Since 1986, four different comets -- Halley, Borrelly, Wild 2 and Tempel 1 -- have been examined in impressive detail by a wide variety of American, European and Russian spacecraft, including one that has actually returned a small dust sample to Earth and another that crashed a large piggyback spacecraft into a comet's nucleus to try and reveal some of its subsurface structure. And in 2014, the still more ambitious European "Rosetta" mission will rendezvous with the nucleus of a fifth comet (Churyumov-Gerasimenko), examine it from just 25 kilometers away (or less) for over a year and a half, and even drop a small survivable lander onto the nucleus' surface.

All this attention is entirely justified, given the fact that comets are the only preserved pieces of the "planetesimals" that were made by accretion out of the initial dust, ices and gas of the primordial pre-Solar System nebula itself, and which in turn merged together to form the planets.



©NASA

So they can give us invaluable data on the composition and structure of that initial nebula, and thus on the earliest evolution of the Solar System. But even after Rosetta there will still be one very important piece missing from our first-stage exploration of the comets -- a piece that would have been provided by the only comet mission to fail thus far.

The reason is that we have known for a long time -- from Earth-based spectra of the gas comas surrounding comets (and our lesser observations of their accompanying dust) that comets vary to a really surprising degree in their composition. They are all composed largely of water ice, but it is mixed with a very wide variety of other frozen gases -- most of them being simple organic compounds -- that also thaw out and sublimate into vapor as the comet approaches the Sun and is warmed by it: carbon monoxide and dioxide, hydrogen and methyl cyanides, methanol (methyl alcohol), methane, acetylene and ethane, ammonia, formaldehyde, sulfur dioxide and others -- many of which have boiling points far lower than that of water and so naturally volatilize into gas much more easily as a previously distant chunk of cometary material approaches the Sun. Moreover, their quantities of these other ices vary greatly from comet to comet.

Our more limited analyses of the "carbonaceous" mineral dust that forms the other part of a comet's substance indicate that it can also vary suprisingly. Earth-bound telescopes and astronomy satellites have taken detailed spectra of the dust cloud thrown up from Tempel 1 by the Deep Impact crash -- and also of the natural dust vented into space by the huge 1997 comet Hale-Bopp -- that strongly suggested the presence of phyllosilicates (clay minerals) and carbonates, such as would be formed if the carbonaceous silicate minerals originally in the grit had been exposed to traces of liquid water.

These have also apparently been seen in our spectra of the protoplanetary disks circling other young stars. But the actual sample of dust returned from Wild 2's nucleus by "Stardust" shows no trace of these water-related minerals. There also seem to be significant differences in the amounts of more complex, less easily vaporizable organic compounds that have been measured in various comets.

What accounts for these major differences? Originally, astronomers expected to see a recognizable pattern. Comets can be divided into two general types: "Oort Cloud" and "Kuiper Belt" comets. Oort comets now orbit -- by the hundreds of billions -- in the vast reaches of interestellar space trillions of kilometers from the Sun (thousands of times farther away than Pluto), but they actually originated among the hundreds of trillions of ice and dust planetesimals in the early outer Solar System.

Most of these crashed together to create the four giant planets, but a smaller fraction made close flybys of the giant planets and were flung outward by their gravity, either escaping from the Sun completely or ending up in their hugely distant Oort orbits -- from which one is occasionally twisted, by the gravity of passing stars and even of the entire Galaxy, onto a path back toward the Sun, and occasionally even back into the central reaches of the planetary Solar System. When they do so, they approach the Sun from every conceivable angle and tilt; half of them even now swing around the Sun backwards as compared to the planets.

However, most visible comets with an orbital period of less than 20 years follow orbits tilted no more than 30 degrees to the ecliptic, and virtually none of them circle the Sun backwards. It's been suspected for a long time that they had a different source -- a more orderly belt of a few billion comets beyond the orbit of Neptune, just a few billion kilometers from the Sun -- and this was confirmed in the 1990s when the growing sensitivity of our telescopes finally allowed direct detection of the Kuiper Belt objects (of which Pluto and its moon Charon were the only previously known members).

Occasionally the gravitational tuggings of the giant planets will cause one of these to veer far enough back toward the Sun to fly by Neptune, which in turn can divert it further inwards to carom around among the four giant planets -- with a certain number of these "Centaur" comets finally flying past Jupiter and being diverted by its powerful gravity all the way into the warm inner Solar System.

