However, the other part of the latest Discovery mission selection process has just been completed: the selection of possible Missions of Opportunity ("MO"s), which involve either piggybacking American instruments on other nation's spacecraft, or finding additional uses for spacecraft that have already been launched (or are at least already officially planned).

These, of course, are a lot cheaper -- their cost limit is only $35 million, as opposed to $425 million at this point for full-fledged Discovery missions -- and on July 3, NASA ended up officially selecting all three of the new Discovery MO finalists -- all of which reuse two highly successful spacecraft that have already completed their primary missions to study two comets.

Those spacecraft are Deep Impact -- which crashed a 370-kg piggyback spacecraft into the nucleus of Comet Tempel 1 two years ago while the main spacecraft flew past the nucleus, observing the effects of the impact -- and Stardust, which flew within 250 km of the nucleus of Wild 2 after a five-year trip, fielded thousands of dust particles from the comet's dust coma in a pad of low-density aerogel capable of preserving the particles largely intact even after they hit it at 6 km/second, and then returned them to Earth two years later in the very first fully successful sample-return space mission from farther away than the Moon. Both craft are still in good operating condition, and Deep Impact was pegged from the very start for a probable flyby of a second comet.

But only fairly recently was it realized that Stardust's orbit, after its close flyby of Earth during which it ejected its sample-return capsule, could allow it to make a scientifically productive flyby of the same comet nucleus previously visited by Deep Impact.

While Deep Impact's cometary crash was a spectacular feat of engineering, requiring a highly accurate autonomous targeting system for the Impactor craft -- and while it was certainly visually spectacular -- its scientific return was not as great as had been initially hoped.

The High Resolution Imager on the Flyby craft was equipped with a 30-cm reflecting telescope that at the time made it the most powerful camera ever put on a Solar System mission, and it had been hoped that the HRI could take high-quality pictures of the crater left behind after the impact, thus both providing direct observation of the compositional layering produced in the comet's upper surface by its repeated exposures to solar heat, and giving us a better idea of the hardness of the surface (both subjects of great debate among astronomers).

Unfortunately, the cloud of fine dust and ice ejecta thrown out by the impact was so huge that it completely blotted out any possible view of the crater until long after Deep Impact had flown millions of kilometers past Tempel 1. What data the mission thus provided on the layering and hardness of the surface had to come from Deep Impact's high-speed observations of the size and form of the ejecta cloud itself.

And while this was initially interpreted as indicating that the comet's surface was a fluffy layer of loose talcum-powder-sized dust grains with virtually no tendency to stick together, Keith Holsapple and Kevin Housen soon pointed out that even this conclusion is open to question -- since the eruption of ejecta was largely produced not by the force of the crash itself, but by the eruption of vaporizing ice from just under the surface of the comet (which is also what drives natural dust and gas jets off the surfaces of comet nuclei).

This probably exerted as much as 20 to 200 times as much force as the impact itself -- meaning that Tempel's top surface material could actually have had a weakly sticky shear strength of as much as several hundred grams per square centimeter, but still be blown massively into space by the explosion of exposed ices boiling into vapor in the vacuum (whose total size and power remain uncertain).

The data provided on the composition of Tempel 1 by the Deep Impact mission was also rather limited. It carried no mass spectrometers to directly analyze the gas or dust particles given off by the comet; instead, it carried an infrared spectrometer to analyze them from a distance.

This could be quite useful, but its ability to detect individual substances was a lot more limited than that of such in-situ spectrometers -- for instance, at this point its spectra of the ejecta cloud have only firmly proved the existence of water, carbon dioxide, and an indistinguishable mixture of various hydrocarbons.

Other spectra of the erupted cloud taken by Earth-based telescopes and astronomy satellites have actually been more sensitive -- especially the spectra obtained by the Spitzer solar-orbiting infrared telescope -- but these could largely be obtained of comets' natural emissions without having to go to the trouble of crashing a spacecraft into the comet.

Still, Deep Impact's crash did indirectly reveal the overall compositional nature of Tempel's sun-warmed outer surface layering, by allowing us to observe the timing with which different types of vaporized ices erupted out of the surface during the first few seconds after the crash.

There seems to be a top layer of dry, loose sunbaked carbonaceous-rock dust only a few centimeters deep. Below that, there's colder material which still has water ice mixed into it, then another layer that also contains colder frozen carbon dioxide as well starting about a meter beneath the surface, and still colder frozen ethane as well starting a few dozen meters down.

(This means that NASA's planned future comet-nucleus sample-return mission may find it easier to sample a comet's interior ices than had been feared; it may only need to plunge a core tube a meter or two down into the surface to sample a lot of them.)

And the observations of the natural, undisturbed surface and coma of the comet with the craft's cameras and IR spectrometer were highly useful in themselves.

The science return from Stardust's returned dust samples, analyzable by giant super-sensitive Earth-based instruments with tremendous sensitivity and detail -- promises to be much higher.

Even some organic compounds (non-biological in origin) were returned from the comet, and even the preliminary study of the dust grains has confirmed that the mineral dust that congealed to make comets contains a lot of high-temperature silicate minerals that must have originally condensed in the innermost part of the forming Solar System and then been spread all the way out into the outer System (a very important fact in understanding the Solar System's formation, and one that had been hotly debated until now).

And the European Space Agency's very big and ambitious 2900-kg "Rosetta" mission -- which is already over three years on its way, and has just made a necessary gravity-assist flyby of Mars -- will be another major stride.

In May 2014 it will actually rendezvous with the nucleus of Comet Churyumov-Gerasimenko, spend at least 19 months hovering near it and slowly orbiting it at distances down to just a few kilometers, map its surface in detail with a variety of cameras and instruments, use onboard instruments to collect samples of the gas and dust given off by the comet for very detailed chemical and mineralogical analysis (although not as detailed as Earth instruments can do with the returned "Stardust" dust sample), and finally detaching its 90-kg piggyback "Philae" lander to actually touch down on the nucleus, photograph and analyze its material on the spot, and even work with the main Rosetta craft to "X-ray" the nucleus' internal structure by detecting long-wavelength radar pulses transmitted clear through the nucleus by Rosetta on its other side.

Later -- some time before the late 2020s -- the US hopes to launch its own comet rendezvous craft as part of the New frontiers exploration program, which would actually land on the nucleus' surface briefly, scoop up a kilogram or more of surface material (and maybe a deeper core sample containing some of the comet's internal ices), and then fly that intact sample back to Earth for much better analysis than can be done with the heat- and impact-damaged dust grains returned by "Stardust".

But even this impressive smorgasbord of comet missions contains one very important element that's still missing, which I'll describe in my next chapter -- along with the way in which the two new low-cost extended missions that reuse Deep Impact and Stardust should help contribute to it.

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