[X] My Assistant

[Guest_BruceMoomaw_*]

It's probably something of a gamble, involving a last-minute course correction utilizing long-range photos of the nucleus' orientation just before the flyby. (You'll notice that the New Scientist article says that just observing the changes in surface features since the DI flyby would be useful as well. Also, of course, we'd get some dust compositional data to go with that for Halley and Wild 2.)

[Phil Stooke]

Hmmm... if the flyby is at a typical speed for such things there would be no chance to delay the closest approach a few hours to allow the crater to rotate into sunlight. That's what I'm chiefly questioning - the kind of last-minute redirection Bruce mentions would allow only a small positional change in the flyby point, but not the timing. If Stardust - sorry, Scarquest (isn't that a diet?) can turn and view the departing side we might get about 50% coverage, as at Wild-2, but that gives us a 50% chance of not seeing the crater.

Phil

[Guest_AlexBlackwell_*]

Hmmm... if the flyby is at a typical speed for such things there would be no chance to delay the closest approach a few hours to allow the crater to rotate into sunlight. That's what I'm chiefly questioning - the kind of last-minute redirection Bruce mentions would allow only a small positional change in the flyby point, but not the timing. If Stardust - sorry, Scarquest (isn't that a diet?) can turn and view the departing side we might get about 50% coverage, as at Wild-2, but that gives us a 50% chance of not seeing the crater.

Phil

This is a good question for both the Discovery 2006 science and TMC review panels to "sanity check." I could easily see the ScarQuest science being a little seductive, especially given that it would be "re-using" a spacecraft.

[Phil Stooke]

Well, I can answer my own question now, because I just had lunch with Don Brownlee and asked him this very question. It turns out that they do indeed think they know the rotation rate of the nucleus well enough to predict where the crater will be at flyby time, and they plan to image some new territory plus see the crater clearly.

I'm surprised about the presumption that they know the rotation rate so well, but I guess I have to accept that.

Phil

[ugordan]

I'm surprised about the presumption that they know the rotation rate so well, but I guess I have to accept that.

Perhaps it's got to do with the Deep Impact observations well before the flyby so they got good light curves. Still, I remain skeptical as you the rotation can be nailed down in such a short observational span.
Any uncertainties now will lead to large rotational offsets 4 years in the future.

[tty]

You would only have to know the rotation period with enough precision to know that the crater is somewhere on the lighted side at flyby time. Tweaking the orbit a little "sideways" to pass over either the "morning", "midday" or "evening" side of the comet could be made quite late based on images taken during the approach.

tty

[Phil Stooke]

Well, tty, your first sentence is absolutely correct - it's just that I'm surprised they know the rotation period that well. But your second is more doubtful, I think. These flybys are so fast that there's no time to get early images, figure out where the crater is, and adjust the flyby position. Their only hope would be to plan a Wild-2 type flyby, on the sunlit side, turning to view both approach and departure sides so the entire sunlit surface is seen.

Anyway, it sounds as if they are reasonably confident that they can see the crater. And from my point of view, if I can see some more of the surface too, I'll be quite happy.

Phil

[Bob Shaw]

Report on Space Daily regarding an article in Science (23 March 2006) on 'main belt' comets - objects in asteroid-like orbits but with comet-like characteristics.

http://www.spacedaily.com/reports/Main_Bel...rths_Water.html

Bob Shaw