[X] My Assistant

[djellison]

Well - impactor TCM 1 went well - <0.3% error and it's on course for a nominal impact time. PI has suggested that the comet is Banana shaped and we're going to hit the end of it which looks a little triangular

[Guest_BruceMoomaw_*]

Actually, the scientific seriousness of the HRI focusing problem has been greatly diminished by the other problem: the fact that the ejecta cloud utterly hid the impactor crater. When you read A'Hearn's and Belton's original article setting forth the science rationale for this mission ( http://deepimpact.jpl.nasa.gov/science/cospar-ms.pdf ) , they DO emphasize looking for layering in the crater walls as one of its most important justifications -- and it's precisely for that reason that they wanted such a very high-powered HRI with a top resolution of "1 meter/pixel", since the layering structure may well be that fine.

Well, of course, the obscuring ejecta cloud has totally ruined that scientific goal for the mission, and would have in any case. And since the Impactor Camera imaged the approach side of Tempel every bit as well as a perfectly focused HRI would have (its resolution and viewfield at 140 km distance was the same as the HRI's at its own minimum distance of 700 km, and at that point the Impactor Camera was still totally non-sandblasted), virtually the only thing we ended up actually losing from the HRI problem was somewhat sharper photos of the OTHER side of the nucleus. Moreover, when you compare the deconvoluted HRI photos of the impact itself with those from the MRI ( http://www.nasa.gov/mission_pages/deepimpa...a/pia02123.html and http://www.nasa.gov/mission_pages/deepimpa...a/PIA02131.html ), it's clear that deconvolution -- even if it didn't completely solve the HRI problem -- has still allowed the creation of photos maybe 3 times sharper than those from the MRI (as opposed to the originally hoped-for 5 times).

But that ejecta cloud -- whose power to blot out the impact crater was always a very strong possibility, even if it hadn't been remotely as big as it actually turned out to be -- raises the question: just how good, really, was the science rationale for this mission? (Hawaii's Jeffrey Bell, playing his usual role as the Skeleton at the Feast, has been raising this point in E-mails to me for years.) It's now clear that, if we really wanted to study the comet's surface layering, it might have been a far more dependable technique to simply put a radar sounder on the craft -- which would also have covered a vastly wider area on the surface, perhaps incuding sites of activity, and could have been used on more than one comet. (The sounder's antenna could have been put in the "shadow zone" protected by the craft's forward dust shield.) Moreover, by eliminating the great mass and cost of the Impactor, we could also easily have added gas and dust-impact mass spectrometers a la CONTOUR and Stardust. In short, this mission -- for less cost -- could have instead been a vastly improved version of the multi-comet CONTOUR mission. Just about the only thing we actually got out of Deep Impact that such an alternative mission couldn't have provided was the data the impact may have provided on the chemical composition of really deeply buried subsurface ices -- and was this enough to compensate for the other stuff we passed up?

This returns me to something I've wondered about for years: just how did Deep Impact actually get selected? Its selection was a total surprise to me; it muscled ahead of other Discovery finalists -- Aladdin, and a VESAT or VESPER Venus orbiter -- which were not only finalists this time but had been during the previous selection, too (which D.I. never had been). Is it possible that Dan Goldin intervened and ordered the selection of Deep Impact as yet another of his harebrained NASA PR stunts with questionable science return (Mars Pathfinder, with its cute but scientifically mediocre rover and not-very-efficient airbag system; the cancelled 2003 miniature Mars Airplane; the proposed all-girl Shuttle flight; the cancellation of a sensible 2003 Pluto probe in favor of a far more difficult and technically sophisticated early Europa Orbiter, with the highly predictable result thaat we got neither)? I already know that NASA HQ virtually ordered the selection of Phoenix in the supposedly "independent" 2007 Mars Scout selection; we were explicitly told so at the first Mars Roadmap meeting.