It's also been obvious for a long time that they vary dramatically in their physical structure -- some are far more active in emitting new gas jets than others when they come nearest to the Sun, and some of them even split up from internal gas pressure during such encounters, or even explode into a veritable cloud of small fragments.

The close-up photographic views we're starting to collect of the surfaces of comet nuclei also show amazing variations -- some of them (like Wild 2) are covered with startlingly steep vertical cliffs and steep-walled craters with flat floors, while others (like Tempel 1) have far gentler features.

But we still don't understand the reasons for these differences. It stands to reason that comets that originally formed at differing distances from the Sun, and thus different temperatures, during the Solar System's earliest days would have different compositions, and that these would tell us a lot about the conditions in the early solar nebula out of which the planets formed.

But we haven't been able to trace back comets of differing compositions to different types of orbits today -- they were apparently shuffled around so much by the gravitational yankings of the four giant planets during the Solar System's earliest days that their current orbits tell us virtually nothing about their places of origin.

On top of that, we have no good idea how much the surface compositions and visible physical structures of comets reflect the varying extents to which they've had their upper layers warmed by the Sun during their flights through the inner Solar System, rather than being related to their starting compositions.

Clearly we need to survey as many different comet nuclei as possible closeup to start resolving these questions. Unfortunately, the only comet probe to fail so far was the only one designed for such a multiple comet survey.

"CONTOUR" (COmet Nucleus TOUR) was the second in the series of Solar System probes designed by the Applied Physics Lab of Johns Hopkins University, which has begun to turn itself into a major rival of the Jet propulsion laboratory in this field. They had already flown the "NEAR" spacecraft that rendezvoused with, orbited and ultimately soft-landed the near-Earth asteroid Eros; and since CONTOUR they've been responsible for the "MESSENGER" Mercury orbiter and the New Horizons Pluto flyby mission, both of which are well on their way to their destinations and so far are working perfectly. But CONTOUR is the one blot on their record so far.

In design, it was a very good example of what can be done to produce a lot of good science with a relatively cheap and simple Discovery-class Solar System probe. It was launched in mid-2002, to go into a solar orbit fairly close to Earth's and with the same one-year period, allowing it to return to the vicinity of Earth at intervals of as little as six months and (if we chose) to make a gravity-assist flyby of Earth that would twist it into a different one-year orbit with a different path away from Earth. This, in turn, could allow it to intercept and fly by a whole series of different short-term comets as they flew into the inner Solar System and passed across or near Earth's orbit, thus making close-up comparison studies of them.

For that purpose, it had the highest-priority instruments for such high-speed flyby observations of comets: two high-resolution cameras, a mapping near-infrared spectrometer to map the nucleus' surface composition and temperature (and to remotely examine some of the gases emitted by it), a mass spectrometer to directly analyze in detail the emitted gases of the comet; and a dust-impact mass spectrometer that would similarly analyze the tiny puffs of vapor produced when solid dust and ice grains in the coma slammed at high speed into a metal target on the instrument during the flyby.

Its two official targets were Encke (which it was to fly past in November 2003) and Schwassmann-Wachmann 3 (the recent split and exploded comet I've mentioned previously) in June 2006. Encke is the only still-active comet at the moment with an orbit that's entirely closer to the Sun than Jupiter's, presumably as a result of its flying close to one of the four inner planets at some point -- its surface seems to have been largely but not completely dried out into an ice-free dust crust as a result.

"S-W 3", on the other hand, could have allowed CONTOUR to get an extremely useful close-up look at the freshly exposed deep interior of a comet as it flew past one of S-W 3's fragments (although, during its 2006 pass, S-W 3 four fragments burst into such a cloud of tinier pieces that the accompanying dust cloud might actually have endangered CONTOUR during that flyby).

After that, it had a set of three possible alternate cometary targets that it could have been chosen to fly by during an extended mission in 2008-09 -- including one that would also have allowed it to make a return visit to Encke in 2013. Alternatively, it could have been chosen instead to make another kind of comet visit at any point after its first Encke flyby -- namely, a flyby of a newly discovered comet that had been identified as incoming by astronomers after CONTOUR was already up and flying.

This could have been a Centaur comet newly entering the inner System after a flyby of Jupiter; or it could have been a flyby of one of the long-period comets from the Oort Cloud. These have such long periods -- anywhere from a "mere" 76 years for Halley, to several thousand for Hale-Bopp, to several million years for many others -- that it's totally impossible to arrange a flyby of any of them (except Halley) before the spacecraft is actually launched.

But by having CONTOUR already in solar orbit, and capable of readjusting its next Earth gravity-assist flyby on a last-minute basis in order to put itself on an intercept trajectory almost as soon as the new comet was sighted by Earth astronomers for the first time, it was calculated that there was a 44% chance of using any one of its Earth flybys to send it on to such a new-comet flyby -- which meant about a 99% chance of finding an acceptable new-comet target for it by 2008. (Had it been successfully launched in 1995, for instance, it could definitely have been re-aimed to make a flyby of the huge and spectacular Hale-Bopp in 1997.)

We would very much like to get another look at one of these hard-to-visit Oort comets for comparison to the short-period ones -- especially given the fact that our current orbital data suggests that Oort comets mysteriously tend to disintegrate after just a few close passes by the Sun, unlike short-period comets.

And CONTOUR was one of the cheaper Discovery missions to boot, at $140 million -- it never came anywhere near making the sort of cost overrun that has plagued the next four Discovery missions. But, alas, none of this came to pass. Instead of being directly fired into solar orbit, the craft was first put into Earth orbit for six weeks, after which its solid-fueled last stage -- a motor actually built into the craft's rear -- was to be fired to send it on its way: a novel launch setup that allowed much more timing flexibility for the initial liftoff.

