[X] My Assistant

[Guest_Analyst_*]

Not every mission is useful only because there are no other missions. Why do you need a flyby spacecraft and not only the impactor, if you have the Rosetta orbiter with 11 instruments watching? As for the extended missions: Both spacecraft have degraded cameras and only one or two other instruments. It is a matter of additional costs: 20 or 30 million are probably o.k., more is not.

Analyst

[tedstryk]

Not every mission is useful only because there are no other missions. Why do you need a flyby spacecraft and not only the impactor, if you have the Rosetta orbiter with 11 instruments watching? As for the extended missions: Both spacecraft have degraded cameras and only one or two other instruments. It is a matter of additional costs: 20 or 30 million are probably o.k., more is not.

Analyst

Well, two angles are better than one. And the impactor will likely need a bus to get there anyway. And Rosetta has a great suite of instruments, but it is by no means comprehensive.

[Guest_BruceMoomaw_*]

Actually, Mars Loon was completely correct in pointing out something that I had stupidly overlooked: the value of a repeat of Deep Impact is massively increased if it produces a fresh crater that Rosetta can examine close up. The impact had better be carried out considerably before Rosetta's arrival, though, or the dust cloud thrown out by it is likely to be disastrous to Rosetta.

[Bob Shaw]

Why not use the Deep Impact bus as an impactor? Then Rosetta can look at the hole it makes... ...assuming that DI could make it at all to Rosetta's target, that is!

Bob Shaw

[Comga]

Why not use the Deep Impact bus as an impactor? Then Rosetta can look at the hole it makes... ...assuming that DI could make it at all to Rosetta's target, that is!

Bob Shaw

It can't. The orbit is all wrong. Also, I believe the targeting software was in the Impactor, which was designed to point its camera right at the target. Besides, while it is a sturdy and healthy craft, it would have to survive for another eight years for Rosetta to observe the impact.

Besides, a large fraction (50%?) of the science return was said to have come from the pre-impact images. The Flyby spacecraft can do this again, at Comet Boethin, and such an extended mission has been proposed.

I respectfully disagree with Bruce on impacting Comet C-G before Rosetta arrives. If they were really worried about the dust, they could always "hide" behind the nucleus. More likely, they would just sit back a good safe distance and still be able to see. It would be interesting to see an estimation of the dust flux as a function of distance, assuming this nucleus is as dusty as Tempel 1.

[mars loon]

It can't. The orbit is all wrong. Also, I believe the targeting software was in the Impactor, which was designed to point its camera right at the target. Besides, while it is a sturdy and healthy craft, it would have to survive for another eight years for Rosetta to observe the impact.

Besides, a large fraction (50%?) of the science return was said to have come from the pre-impact images. The Flyby spacecraft can do this again, at Comet Boethin, and such an extended mission has been proposed.

I respectfully disagree with Bruce on impacting Comet C-G before Rosetta arrives. If they were really worried about the dust, they could always "hide" behind the nucleus. More likely, they would just sit back a good safe distance and still be able to see. It would be interesting to see an estimation of the dust flux as a function of distance, assuming this nucleus is as dusty as Tempel 1.

Actually the back-up plan for Deep Impact was to use the flyby as an impactor on Comet Temple-1 if the impactor missed. The newly proposed "DIXI" extended mission plan, as i mentioned above, is to fly by Comet Boethin in 2008. A funding decison will be announced around September 2006.

Bob, as comga wrote, this DI cant fly to Comet 67P/Churyumov-Gerasimenko. A DI target burn to enable the Comet Boethin flyby has already been completed last July 2005 shortly after the July 4 impact.

Bruce, thanks for the complement above. I am glad we can agree on this new Discovery DI clone proposal to impact Comet 67P/Churyumov-Gerasimenko. In fact 2 teams have proposed this concept.

Analyst, the Deconvolution has achieved the full image resolution expected with the DI HRI and all the instruments are working perfectly.....

and tedstryks follow-up comments are right on target

[Comga]

Actually the back-up plan for Deep Impact was to use the flyby as an impactor on Comet Temple-1 if the impactor missed.

I believe that may have been a contingency if the Impactor died early in the flight. By the time the Impactor hit or missed, it was well too late to retarget the Fly-by.

"I am glad we can agree on this new Discovery DI clone proposal to impact Comet 67P/Churyumov-Gerasimenko. In fact 2 teams have proposed this concept. "

I must be falling behind. Which two groups proposed this concept?

Impactor-only missons to Churumov-Gerasimenko have been discussed, but I don't know if anyone proposed it or will propose it.

"Analyst, the Deconvolution has achieved the full image resolution expected with the DI HRI and all the instruments are working perfectly..... "

Yes, indeed. There is a heck of a difference between the Stardust Nav-Cam and the Deep Impact High Resolution Instrument, which met its resolution and SNR requirements after deconvolution. Not to mention the IR spectrometer and the filter wheels AND the Medium Resolution Instrument, which alone is more powerful than the Nav Cam.

