NASA Dawn Asteroid Mission Told to "Stand Back Up", Reinstated! |
NASA Dawn Asteroid Mission Told to "Stand Back Up", Reinstated! |
Mar 28 2006, 07:58 AM
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Senior Member Group: Members Posts: 3419 Joined: 9-February 04 From: Minneapolis, MN, USA Member No.: 15 |
Just 'cause I said I would...
Hopefully, though, this whole episode has made its point -- NASA isn't afraid to tell overbudget missions to stand down. I just *really* wish we could get the magnetometer back on the beastie, though... -the other Doug -------------------- “The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Guest_BruceMoomaw_* |
Apr 2 2006, 08:13 PM
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Guests |
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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Apr 2 2006, 08:57 PM
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Senior Member Group: Members Posts: 2488 Joined: 17-April 05 From: Glasgow, Scotland, UK Member No.: 239 |
Bruce:
'Harder metal cores?' That's an interesting one. If the proto-asteroids were differentiated, presumably by virtue of the radiactive decay of Al, then would hot cores be more or less likely to be disrupted than undifferentiated bodies after big impacts? Similarly, if the early asteroid belt saw many impacts, would that not mean that some bodies might still have been warm after the last whack when they ran into something else? Obviously, these days, a monolithic lump of cold nickel-iron (or some variant thereof) is going to be hugely more robust than a rubble pile - but what about *warm* objects? And that's got me wondering about the effect of T-Tauri solar winds on both icy and rocky/metallic bodies... ...induced currents in rotating irons, leading to poor structural cohesion, maybe? Bob Shaw -------------------- Remember: Time Flies like the wind - but Fruit Flies like bananas!
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