Paradoxically, it was at first thought that such short-period Kuiper comets would actually contain colder frozen gases than the Oort comets, because they had actually been originally formed near or beyond Neptune, rather than closer in to the Sun in the midst of the four giant planets like the Oort comets. However, no such pattern has shown up -- short-period comets and long-period Oort comets seem about equally rich in very low-temperature frozen gases like carbon monoxide, methane and ethane.

And new computer simulations of the long-time orbital evolution of Solar System objects suggest that most (and maybe even all) Kuiper comets first formed, like the Oort comets, closer in toward the Sun in the midst of the four giant planets, but were flung less dramatically outwards from the Sun by the gravitational tuggings of those planets. (Some Kuiper Belt objects follow relatively neat, circular orbits around the Sun, and may still have formed out there to begin with, unlike the Kuiper objects with more elongated or tilted orbits -- but simulations show that those orderly Kuiper objects are also much less likely ever to wander back in towards the Sun so that we can detect the cometary gases boiled off them.) So, the current orbits of comets tell us almost nothing about how close to the Sun they first formed.

But even more puzzling, however, is the fact that our data up to now also shows little correlation between the amounts of the different low-temperature ices in comets -- comets that are rich in carbon monoxide ice are often poor in methane and ethane ice, or vice versa. So far, there seems to be little evidence that the distances from the early Sun (and its warmth) at which comets originally formed had any consistent effect on the relative amounts of the various different ices out of which they are made. So what in the world did cause them to vary in composition as dramatically as they do?

Moreover, most comets (and most Kuiper and Oort objects in general) are thought to each consist of multiple separate planetesimals that at some point banged into each other and stuck, like the weapons in a giant snowball fight. In that case, one would expect the ices making up any individual comet to vary greatly in mixture from one place on the comet to another, just as entire separate comets thus vary greatly. But the evidence on this point, too, is contradictory.

When Deep Impact flew by Tempel 1, it found that one of the two major natural jets on its surface was emitting mostly water vapor, while the other was emitting large amounts of carbon dioxide -- which would indeed fit with the "conglomerate makeup" theory of comets. But two comets -- the Oort comet "C/1999 S4 (LINEAR)", and the short-period comet Schwassmann-Wachmann 3 -- have been examined in detail by Earth-based spectrometers both before and after suddenly breaking up into multiple pieces (as comets frequently do), and the ratios of the different gases vaporizing out of their suddenly exposed interior parts seem identical with the ratios of different gases that had been vaporizing off their original outside surfaces, indicating that their mixture of different ices was actually very uniform from place to place.

Finally, there's still another puzzle -- what looks like a contradictory indication that Oort comets and short-period comets might have different compositions after all. Most of the Oort comets that we've seen have incredibly elongated orbits with periods of up to several million years, indicating that they are still following the initial orbital path by which they first fell out of the distant Oort Cloud back into the main Solar System.

A smaller number of them, however, have obviously at some time in the past made a close flyby of one of the giant planets that shortened their orbit, such as Hale-Bopp (whose orbital period is now only about 2500 years) and Halley (whose period has been trimmed down to a mere 76 years). But our orbital simulations indicate that there should be a much bigger number of these orbit-trimmed Oort comets than we're actually seeing -- which suggests that Oort nuclei are made out of some very fragile substance and usually burst apart after at most a few dozen flights through the warm inner Solar System.

Short-period comets, however, have much more staying power -- our mathematical backtracking of their orbits indicates that they usually survive for hundreds or even thousands of passes through the warm inner Solar System before bursting apart or drying out completely. They seem to be made of tougher stuff. But how can this be, if (as all other indications show) they were originally formed in just the same zone of the early Solar System as the Oort comets?

One possible explanation may be that Kuiper comets, during their initial "Centaur" days, tend to originally wander very gradually inwards from the Kuiper Belt through the realm of the four giant planets and into the inner System over a period of thousands of years -- whereas Oort comets plunge directly from the supercold regions of the outer Solar System (or near-interstellar space) into the warm inner System.

It may be that this much slower gradual initial warming of Kuiper comets does something to "toughen" them by allowing them to vent a large part of their new inner gases more gradually, so that they avoid an internal pressure buildup of the type that Oort comets undergo. But, once again, we don't know. It may be instead that there's just something currently wrong with our computer calculations of the number of orbit-trimmed Oort comets that should exist.

As an additional puzzle, the clear closeup photos that we've now gotten of three comet nuclei -- Borrelly, Wild 2 and Tempel 1 (the "Giotto" probe's much fuzzier photos of Halley's nucleus aren't nearly as useful) show some similarly baffling physical differences between them.

