we don't seem to have any proper slot for Jovian news -- including yesterday's totally unexpected announcement that Amalthea's density is so low as to suggest that it's a highly porous ice object; maybe a captured Kuiper Belt Object reduced to rubble by infalling meteoroids. As Jason Perry says, this might explain those previously mysterious light-colored patches on Amalthea -- they may be its underlying ice, exposed by impacts that punched through the layer of sulfur spray-painted onto it by Io.

Scott Bolton has been pretty talkative to me already about the design of Juno. It certainly won't be as good in the PR department as Galileo or Cassini, but it DOES carry a camera -- as much for PR as for Jovian cloud science, according to Bolton. And since the latitude of periapsis of its highly elliptical orbit will change radically during the primary mission, I wonder if they might be able to set up at least one close photographic flyby of Io and/or Amalthea? (I believe, by the way, that this selection is a bit ahead of schedule -- and it certainly indicates that NASA's science program under Griffin won't be a complete slave to Bush's Moon-Mars initiative.)

[Guest_BruceMoomaw_*]

As for Van Kane's comments on the "Io Observer": I don't understand it could flyby Io at two different positions in its orbit if its periapse was lower than Io's -- the only way it could pull that trick is if it had a circular orbit IDENTICAL to Io's except for being highly inclined (a trick which Cassini, in fact, will pull with Titan later in its primary mission, but which would probably sharply increase the amount of radiation the Io Observer would be exposed to). And, indeed, the description of it in the White Paper says flatly that it would continue reexaming regions on the same hemisphere of Io close-up over and over, at intervals of a month or less, to look for detailed changes. That mission by itself, however, could do a hell of a lot scientifically.

The really interesting question is whether you could combine this with another New Frontiers mission that the Decadal Survey and Solar System Roadmap groups have expressed interest in: a "Ganymede Observer" (actually, at this point a Ganymede-Callisto Observer) which would emulate Galileo by making repeated flybys of those two moons. The question is whether you CAN combine these two mission, to thus create a "Galileo 2" which could observe the entire Jovian system except for Europa in more detail -- the problem is that to get enough radiation resistance for Io Observer you probably need a polar orbit, which in turn means that you'd have to have an awful lot of delta-V to change the orbit's apoapse to make later flybys of Ganymede and Callisto. You MIGHT be able to pul it off with some particular clever gravity-assist flybys of the various moons, but the last time I brought it up this question was still wide open.

On another subject: there are really serious mass problems with flying a Jupiter Multiple Entry Probe mission, even separately from Juno (e.g., with the probes dropped off by a flyby). Given the phenomenal difficulty of Jupiter entry, they would all need big honking heat shields (and even all the companies which manufacture the substance used in the Galileo probe's heat shield have long since stopped doing so and would have to radically retool!). And when you add that to their need for a stout pressure hull (unlike the vented Galileo probe) to get down to the 100-bar level as they wish, that's a lot of mass. Add that to the fact that their main purpose was to map water, ammonia and H2S and winds at different locations and deeper depths -- and that Juno's orbital instruments, by itself, will do a lot of that -- and I think that the Galileo probe and Juno together have removed a lot of the justification for any more near-term Jupiter entry probes.

Now, the heat shield mass needed to enter any of the other three giant planets (including Saturn) is MUCH smaller -- and in the case of Uranus and Neptune, even at the great depths you want the probes to get down to (hopefully, as much as 1000 bars to reach the water cloud), temperatures are no problem at all and you could probably make the probe vented in structure, as both the Galileo probe and the deepest robotic submersibles on Earth are (although a strong radio data signal from that depth is a problem for any entry probe). For these reasons, I suspect that we're going to see deep entry probes for at least one of the other giant planets -- maybe even all of them -- before we see another Jupiter entry probe.