Nasa Picks "juno" As Next New Frontiers Mission Options

[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_*]

Actually, there are a hell of a lot of things they very badly want to get more closeup looks at Io for -- and they're listed both in Spencer's white paper ( http://www.lpi.usra.edu/opag/io.pdf ) and in William Smythe's previous essay "Getting Back to Io", which can be found at members.fortunecity.com/volcanopele/gbtio-1.doc .

On rereading both these essays, by the way, I've come to realize that I have very seriously underestimated the potential range of flexibility of orbital design for an Io Observer. Indeed, Spencer's paper actually points out that, if the Observer had the same radiation hardness as Europa Orbiter, it could make 100 Io flybys from an EQUATORIAL orbit, despite the fact that this would maximize its radiation dose on each flyby. So, simply by utilizing the radiation-hardened technology -- and, indeed, the spacecraft design -- of Europa Orbiter (and with much less fuel), we could devise a perfectly good combined Io/Ganymede/Callisto multiple flyby mission -- that is, our yearned-after "Galileo 2". (Indeed, with radiation hardness like that, we could use it to take a few close looks at Amalthea as well.)

[JRehling]

Actually, there are a hell of a lot of things they very badly want to get more closeup looks at Io for -- and they're listed both in Spencer's white paper ( http://www.lpi.usra.edu/opag/io.pdf ) and in William Smythe's previous essay "Getting Back to Io", which can be found at members.fortunecity.com/volcanopele/gbtio-1.doc . 

Indeed. Finding some impact craters is one of the ones that excites me more, because that would provide some sharp-shooting information regarding the cratering rate -- information that could be nigh impossible to find elsewhere in the jovian system.

On rereading both these essays, by the way, I've come to realize that I have very seriously underestimated the potential range of flexibility of orbital design for an Io Observer.  Indeed, Spencer's paper actually points out that, if the Observer had the same radiation hardness as Europa Orbiter, it could make 100 Io flybys from an EQUATORIAL orbit, despite the fact that this would maximize its radiation dose on each flyby.  So, simply by utilizing the radiation-hardened technology -- and, indeed, the spacecraft design -- of Europa Orbiter (and with much less fuel), we could devise a perfectly good combined Io/Ganymede/Callisto multiple flyby mission -- that is, our yearned-after "Galileo 2".  (Indeed, with radiation hardness like that, we could use it to take a few close looks at Amalthea as well.)

I'm still enamored of the orbit that makes use of two Io intercepts. If they were 180 degrees apart, then flybys could alternate in terms of geometry/lighting, and 50 "survived" perijoves would mean 25 looks at each hemisphere. Europa and Ganymede would cycle through 4 and 8 relative positions when the craft crossed their orbital radii, and that might mean regular flybys in fixed lighting conditions, if dumb luck wills it. If they were 120 degrees apart, then you could set the apojove so that the flybys would cycle through three relative Io positions: one third (~17) each with a flyby at two positions, and a third would have no Io flyby, but would likely enable some nice geometry for Europa/Ganymede (they would cycle through six and twelve positions -- it seems assured that at least four decent flybys of Ganymede, albeit in the same lighting condition, would take place).

I am ignoring the fact that the Galileans can yank a craft's orbit around. Those nudges could either be utilized to willfully vary trajectory geometry or could alternately be overcome with a little propulsion...

Indeed, if Europa Orbiter has a Galileo-2-ish midgame and an Io Observer had some of that worked into its primary mission, there'd be not much need for a dedicated-to-general-purpose Galilean/Jupiter craft in jovian orbit anytime soon. I suspect that, all things being equal, Io would end up trumping Ganymede/Callisto as a single priority, although the latter and larger pair would get the best coverage from any orbiter that was not specifically intended to be Io-devoted.

[Guest_vjkane2000_*]

Indeed. Finding some impact craters is one of the ones that excites me more, because that would provide some sharp-shooting information regarding the cratering rate -- information that could be nigh impossible to find elsewhere in the jovian system.
  I'm still enamored of the orbit that makes use of two Io intercepts. If they were 180 degrees apart, then flybys could alternate in terms of geometry/lighting, and 50 "survived" perijoves would mean 25 looks at each hemisphere. Europa and Ganymede would cycle through 4 and 8 relative positions when the craft crossed their orbital radii, and that might mean regular flybys in fixed lighting conditions, if dumb luck wills it. If they were 120 degrees apart, then you could set the apojove so that the flybys would cycle through three relative Io positions: one third (~17) each with a flyby at two positions, and a third would have no Io flyby, but would likely enable some nice geometry for Europa/Ganymede (they would cycle through six and twelve positions -- it seems assured that at least four decent flybys of Ganymede, albeit in the same lighting condition, would take place).

  I am ignoring the fact that the Galileans can yank a craft's orbit around. Those nudges could either be utilized to willfully vary trajectory geometry or could alternately be overcome with a little propulsion...

  Indeed, if Europa Orbiter has a Galileo-2-ish midgame and an Io Observer had some of that worked into its primary mission, there'd be not much need for a dedicated-to-general-purpose Galilean/Jupiter craft in jovian orbit anytime soon. I suspect that, all things being equal, Io would end up trumping Ganymede/Callisto as a single priority, although the latter and larger pair would get the best coverage from any orbiter that was not specifically intended to be Io-devoted.

I would love a mission that does everything you say (alternating encounter points, etc), but I worry about the orbital mechanics. Either each flyby would have to use a gravity assist to tweak the timing of the next enounter, or fuel would have to be burned to do so. This would be complicated by the fact that at least some of the encounters would have to use gravity assists to walk the orbit so that the movement of Jupiter (taking Io with it) doesn't move the encounter point.

The alternative idea of using polar orbits has its own problems. Roughly 1/3 of the surface below the encounter point can be imaged well. If you come in at Io's poles, half of this area is in darkness (although you should be able to pick up 1/2 of the other pole on the way out). In theory, you could just pass by Io on the sunward side, say 10,000 km out, and see the poles and the sunlit side. Unfortuantely, you then lose the gravity assists that allow you to walk the orbit to counter Jupiter's movement around the sun. Not too long after you entered orbit, you'd be passing on the dark side of Io.

Anyone know enough about celestrial billards (ie, using gravity assists) to know if these problems can be reasonably handled?