Turns out I misread that white paper -- Europa Orbiter's planned level of radiation hardness would allow an Io Observer to make not 50 Io flybys, but 100.

Cost is, of course, a major issue for any Jupiter mission. The Io mission I described isn't much less complicated than Juno, but also not much more complicated than Messenger or Dawn. With the new spending limits on Discovery and the lack of good missions closer in, I think we'll see more Jovian missions proposed for the discovery program.

A difficulty with eccentric orbit orbiters that zoom in to observe inner satellites at periapsis only is that if the mission is not very long, you observe only a limited range of lighting conditions, and with tidally-locked rotations, you observe a limited range of longitudes. This is why we've seen one side of Io much better than the other (from Galileo) and keep seeing similar longitudes of Titan -- even as late as next spring, we still won't have decent coverage of all Titan longitudes, despite many flybys.

If an orbiter could go into an eccentric polar orbit around Jupiter, it might be able to observe Io (or Europa) both coming and going (when the satellite is nearer to the Sun than Jupiter, or farther). But that's not going to be cheap in delta-v.

I'm not sure what tradeoffs are possible, but getting multiple close flybys of the same hemisphere doesn't get you a lot that a single flyby doesn't. (See Mariner 10.) Of course, the long-duration observations from farther out are all fine and good.

I'll chime in that the capability for continuous observation of Io, at least to the point of observing the several larger hotspots, from Earth had gotten pretty good, although I'm not aware of the details of any such surveys that may be ongoing.

The Io working group specifically said they wanted to repeat the encounter with the same geometry so that any changes between passes represent real changes and not just lighting changes.

However, if the perijove was inside Io's orbit, then the spacecraft's orbit crosses Io's orbit in two locations. It would be possible to vary the orbital timing slightly to alternative which crossing you encounter Io at. You can't get exactly opposite hemispheres, but you do increase your coverage. The downside, however, is that you are going deeper into the radiation belts and increasing the radiation dosage of each orbit. I don't know if the 40Krad Galileo dosages per orbit included going inside Io's orbit or not.

Thanks for the link, Bruce. Interesting that Danztler considers the MSL 2009 launch date "FIXED."

When he is saying the Science Mission Directorate budget is being "rebalanced," what is the implication? Does this point to a reduction in the science portion of Mars exploration?

Damned if I know, especially with this president -- who may hold out stubbornly for continuing to have the manned program drain off money from NASA's science program like Dracula. The one upcoming change I'm virtually certain of is that Europa Orbiter will be funded in 2007. But, looking at the current science budget and the retinkerings with it that Griffin has done just in the last month, I would be inclined to suspect some money will indeed be drained out of the Mars program -- and also that the Webb Telescope will be substantially delayed both to make sure that when it does fly it will still be big enough to do a proper science job, and to make sure that there will be enough money for the Hubble repair-or-reflight and to reduce the rapidly growing new delays in the two big extrasolar-planets missions (SIM and TPF-C).

(Once again, all these problems could be radically reduced if Griffin decided to go for a Hubble unmanned reflight rather than a Shuttle repair mission -- since such a reflight could be delayed as long as necessary, and would probably cost a little less into the bargain -- but, ah, that would once again expose the manned space program as a literally total fraud, and we can't have that. Too many incumbent politicians' and bureaucrats' reputations hinge on not admitting that they made a huge mistake, which is also why the Iraq War will drag on for a couple more years before we admit that we've lost it. But I digress.)

Bruce:

Are you talking about the Hubble II complete new vehicle idea, rather than a repair? The arithmetic is impossible to argue with (which is presumably why the DoD do it that way for their spy satellites).

It still leaves the vexed question of what to do with the original Hubble - unmanned rendezvous appears still to be a bit of a black art in the US (perhaps subcontracting the de-orbit to those guys with the Ariane mini space-tug would be a better idea, or even finding out how NPO-Energia would feel about selling a Progress de-orbit mission to NASA (bet they'd *love* to sort out NASA'a pride and joy after being 'persuaded' to dump their good ol' Mir in the ocean!)). Of course, you could always stick some gyros, comms and solar cells on a PIRS airlock module atop a Progress bus, boost the whole shooting match a couple of hundred miles higher and wait for Burt Rutan...

Bob Shaw

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.

"Are you talking about the Hubble II complete new vehicle idea, rather than a repair? The arithmetic is impossible to argue with (which is presumably why the DoD do it that way for their spy satellites)."

Oh, yes (although the DoD does have another reason adequate by itself, namely that they can't launch Shuttles into polar orbit). And the design for Hubble II is already full-ripe: see http://www.pha.jhu.edu/hop/ . The Aerospace Corporation study commissioned by NASA itself to compare the the three ways to deal with the Hubble problem ( http://www.npr.org/programs/morning/featur...bble/hubble.pdf ) concluded that HOP was not only as cheap as a Shuttle repair mission, but about 20% more likely to succeed (and without risking any lives) -- and, as you'll see when you look at HOP's design, it would carry, in addition to the two new instruments planned for Hubble, at least one and maybe two radically new instruments giving it greatly expanded capabilities beyond Hubble. Two days later, the National Academy of Sciences muddied the waters by stating in its own report that a Shuttle repair mission would be better -- but their reasons for doing so are incredibly vague; indeed, they only mentioned the reflight possibility at all in one short paragraph on one page! ( http://www.nap.edu/catalog/11169.html ; pg. 39). And my attempts to extract more information from the N.A.S. on why they made that statement have been totally unsuccessful.

