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NASA Dawn Asteroid Mission Told to "Stand Back Up", Reinstated!
Guest_Myran_*
post Mar 30 2006, 06:36 PM
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JRehling wrote: The total mass of the main belt asteroids is less than the Moon,


Thats correct, though the asteroid belt could have been more massive in the past with many bodies ejected, perturbed by Jupiter or broken up from collsions the ejecta in chaotic orbits etc. But I dont have any impression theres any consensus on how much more massive it might have been. But feel free to correct me on that if im mistaken.

Yes Vesta are interesting since it is special in more than one way, yes it have taken a huge impact, and spectroscopy hints it have had some volcanism so its should have undergone differentiation.

But you cut to the core (oops) of what I was thinking there, the question if most meteorites comes from one or few bodies. Or if they are offsprings of a wide range of parent bodies.
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tedstryk
post Mar 30 2006, 08:39 PM
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I think that it depends on how one defines samples...if you just want a main belt asteroid, we have done that. And heck, if we operate on that level, Pluto, even with its new neighbors in the outer solar system, would round out our exploration. But in reality a much broader sampling of worlds is needed.


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JRehling
post Mar 30 2006, 11:39 PM
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QUOTE (Myran @ Mar 30 2006, 10:36 AM) *
But you cut to the core (oops) of what I was thinking there, the question if most meteorites comes from one or few bodies. Or if they are offsprings of a wide range of parent bodies.


It seems that Vesta is one proto-planet that survived (mainly) and Ceres, which is considerably bigger, is one proto-planet that never lost too much matter to impact. Pallas is another. Those three were never all-in-one, so there was no "Pangaea" of the asteroid belt, and the number of parent-worthy bodies (Ceres and Pallas having never been seriously fissioned) must have been at least four or five. The question is how many others were there? Was five the grand total? Or was it twelve? I was wondering if meteoritics had constrained this, but no one is piping up.

I would think that isotopic analysis might cluster the massive collection of meteorites into a few groups, but I could easily imagine that this hope would be dashed by reality. Just wondering if anyone had pointers to results along these lines.
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Mariner9
post Mar 31 2006, 03:47 PM
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I remember in one of the earlier Discovery mission proposals I read about a mission similar to CONTOUR which would flyby multiple asteroids. It wouldn't seem all that cost effective to only fly by 2-3 asteroids, but I know that in the early days of DAWN they were suggesting that they could fly past as many as 10-12 targets on the way there. I always found that number as overly optimistic, Dawn will have enough on it's plate just getting there, so if they manage 1-2 extra flybys it would be impressive enough. Something I read recently suggested they were now using lower numbers whenever the topic came up.

But has anyone given any serious consideration to taking a Dawn type ion propelled spacecraft and using all of the delta-vee specifically for as many fly-bys as possible? I would think with a couple Earth, Venus, or Mars flybys thrown in to make the orbit more elipitcal, they could potentially survey a very large number of targets.

It occurs to me that the instrumentation on DAWN might not be the best for fast flybys, but with the right package this might yield a lot more targets and results than a simple ballistic probe like CONTOUR. And that might make it worth it.

All of this is assuming that the Discovery office at NASA isn't really gun shy about ion drive missions right now.
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JRehling
post Mar 31 2006, 05:47 PM
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QUOTE (Mariner9 @ Mar 31 2006, 07:47 AM) *
But has anyone given any serious consideration to taking a Dawn type ion propelled spacecraft and using all of the delta-vee specifically for as many fly-bys as possible? I would think with a couple Earth, Venus, or Mars flybys thrown in to make the orbit more elipitcal, they could potentially survey a very large number of targets.


Yeah, if you pick targets according to trajectories that net the highest sheer number, without any particular targets offering constraints, I would expect some fantastic possibilities.

Thinking outside the box, I wonder about using a Jupiter gravity assist to create a retrograde orbit that could be later circularized or near-circularized with propulsion and/or Earth gravity assists. It seems to me that flying backwards through the asteroid belt would very much increase the number of flyby possibilities, the same way that you would pass close to a LOT more cars on the highway driving 70 mph against traffic than you would driving 90 mph with traffic. In fact, this would mean no propellant would be needed to catch up to asteroids, or to lag behind them, and all of the propellant (once you achieve that orbit) could be used in "lateral" motion to create flybys. With 10,000 targets, and a full lap relative to the field taking place in about 2.5 years, the craft in retrograde orbit would fly by the radial vector of another asteroid every 2.5 hours! Assuming the asteroid belt is 1 AU wide, totally planar and with uniform distribution of the asteroids within it, the craft would fly within 0.001 AU (149,000 km) of an asteroid about three times a year even if you did nothing to aim for any targets! I would think that a campaign of lateral manuevers aiming for targets well in advance could lead to a mission ultimately achieving hundreds of flybys in a main mission of ten years.

