Comets are often described as "dirty snowballs" of ice and dust, which suggests something that looks like what kids throw at each other in winter. But NASA’s Alice instrument package installed aboard the ESA Rosetta probe currently orbiting comet 67P/Churyumov-Gerasimenko has sent back its first science data, which shows that the comet is about as white and fluffy as a lump of coal.
Alice is one of eleven instruments carried aboard Rosetta and one of three instrument packages supplied by NASA for the unmanned orbiter. Essentially, it’s a miniature UV imaging spectrograph that looks for thermal markers in the far ultraviolet part of the spectrum in order to learn more about 67P/Churyumov-Gerasimenko’s composition and history.
It does this by looking specifically for the markers associated with noble gases, such as helium, neon, argon, and krypton. Since the temperature at which these gases sublime is known, it’s possible to measure their presence and calculate the temperature of the comet in its past as well as its present characteristics.
According to NASA, Alice weighs less than 9 lb (4 kg) and uses only four watts of power, but it’s 1,000 times better at data gathering than similar instruments of just a generation ago. Also, it’s closer to a comet than any such device has ever been.
The upshot of all this high-tech imaging is the surprising discovery that 67P/Churyumov-Gerasimenko doesn’t look so much like a dirty snowball but something that came out of a coal cellar. In fact, it is black. Really black. According to NASA, it’s darker than charcoal. And though Alice has detected oxygen and hydrogen in the comet’s coma, the patches of barren ice that NASA scientists had expected aren't there. Apparently, this is because 67P/Churyumov-Gerasimenko is too far away from thee warmth of the sun to turn the ice into water vapor.
"We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows," says Alan Stern, Alice principal investigator at the Southwest Research Institute in Boulder, Colorado.
Launched in 2004, Rosetta reached 67P/Churyumov-Gerasimenko by a circuitous route involving three flybys of Earth, one of Mars, and a long detour out beyond Jupiter as it built up enough speed to catch up to the comet. During this time, it passed close to the asteroids Šteins and Lutetia, and went into a 31-month hibernation to conserve resources until the comet rendezvous on August 6 to start a year-long mission to study the comet to gain new insights into the early history of the Solar System. The first attempt to land on the comet nucleus is set for November.