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Bruce Moomaw

Actually, it's the fifth US comet mission -- we musn't forget the unfortunate CONTOUR (which would already have contributed quite a bit of knowledge had it succeeded). And one can make an excellent case that the two Vegas contributed much more science than our own ISEE-3 (even though that was the very first comet mission).

That being said, I retain some serious doubts about the science rationale for this mission -- especially since A'Hearn and Belton, in their original articles laying out the case for it, emphasized the importance of getting very high-resolution photos of the crater wall to examine surface layering. They said, explicitly, that that was the whole reason for having such a high-resolution camera -- we needed photos of the crater wall with a resolution of "about one meter". As things turned out, we can't even find the damn crater (which also pretty much eliminates any actual science loss from the moderately out-of-focus HRI camera) -- but even if the ejecta eruption had been vastly less, the odds would have been excellent that it would have blotted out our views of the crater.

So, in the end, pretty much the only thing the very difficult, heavy and expensive Impactor added to this mission was some additional data on the physical hardness and chemical composition of the deeply buried interior ices of the comet -- and we could have gotten a lot of that (for more than one comet) by probing the comet's interior with a radar sounder and using the craft's near-IR spectrometer to analyze the gases at the bases of its natural jets. I've always been a bit puzzled by the selection of this as a Discovery mission -- it came out of nowhere to vault over two other Discovery missions that had been finalists in BOTH of the most recent selection cycles -- and I can't help wondering whether Dan Goldin ordered its selection as yet another of his scientifically harebrained NASA PR stunts.

Oliver Morton

That's very interesting, Bruce; I hadn't followed it that closely, and hadn't clocked the value laid on the hi res imagery of the crater. Let's hope some of the people covering this have, and ask the questions (it reads as if David Chandler knows what's what, in that he stresses the lack of crater data.)

Something Deep-Impact-ish was suggested at the very first Discovery meeting, in Capistrano, and was quite enthusiastically discussed, so one reason for its selection may have been that a lot of people just liked the idea. You don't *have* to be Dan Goldin to find appeal in its drama. And they may have done a great job of the pitch/proposal. But what you say does sound a bit odd. What were the passed over finalists -- a Venus infrared mission and something else?

As to Contour, I thought about it, but decided, in the context of a successful flyby, to limit myself to missions that had actually got to the comet.

Bruce Moomaw

The two passed-over missions were Aladdin (the mission to fire tiny impactors at Phobos and Deimos to collect samples of both moons' surface soil for return to Earth) and VESPER (the Venus Express-type Venus orbiter). This was actually VESPER's first appearance on the finalist list, but I include it because its somewhat less capable counterpart VESAT had been a finalist (along with Aladdin) in the previous round. I lump VESPER and VESAT together together as "the Discovery atmospheric Venus orbiter". (Poor Kevin Baines proposed VESAT for four straight rounds of Discovery selections, and kept almost but never quite hooking the brass ring. As for VESPER, its original name was "Venus 2000". I looked it up on Google under that name once and was deluged with ads for sex toys, which I imagine explains the name change.)

The paper in which A'Hearn and Michael Belton first lay out the science rationale for Deep Impact -- emphasizing a very high-resolution search for layering in the crater wall -- can be found at http://deepimpact.jpl.nasa.gov/science/cospar-ms.pdf . Three very recent and extremely detailed papers on the mission -- just published in "Space Science Reviews" -- are also available on Belton's website at http://www.beltonspace.com/ . They seem to deemphasize the search for layering slightly, but not much -- and the main one leaves me as puzzled as ever as to why the Impactor was considered so important.

Bruce Moomaw

One other note: a simulation of the sort of views of the impact crater that A'Hearn and company were hoping to get can be found at http://deepimpact.jpl.nasa.gov/tech/hri.html . Not quite what they actually did get.

Bruce Moomaw

A major surprise today. Contrary to my confident earlier beliefs, it starts to look as though the impact actually vaporized very little ice from the comet's interior -- virtually all that huge ejecta cloud was simply dry, very fine surface dust kicked into space by the impact itself. At any rate, that seems to be the case from both the craft's own observations and those of the SWAS satellite (which was roused from long-term hibernation for this particular mission):

Hawaii's Jeffrey Bell will be delighted -- he was cackling to me right after the impact that the entire huge ejecta cloud could be explained by this alone. I didn't believe him. But, as he said at the time, this also calls the science rationale for the mission into still further question: it seems to make it even more certain that any impact which produced a crater big enough to be seen would also produce a cloud of ejecta big enough to completely blot it out from the main craft's cameras. About all we've really learned from this impact is that Tempel 1 has a very thick but very loose surface dust layer, as opposed to a thinner and/or cemented one -- and couldn't we have learned that just as well (and more besides) just from equipping the main craft with a radar sounder?

By the way, NASA TV didn't bother to cover today's DI press conference at all -- they were too busy covering really exciting stuff like the Shuttle...and...the ISS...ZZZZZZZZZZ


I'd like to echo Bruce's comments regarding the science return from the Vega missions - it was excellent, and don't forget that the two craft also released the rarely-mentioned balloons into Venus's atmosphere on the way to Halley. The images of Halley's nucleus from Vega 1 & 2 weren't a match for Giotto, but they had an extensive suite of non-imaging instruments that returned very valuable data.

Also, don't forget the ESA/NASA Ulysses mission. Although clearly not a comet mission, its two unexpected cometary ion tail crossings have demostrated that even a relatively small comet's presence can be sensed several AU away as a pulse of cometary ions and structured magnetic field lines in the solar wind.

David Chandler

I'm just belatedly catching up with this post. I wanted to respond to the issue of what one might really have hoped to learn from this kind of impact, as opposed to say a flyby with radar. I think the essence of it was summed in an analogy someone made at one of the press conferences -- I think it was Pete Schultz, but I'd have to check -- comparing it to a geologist trying to figure out a new location without a rock hammer. This mission for the first time allows them to break a rock, to expose a fresh surface. Everything on the surface has been heavily processed through solar and impact weathering, so under the surface is where the potentially interesting information is. If the nucleus really is made of relatively primordial material from the solar nebula, then it's only by exposing brand-new interior material that we can hope to determine not just in general, but precisely what kinds of organic molecules this planet received as a starting point for that crucial next step toward replication. Did we get all the amino acids for free? or stuff even more complex than that? I don't know if they're going to be able to tease any of this out of the actual data, but that's my sense of what the hope was, and it seems like useful information to me.
And, if so, that would mean that the really interesting data is all spectrographic anyway, and may take some time, but might, one hopes, be less affected by the glare of the surface dust.

Bruce Moomaw

Oh yes; it will be extremely interesting to look at the spectral data. But, again, how much of that data could they have obtained just by using the near-IR spectrometer to examine the gases at the base of the comet's natural emission jets, or near its surface? (Especially on a more active comet, since Tempel 1 was known in advance to be pretty non-flatulent.)

I still remain uncertain just how scientifically worthwhile this mission actually was as compared to possible alternative missions. They may have a good case for it, or they may not. I have, in fact, E-mailed A'Hearn to ask him such questions, but so far no response. Looks like I'll have to call him. In any case, certainly its total scientific cost-effectiveness per dollar can be raised by extending it cheaply to a second comet flyby.

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