Feasible?
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Guest_BruceMoomaw_*
post Mar 31 2006, 08:12 PM
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It's certainly an idea. One might even combine it with a few small passive impactors and a flash spectrometer on the main craft to try to get element measurements during the flyby (which, unfortunately, are just about impossible to get any other way during flybys -- gamma rays are inherently too weak for the purpose without an orbiter, and out in the Belt solar radiation is too weak to generate adequate fluorescent X-rays for meaningful element data during a flyby).
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Bob Shaw
post Mar 31 2006, 10:57 PM
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QUOTE (JRehling @ Mar 31 2006, 06:47 PM) *
Thinking outside the box, I wonder about using a Jupiter gravity assist to create a retrograde orbit that could be later circularized or near-circularized with propulsion and/or Earth gravity assists.

Feasible?


Now *that* is a really clever idea!

Simple, cheap, and (almost) free.

Any serious orbital dynamics guys out there? Or girls, I care not which!

Bob Shaw


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David
post Apr 1 2006, 12:06 AM
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Not me, but one thing occurs to me: aren't these byflights going to be awfully fast?
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Guest_BruceMoomaw_*
post Apr 1 2006, 12:16 AM
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Oh, yes. Might be worth doing anyway, though, if you can't get element measurements even during regular slower prograde flybys. By contrast, near-IR mineral spectrometers (along with cameras) could probably get good mineralogical data even during such a high speed flyby, and as John says you might get a much larger number of them during the probe's operating lifetime.

Perhaps the most important question is the extent to which near-Earth asteroids (which are so much more easily accessible and samplable) provide an adequate sample of the different types of Main Belt asteroids. They seem to be diverted from the Main Belt into the inner System only when they wander into one of the narrow zones in which Jovian gravitational resonances divert them inward -- but the Yarkovsky Effect seems to have moved a substantial number of the smaller asteroids (and their meteoric fragments) inward or outward from their original orbits over the Solar System's lifetime, and so has presumably provided these "escape hatches" from the Belt with a hefty shovelful of asteroid samples from all sorts of different zones in the Main Belt. There is apparently still some debate over just how well Yarkovsky works, though -- and it does not work on significantly larger asteroids.
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JRehling
post Apr 1 2006, 12:32 AM
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QUOTE (David @ Mar 31 2006, 04:06 PM) *
Not me, but one thing occurs to me: aren't these byflights going to be awfully fast?


Yeah. About 50 km/s, which is about 3 times the relative velocity of, say, New Horizons at Pluto. But these worlds are pretty small, too, so you could aim for a daytime-side flyby, and reliably get a good "full asteroid" view during the minute or so that you have to squeeze off some multicolor frames. Looks like short exposure time, probably a pushbroom design, etc... we see some mission design specs that the high speed would require. Operational precision would be required. All told, though, if you're getting hundreds of flybys and a few are botched, you're still doing pretty well.
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Guest_BruceMoomaw_*
post Apr 1 2006, 12:48 AM
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QUOTE (The Messenger @ Mar 29 2006, 04:08 PM) *
Can somebody provide me with a good reference on [Dawn's] scientific objectives and why-fors? I don't dare Google "Dawn Love Numbers" wink.gif


One of the references Alex provided -- "Dawn: A Journey in Space and Time" -- is absolutely terrific for that purpose ( http://www-geodyn.mit.edu/russell.dawn_space_time.pdf ). It's dated mid-2003, but is still up to date in practically every respect. (It was written at a time when Dawn still had its magnetometer, and of course the launch date has since been changed, presumably with detailed effects on the schedule and duration of orbital activities around Vesta and Ceres.) Alas, there is no reference to "Love numbers". I'm currently trying to accumulate information on just how likely it is that Ceres might have a subsurface liquid-water ocean, given that it undergoes almost no tidal heating. Apparently McCord and Sotin did a piece in the May 2005 "JGR-Planets" ( http://www.agu.org/pubs/crossref/2005/2004JE002244.shtml ) concluding that the heat from the radioisotopes in Ceres' rocky core were probably adequate to keep some of its ice layer liquid for about 2 billion years -- and if Ceres also has a significant amount of ammonia in its interior (there's one extremely tentative hint of possible ammoniated clays in its near-IR surface spectra), they say it might have a liquid-water layer to this day, but that the odds are against it.

Two notes about Russell's article:

(1) The table on page 41 lists some possible asteroid flyby targets for Dawn's mission as it was then planned. Three of them are tiny things only a few km across; the fourth was 197 Arete, 30 km wide. The distances by which Dawn would have missed them if it expended none of its ion-drive fuel for minor course corrections to make close flybys of them are listed in "Gm" -- presumably "gigameters", or "millions of km". But page 12 notes: "Because of the efficiency of SEP, the number of candidate flyby targets is expected to be large, with many opportunities for low-velocity encounters. Groundbased observations of these targets will be critical to a selection of targets on the basis of science return as opposed to simple dynamical convenience." Even with Dawn's fuel load reduced, we can still hope for a fair number of such flybys.