I imagine, though, that we'll see the Shuttle repair mission instead -- after all, NASA, like any con artist finally backed into a corner, is frantically grabbing at anything they can, and the supposed "usefulness" of a Shuttle repair mission to Hubble is the only argument they have left for that miserable white elephant that still has an ability to take in otherwise knowledgeable skeptics (such as Paul Krugman).

"It still leaves the vexed question of what to do with the original Hubble - unmanned rendezvous appears still to be a bit of a black art in the US (perhaps subcontracting the de-orbit to those guys with the Ariane mini space-tug would be a better idea, or even finding out how NPO-Energia would feel about selling a Progress de-orbit mission to NASA (bet they'd *love* to sort out NASA'a pride and joy after being 'persuaded' to dump their good ol' Mir in the ocean!)). Of course, you could always stick some gyros, comms and solar cells on a PIRS airlock module atop a Progress bus, boost the whole shooting match a couple of hundred miles higher and wait for Burt Rutan..."

Once again: since they can't possibly cram a Deorbit Motor into the Shuttle cargo bay along with the other equipment they'd need for that repair mission, that will be separately built and robotically launched to Hubble in any case (for about $400 million). Orbital Services is absolutely confident that they can build that simpler mission, and succesfully dock it with Hubble. Of course, a few heretics like Jeffrey Bell are also asking why we need to spend $400 million to eliminate the miniscule chance that any piece of Hubble would hit anyone on the head in any case...

[quote=BruceMoomaw,Jun 19 2005, 03:58 PM]
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
[/quote]

Imagine the way Halley's Comet's orbit intersects the Earth's (assuming the two orbits were nearly coplanar). They intersect in two places. On some passes, a close encounter will happen at one intersection, on some, at the other. On many, no encounter at all. Of course, there's no design behind the natural case. Clever choice of orbital period could allow close encounters to occur at both locations, on different orbits.

[quote]
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
[/unquote]

If Io were only to be observed from afar, then a perijove near Ganymede would suffice save a whole lot of delta-v.

[quote]
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.

[/quote]

Interesting, and Saturn would probably serve much the same scientific purposes, if the two giants ended up with similar ratios of raw material.

(1) "Imagine the way Halley's Comet's orbit intersects the Earth's (assuming the two orbits were nearly coplanar). They intersect in two places. On some passes, a close encounter will happen at one intersection, on some, at the other. On many, no encounter at all. Of course, there's no design behind the natural case. Clever choice of orbital period could allow close encounters to occur at both locations, on different orbits."

Ah, I see -- you're assuming that most of the Observer's orbit would be OUTSIDE Io's orbit, whereas I was assuming that it would be in a polar orbit INSIDE Io's orbit (like Juno) and intercept Io only at each of its apoapses. Put it in a polar orbit mostly outside Io's orbital distance -- and put its periapse at one of Jupiter's poles, so that it can intercept Io either while going inward or coming back out -- and you can indeed have it intercept Io's orbit at two different spots. I need to talk with Spencer and Smythe as to whether this is the sort of orbit they might have in mind for the Observer. (I lack a good visual imagination when it comes to orbits -- or, indeed, anything else.)

By the way, one fact I forgot to mention: in response to Van Kane's other question, Galileo only ventured slightly inside Io's orbit during each of its Io flyby orbits.

(2) "If Io were only to be observed from afar, then a perijove near Ganymede would suffice to save a whole lot of delta-v."

True -- but any orbiter can do that (Europa Orbiter will regularly do it from as close as Europa's orbit). The central goal of Io Observer will be to take repeated close-up looks at Io -- so the problem, once again, is whether there's any practical way to combine its mission with that of the Ganymede-Callisto Observer (hereafter called, by me, the GC Observer).

(3) "Interesting, and Saturn would probably serve much the same scientific purposes, if the two giants ended up with similar ratios of raw material."

This is going back a ways; but, given the difficulty of a Jupiter entry, back in the early 1970s serious consideration was given to making Saturn, rather than Jupiter, the target of the first giant-planet entry probe. (Who knows? We might have ended up sending Galileo to Saturn instead, and then ended up bitching even more about the consequences of its high-gain antenna failure.)

I've always thought that the best deorbit mission for Hubble is a $20 million check dispatched by the USPS to Lloyds of London....

Delta-v could vary sharply from one of these possibilities to another, and in the absence of some mission-critical geometry need, it would, I expect, be a powerful factor. Orbiting close to Jupiter costs delta-v. Inclined orbits cost delta-v. I guess that something in a mainly-outside, coplanar orbit would win in every way but one, which is that it would probably take more radiation/pass, and thus live a shorter life. But if the point is to have a long duration of temporal coverage, a longer orbit with increased apojove would provide that, at the cost of time resolution (more flybys).



I've looked at some Cassini images of saturnian moons at typical Ganymede-to-Io distances, and the resolution is really not bad. Europa-to-Io would be better. What is the point of such a mission: to track lava flows at decameter resolutions?

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.)

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.