[Guest_BruceMoomaw_*]

I believe that may have been a contingency if the Impactor died early in the flight. By the time the Impactor hit or missed, it was well too late to retarget the Fly-by.

"I am glad we can agree on this new Discovery DI clone proposal to impact Comet 67P/Churyumov-Gerasimenko. In fact 2 teams have proposed this concept. "

I must be falling behind. Which two groups proposed this concept?

Impactor-only missons to Churumov-Gerasimenko have been discussed, but I don't know if anyone proposed it or will propose it.

"Analyst, the Deconvolution has achieved the full image resolution expected with the DI HRI and all the instruments are working perfectly..... "

Yes, indeed. There is a heck of a difference between the Stardust Nav-Cam and the Deep Impact High Resolution Instrument, which met its resolution and SNR requirements after deconvolution. Not to mention the IR spectrometer and the filter wheels AND the Medium Resolution Instrument, which alone is more powerful than the Nav Cam.

(1) The plan to hit Tempel 1 with the main craft was indeed an emergency expedient to be carried out if the Impactor failed to pass its functioning tests before release.

(2) I'm very far from saying that a repeat of DI should be the next Discovery selection -- I'm just saying that it would be considerably more scientifically useful than I initially thought. If I had to make a guess as to the two front-runners for the next Discovery selection, I should say a CONTOUR repeat (maybe jazzed-up with a different spacecraft), or the U. of Arkansas' "Hera" scheme for a large-scale sample return from 3 different near-Earth asteroids.

(3) Even with deconvolution, the resolution on Deep Impact's HRI was only about half as good as had been planned -- although this still made it far better than the MRI, let alone the Stardust camera (which had some troubles of its own, since they were never able to completely clean all the early contamination off its optics).

[djellison]

I'm just saying that it would be considerably more scientifically useful than I initially thought. I

You were all OVER the selection of DI after the mission was done, saying that the science return was poor because they said that seing the crater was the important part of the science and everyone should have known it would have been obscured by ejecta.

Why the change of tune?

Doug

[Guest_BruceMoomaw_*]

Because, my dear Douglas, in this case we have another spacecraft hanging around to take a good close (in fact, VERY close) look at the impact crater after the new DI leaves -- which we didn't have last time. No doubt, if this second mission is flown, the new DI main spacecraft won't be able to get a good look (or ANY look) at its impact crater, any more than the first one was (although that was its most important scientific goal) -- but Rosetta will be able to examine it in exquisite detail. It might even be able to touch down its lander on the nearby ejecta blanket.

I don't know if this is enough to justify picking this mission as a Discovery selection, but it certainly greatly strengthens the case for it compared to the first DI.

[Comga]

Rosetta will be able to examine it (the crater) in exquisite detail. It might even be able to touch down its lander on the nearby ejecta blanket.

Only if the Europeans become so interested in the manmade crater that they change their plans. The Philae lander is scheduled to be deployed well before an impact is feasible. However, after seeing what JAXA did with Hayabusa, Rosetta itself should be able to get close enough to examint it, as you say, exquisite detail.

Anyone care to estimate the limiting resolution of the Rosetta instruments and the standoff distance for that resolution? (Actually, if Rosetta were to get too close, one could push the resolution and recover some of the defocus blur through deconvolution. Ironic isn't it?)

[djellison]

Is NASA HQ is really going to select a mission from which the bulk of the science is done by a foreign spacecraft with foreign instruments and foreign PI's? I'm sure there's plenty of US involvement within Rosetta - but I can't see the cheque-writers going for it in any way.

Furthermore - the ejecta would surely be a big risk for Rosetta, look at the HST images from the first DI, and any damage caused would of course have been entirely predictable and obvious eh Bruce.

Doug

[Guest_BruceMoomaw_*]

Once again, the latter would depend (as "Comga" points out) on whether Rosetta was at a safe distance from Comet C-G during the impact itself, and closed in for the closeup view only after the coast had (literally) cleared.

I'll agree that the odds are against the selection of this mission -- and one reason (connected with what you said) is that, for proper science return, it depends on the success of two spacecraft, both of them carrying out complex assignments. Still, it can't be ruled out completely -- especially because it would utilize an virtually exact copy of a spacecraft design which we already know works.

[edstrick]

Rosetta could well stand off from the comet several thousand kilometers to watch an impact from safe distance.

Also..... Note that impact ejecta would hit a station-keeping spacecraft at zero-relative-velocity-plus-ejection-velocity. Fine dust ejecta might be travelling (armwaving guess here) a kilometer per second, while coarse ejecta would be travelling much slower, much of it not escaping the comet at all.

[ugordan]

Plus, at several hundred km distance, the impact ejecta would probably be very diluted and would not pose any real hazard to the S/C. Even a kilometer per second sounds to be on the high side here, I wonder if fluffy, dusty and compressible material would readily reach supersonic velocities like those.