(2) The minimum "performance floor" version of the Dawn mission (pg. 13-14) called for it to orbit Vesta, but just to fly by one hydrated C-type asteroid similar in overall composition to Ceres -- perhaps the 100-km wide "50 Virginia". This confirms something I said earlier: where asteroids are concerned, size matters a lot less than composition. During the quarter-century in which I've been reading proposals for asteroid exploration, Vesta has always been right at the top of every list because of its unique composition, which seems to be right at the far end of the spectrum of asteroid differentiation levels. But, until Dawn came along, I had NEVER seen Ceres listed as any kind of high- or even medium-priority target, despite the fact that it's the unquestioned King of the Asteroid Belt -- the reason being simply that, compositionally, it was regarded as just one of a fairly large number of hydrated C-type asteroids. It was starting to look as though, by 2020, Ceres would be by far the biggest object in the Solar System anywhere this side of the Kuiper Belt to be unreconnoitered.
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Guest_AlexBlackwell_*
post Apr 1 2006, 01:36 AM
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QUOTE (BruceMoomaw @ Apr 1 2006, 12:48 AM) *
One of the references Alex provided -- "Dawn: A Journey in Space and Time" -- is absolutely terrific for that purpose ( http://www-geodyn.mit.edu/russell.dawn_space_time.pdf ). It's dated mid-2003, but is still up to date in practically every respect. (It was written at a time when Dawn still had its magnetometer, and of course the launch date has since been changed, presumably with detailed effects on the schedule and duration of orbital activities around Vesta and Ceres.)

Actually, after having read it, I realize this particular link is a preprint/postprint of the Planetary and Space Science paper.
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Guest_BruceMoomaw_*
post Apr 1 2006, 02:22 AM
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I've considered trying to look into the question of what really lay behind the reinstatement of Dawn -- but the more I think about it, the more such a project strikes me as an exercise in futility. Every single person I talked to would be cheerfully willing to lie for their own reasons about what actually went on, and -- not having access to any instruments of torture -- there would be no conceivable reliable source I could track down on what really happened.
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tedstryk
post Apr 2 2006, 04:53 PM
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QUOTE (BruceMoomaw @ Apr 1 2006, 12:48 AM) *
But, until Dawn came along, I had NEVER seen Ceres listed as any kind of high- or even medium-priority target, despite the fact that it's the unquestioned King of the Asteroid Belt -- the reason being simply that, compositionally, it was regarded as just one of a fairly large number of hydrated C-type asteroids. It was starting to look as though, by 2020, Ceres would be by far the biggest object in the Solar System anywhere this side of the Kuiper Belt to be unreconnoitered.


I think the recent HST data changes a lot of that. It is the only main belt asteroid that, were it a moon of one of the outer planets, not be one of the "irregulars." Thus, there is the possibility that it is differentiated, which would also be unique - asteroids like Vesta might be fragments of what used to be a larger world, but Ceres seems to be more than a fragment.


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Guest_BruceMoomaw_*
post Apr 2 2006, 08:13 PM
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That, in turn returns us to the question I mentioned a bit earlier: to what extent have all the asteroids, including the biggest ones, been busted up by impacts? Especially during the Belt's earliest days, when it is generally thought to have had MUCH more material in it -- perhaps including dozens of large protoplanetary "embryos", some of them perhaps as big as the Moon or Mars? Most of that material was eventually completely ejected from the Belt by Jupiter's gravitational perturbations, but in the meantime the situation there must have been wild indeed.

The trouble is that the evidence seems contradictory. The existence of a fair number of M-type asteroids -- some of which still appear to be iron-nickel, even in the latest revised near-IR compositional studies -- suggests that they were created from small, initially differentiated asteroids whose outer silicate mantles were completely stripped away, leaving their harder metal cores. There are also the asteroid "families" whose trajectories clearly show that they are the fragments of a bigger original body that got blasted to hellandgone at some point -- and eight of those families are big enough that they must have come from bodies over 200 km wide. But at the same time Vesta seems mostly covered by a basalt crust -- which must be the product of a particularly strong and complete differentiation process -- and most of that outer crust is still on it, instead of having been blasted away by impacts to expose the asteroid's underlying non-basalt mantle. (There are also the other really big remaining asteroids -- Ceres, Pallas, Hygeia. Did they also avoid a large number of impacts, or were they bashed into rubble but were still big enough that their gravity recompressed them again into solid material and spherical forms? If so, then the original ice in them could have been melted by those impacts, and the large part of it that they retained would then have refrozen again as their upper layer.)

There seem to be at least three different theories to explain this seeming contradiction on the Belt's collisional environment, and I'm currently trying to slog through abstracts and papers on the subject to try and get everyone's views properly sorted out. Once I get this done -- on top of the other things I'm trying to do right now -- I'll have more comments.
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