Wild 2's surface, as seen by Stardust, was utterly bizarre. The nucleus was pockmarked with strange-looking craters that -- unlike the bowl-shaped meteor impact craters on other worlds -- had flat floors but very steep wall slopes as deep as 200 meters, making the craters look as though they had been punched out by a cookie cutter.

There is still debate over whether these might be impact craters, but their shape suggests instead that they may be "sublimation pits" -- in which a small initial depression (a small normal impact crater or a gas-vent hole) becomes gradually wider as the ice mixed into its walls vaporizes away under the solar warmth on perhelion passes, with the dark rock grit than was mixed in with it then sliding to the bottom of the slope (or being blown some distance beyond the foot of the slope, under the comet's extremely faint surface gravity, by the pressure of the vapor from the sublimating ice).

On flat surfaces, on the other hand, after the surface ice vaporizes away, the remaining "lag deposit" of rock dust would simply continue to sit on the surface, forming a blanket that soon becomes thick enough to choke off further vaporization of the ice underneath it. And so the initial crater or vent hole would continue to grow steadily wider as more and more of its walls crumbled away, but without the hole getting much deeper -- and the initial difference between the steeper slopes of its upper walls and the lesser slopes of its lower walls and floor would also become greater and greater, turning any initial bowl-shaped depression into that flat-bottomed, steep-walled cookie-cutter-type hole.

Such sublimation of ice off slopes, as the process proceeded steadily onwards, would also produce other effects -- such as initial sublimation pits growing wider and wider until they actually ate away most of the comet's initial layer of surface, leaving behind only isolated mesas with steep walls but flat tops, and even leaving behind isolated steep pinnacles. All of these have been seen on Wild 2 -- there is even one place along the edge of the biggest and steepest crater seen on Wild 2 where this methodical eating away of ice by solar warmth seems to have produced an overhanging lip!

While the photos of the surface of Comet Borelly's nucleus taken by Deep Space 1 in 2001 are a good deal fuzzier because the probe was farther from the comet, they also show clear signs of this same sort of thermal erosion. There are indeed steep-walled isolated mesas up to 100 meters tall on the surface of Borrelly, and other areas where the ground seems to have been eaten away in roughly circular depressions -- although we don't see anything as dramatic as the steep-walled craters of Wild 2. Both comets look as though their surfaces are peeling away in patches like the skin of a bad sunburn victim.

But the surface of Tempel 1, when Deep Impact viewed it, was radically different. Whereas Wild 2 has large numbers of slopes of up to 70 degrees (to say nothing of that overhang), virtually all the slopes on Tempel are relatively gentle -- there seem to be none over 26 degrees. There is one big area with a smooth flat top which has a steeper-sloped edge, which could be due to the same kind of methodical nibbling away at slopes that we see on Wild and Borrelly -- but its edge seems to be only about 30 meters high.

We see some other evidence for the kind of layer-by-layer sunburn-like peeling that we see on Wild and Borrelly, but it is always much shallower and its edge slopes are far gentler. And the craters on Tempel are also far less strange-looking than those on Wild -- some of them are gentle depressions with rounded walls, while others have sharp steep edges but have inner walls that are very low, with the craters' flat floors being at almost the same level as the plain beyond the crater. (Deep Impact's Impactor crashed near two such craters.)

Some scientists think that we are actually looking at what were originally standard meteor impact craters on Wild and Tempel, which were then extensively modified in form by the thermal slope-ice erosion that I've mentioned. But it is rather hard to see how impact craters could endure very long on the nuclei of these comets, since their surfaces are eroded so fast by ice sublimation; it makes more sense to assume that we are looking on both nuclei at sublimation pits that grew from what were originally far smaller impact craters or gas vents.

Wild 2 was a Centaur-type object that never came close to the Sun than Jupiter's orbit until only 33 years ago, at which time it made a close flyby of Jupiter that flipped its orbit into an entirely new one taking it into the inner Solar System. Since then it's made only five orbits into the warm inner System -- so it's been proposed that only a few meters of its icy surface will have vaporized away, allowing the big impact craters that had formed on Wild's surface during its ages in the cold outer Solar System to still exist on its surface at this point.

But most such Jupiter-crossing comets have been shown by computer calculations to switch back and forth repeatedly between periods where they do stay in the cold outer System, and other periods where a Jupiter flyby flips them into the inner System for a while before a later Jupiter flyby flips their orbit back into the cold outer System again -- so it's likely that Wild's surface has actually been eroded away by the Sun's warmth for a total period much, much longer than just 33 years. This returns us once again to the idea that its big craters must be sublimation pits, rather than big meteor impact craters that were somehow preserved on its surface despite the major erosive effects of the Sun's heat.