However, when CONTOUR's kick motor was finally ignited, it was never heard from again. This frustratingly happened (like the failures of Mars Observer and Mars Polar Lander) during one of the very few periods when the craft was out of radio contact with Earth, so we'll never know the reason for absolute certain; but a postmortem indicates that it probably happened because the extent to which the exhaust plume from the big internal solid rocket motor would heat up that end of the craft had been seriously underestimated, perhaps causing some of its parts to even melt.

This design problem would have been very easy to correct for a repeat attempt -- all that was necessary was canceling that long Earth-orbit period, and fastening the kick motor onto the probe's rear instead for a direct launch into solar orbit. (Indeed, most of the electronics designs for CONTOUR have been used again for the New Horizons Pluto probe, where they're working splendidly.)

And CONTOUR 2 was indeed proposed by the Applied Physics lab for the next Discovery selection in 2004 -- a mission that would have taken off this year to fly by two short-period comets in four years, and then gone on to fly by one or even two more comets during its extended mission (or, alternatively, to make that flyby of a new "fresh" comet).

But this round of Discovery selections was a fiasco -- NASA had concluded by then that the maximum-cost cap that had been set at that point for Discovery missions was too low to fly any of the proposed missions with acceptable odds of success, including CONTOUR 2. And so no Discovery mission at all was accepted during that round.

The next round of Discovery proposals was invited -- with the cost cap raised further -- for a mission to be launched by 2013. But, alas again, it turned out that this time there were no good multiple-comet flyby opportunities for a launch in that period, so CONTOUR 2 was not proposed again in that round (in which three other missions have been picked as finalists, with the final choice to be made in October).

At some point, either CONTOUR 2 or a similar multiple-comet visit is almost certain to be flown; it's too scientifically important not to be. But during that new Discovery selection, two proposals were made to recover at least some of the science goals of such a CONTOUR mission by reusing the already-successful comet probes Deep Impact and Stardust for visits to additional comets.

Neither Deep Impact nor Stardust has the full set of four top-priority comet instruments that CONTOUR carried; but each of them has two of those instruments, and since each such added flyby would cost only $25-30 million, both of them ended up being accepted. It still looks like a good scientific bargain cost-wise.

Indeed, an extended mission for Deep Impact, visiting a second comet, had been considered probable from the very start. There were two possible targets, with the accepted one being Boethin -- a comet with a current period of 11 years whose nucleus seems to be only about half as big but twice as active as Tempel 1's. Deep Impact will make a gravity-assist flyby of Earth this December 31 to readjust its orbit for the Boethin flyby on Dec. 5, 2008.

Of course, it doesn't carry a second Impactor craft to arrange another dramatic comet crash -- but this actually frees up its instruments for some other kinds of observations that it couldn't make at Tempel 1 because of the need to focus most of the time on that impact. Deep Impact focused its two high-powered cameras and its near-infrared mapping spectrometer completely on the crash site from the time of impact (when the main craft was still 8600 km from the nucleus) until it was only 700 km away. At that point, the very thin but huge cloud of dust thrown up by the impact was regarded as hazardous enough that the craft re-aimed itself to point its dust shields forward along its flight path, forcing it to aim its instruments away from the nucleus during its closest approach of 500 km and for 22 minutes afterwards, after which it could resume looking back at the nucleus and dust cloud as they receded into the distance behind the craft.

This time, the craft will fly within 700 km of Boethin, at a distance and an angle that safely allows it to keep its instruments aimed at the nucleus throughout the entire flyby without ever having to swivel into the "shields forward" position. Thus it will be able to map much more of this comet's nucleus at very close range. (By contrast, during all its close-range time at Tempel, it was squinting at the unexpectedly thick cloud of impact dust that completely blotted out the new crater that it was looking for -- although the camera on the Impactor craft itself took photos showing details as small as three meters just before the crash.)

Its High-Resolution Imager camera is hooked up to a powerful 30-cm telescopic mirror. In a bizarre replay of the Hubble Telescope's initial accident, it turns out that the curvature of that mirror wasn't properly calibrated during its initial tests, and so the HRI is mildly out of focus. But a mathematical technique called "deconvolution" can compensate for much of that fuzziness.

This technique works much better for relatively bright scenes like the comet's surface than it does for dimmer objects against the background of space (which is why it wasn't very useful for improving Hubble's photos before the 1993 Shuttle repair team installed its focus-correction mirror). In the case of Deep Impact, it should allow the HRI to take pictures with a resolution as good as 3 to 4 meters, a bit better than the resolution that CONTOUR's cameras would have gotten during its much closer flights past its comet nuclei.

Its medium-resolution camera will simultaneously take pictures covering an area five times wider, and the infrared spectrometer hooked up to the HRI's big mirror can be kept in the spacecraft's shadow during the whole flyby this time, improving its own sensitivity. (That spectrometer, free of the duty to look at an artificial impact cloud, will be able to concentrate more on the natural gas and dust jets spurting out of the comet, to compare the gases originally coming out of them with the other gases that they're changed into by sunlight-driven chemical reactions soon after they're ejected from the comet -- a process very important in comets.) In short, even given the fact that Deep Impact carries only two of the four instrument types that CONTOUR carried, a visit to another comet nucleus for comparison studies at a cost of only $30 million seems very worthwhile.

But such a reuse of Deep Impact was always probable -- it's still a nice fresh spacecraft that took only six months to get from Earth to Tempel 1 and will be only four years old when it flies by Boethin. The reuse of Stardust is a lot more surprising, and I'll describe that in my final part of this report -- along with an even more ingenious use of Deep Impact's camera to actually provide data on planets around other stars.

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