Part of the crack in the comet's crust imaged by one of Rosetta's high-resolution OSIRIS cameras. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Jan 25, 2015 Cracking stuff as a comet gives up its secrets

Sen—A flurry of new findings were published this week about 67P/Churyumov-Gerasimenko, the comet that Europe’s Rosetta spaceprobe has been closely studying over recent months.

One interesting discovery is that a lengthy crack has formed around the neck of this comet’s nucleus, leaving some to speculate that it could be on the way to breaking up into two or more pieces.

It is hardly surprising though. One can only imagine the forces that must be exerted on this oddly-shaped collection of rock and ice as it makes its regular elongated orbit around the Sun, making it flex under the strain of regular bouts of warming and cooling.

Comets have been seen to fragment before, when they were still regarded as ghostly apparitions that appeared in the sky, and long before any machine could get close enough to learn what was actually happening.

The most celebrated case must be that of Biela’s comet, an object that was first discovered way back in 1772 by Montaign from Limoges, France. Subsequent rediscoveries were made on return orbits in 1805 and 1826, the latest of them by German astronomer W. von Biela who recognised they were one and the same comet.

Like Rosetta’s comet, 67P, Comet Biela was found to have a very short orbital period, taking around six and a half years to orbit the Sun. (No doubt it once orbited over a timescale of many thousands of years when it travelled in from the outer Solar System, but its orbit was disturbed, as with so many, by mighty planet Jupiter).

On a subsequent return in 1846, astronomers were startled to find that Biela had become not one, but two comets, with two distinct bright nuclei visible in their telescopes. They were seen again in 1852 for the very last time, after which it was assumed that the comet had broken up.

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More evidence from Rosetta of the crack running through the neck of Comet 67P/Churyumov-Gerasimenko. Image credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

That was not the end of the story however, for astronomers had been becoming aware of a new shower of shooting stars, or meteors, during the 19th century. They culminated in November 1872 with a meteor storm, with too many to count falling across the sky, just when the comet had again been due. 

A similar storm was seen in November 1885, dubbed the Andromedids, after the constellation of Andromeda from which they appeared to come. These meteors were clearly remains of Comet Biela, and a link between comets and meteors was established.

Other comets have also been observed to fragment, notably Comet Shoemaker-Levy which was torn apart by Jupiter’s mighty pull to form what resembled a string of pearls before they smashed into the same planet’s cloudtops, giving it a series of “black eyes”.

Comet ISON ignored the fanfares building it up to becoming the “Comet of the Century” and disintegrated completely as it rounded the Sun in November 2013.

Time will tell us whether 67P will suffer a similar fate to Biela. The new studies will no doubt help scientists wondering whether the comet got its rubber-duck-shaped, twin lobes because two chunks of cosmic debris collided, or whether one large object has simply eroded around the middle because that is where most of the jets of gas and dust are being given off.

What is clear is that the idea popular in the last century that a comet is an “icy snowball” has long gone out of the window. And the other common model of those times that it is a “flying gravelbank” no longer holds either.   

Comets are more cohesive and resilient than that, as demonstrated by recent encounters of a couple of them with the Sun. A previous Comet Lovejoy (not the one currently gracing our skies, but an earlier discovery by Australian comet-hunter Terry Lovejoy) astonished astronomers in December 2011 by flying through the Sun’s atmosphere, just 120,000 km above the surface, and surviving a furnace of several million degrees.

Building on earlier encounters by ESA’s Giotto probe with Halley’s Comet in 1986, and NASA’s Deep Impact with Comet Tempel I in 2005, Rosetta is finally helping planetary scientists learn what these peculiar bodies, left over from the formation of the Solar System, are made of.

What an exciting time this is to be discovering just what these ancient bodies, once feared by the ancients as harbingers of doom, are really all about.