In any case, we're left with the question: why are the slopes on Tempel 1 so much gentler and less tall than those on Wild 2 and Borrelly? Is it just that its top surface material is far more crumbly than that on those other two comets? I've mentiond earlier that the conclusion was initially drawn from the shape, size and timing of the ejecta cloud thrown up by Deep Impact's Impactor that the comet's surface must be incredibly soft, fluffy and non-conhesive, something like dry talcum powder -- but I've also noted that Kevin Housen and Ken Holsapple think that this underestimates the effects of newly vaporized gases erupting from underneath after the impact, so that the surface material may actually be somewhat sticker and firmer than that.

However, this could still leave it far more crumbly than the surface material on Borrelly and Wild, and thus incapable of sustaining their kind of steep, tall slopes and cliff walls -- even given the extremely faint surface gravity on all three comet nuclei.

But in that case, why is Tempel's surface softer -- and why does it look as though Borrelly's slopes are also somewhat less steep than the surrealistically steep cliffs on the surface of Wild 2? Were the three comets made from the start of ice and dust mixtures of different compositions, and thus different firmnesses -- with the three comets varying in their ice-dust ratios, or Tempel's ices perhaps being made out of more volatile gases that boil away into vapor more easily than the surface ices of Wild and Borrelly? Or have their surfaces been affected by different complex sequences and amounts of solar heating over their histories, since their orbits have all likely changed repeatedly in the manner that I've described?

Finally, Arizona comet expert Michael J.S. Belton has suggested that some of the layering seen on all three comets may not be thermal peeling, but may actually be the result of the collisions between smaller blobs of ice/dust that created them in the first place in the outer System or in the Kuiper Belt. He posits the "Talps" theory (that's "Splat" spelled backwards), in which -- when one such snowball hits a bigger one -- it is flattened out into a sheet that spreads around part of the surface of the bigger snowball.

This could explain, in particular, what seems to be one layer of material that actually runs through the middle of Tempel's nucleus, so that we see a cross-section of the layer as a straight-edged 200-meter-wide band running across one part of the nucleus. Once again, though, in that case we would expect to see different patches of any individual comet nucleus being made out of a different mixture of various ices and dust -- and while Deep Impact did see some sign of such compositional patchiness on Tempel 1 (where one of the comet's surface jets expels just water vapor while the other expels a lot of carbon dioxide), we don't seem to see it in the separate fragments of other comets that have ruptured into pieces.

In short, our first space probes have shown that different comet nuclei vary as dramatically in their physical makeup and structure as they do in chemical composition. To get any good understanding of what causes these differences -- and what they may say about different comet's' actual original formation conditions in the early Solar System (as distinguished from the complex effects that comets' different orbital histories, and thus their varying degrees of warming by the Sun, may have had on their surface appearance) -- we are clearly going to have to look at a much larger sampling of different comet nuclei, just as the great variations between the Asteroids means that we must get a closeup look at a large sampling of them to understand them properly. Just looking at a few comet nuclei -- even when you look at one of them in such spectacularly close detail as Europe's "Rosetta" comet rendezvous-and-landing mission will do in 2014 -- won't be adequate.

And the one comet probe intended so far to look at a whole multiplicity of different comets failed disastrously in 2002. In the last part of this series, I'll examine how comet scientists hope to recover from that failure -- and how the new extended missions of the Stardust and Deep Impact probes can help us do that.

The most powerful of those was Deep Impact's "High-Resolution Imager" (HRI), which at the time had by far the most powerful optic system ever carried on a Solar System probe: a reflective telescope with a 30-cm wide mirror, which was intended to take pictures of the comet nucleus' surface with a resolution of only 2 meters per pixel from a range of 700 km. Unfortunately -- in an eerie replay of the Hubble Telescope's initial mirror problem -- it was discovered after launch that the mirror was slightly out of focus.

The mathematical image-processing known as "deconvolution" largely compensated for this for photos of the relatively bright comet nucleus surface itself, but deconvolution is less effective in deblurring images of dim light sources because it amplifies random noise in the photo (which is why it could do little to correct Hubble's problem before astronauts installed a focus-correcting mirror on the telescope).

However, Drake Deming of the Goddard Space Flight Center came up with an ingenious proposal for a Deep Impact extended mission that not only managed to use Deep Impact's HRI for genuine astronomy studies, but actually makes some lemonade out of the lemon of its focusing problem.