On the Astronomy Picture of the Day site for October 16, we get to see a stunning image (cropped version above) that’s unique in many ways. The Rosetta spacecraft is a probe designed and launched by the European Space Agency (esa) to rendezvous with comet 67P/Churyumov-Gerasimenko, and presently riding just 16 km (10 mi) away from said comet. It has another craft attached, called Philae, which will separate on November 12th to actually land on the surface of the comet. But while still attached, the cameras on Philae were used to get this image of the comet and the solar panels of the Rosetta craft in an excellent composition. 67P/Churyumov-Gerasimenko has a curiously dumbell-shaped body, and is now close enough to the sun that it is starting to generate the jets of vapor that produce the coma and tail, the glowing (actually reflective) haze that we typically imagine comets to have – in truth, they only have this when close enough to the sun for the ice to sublimate into vapor. But seriously, don’t just look at my cropped version up there – go to the source page and click on the image for the much larger version.
A month ago, Rosetta sent back an even more dramatic photo, a wonderfully detailed look at the surface of the comet – click on that one, too, because it’s a wonderfully stark and forbidding image. For years, comets were believed to be “dirty snowballs,” made of mostly ice but with a generous helping of dust, grit, and rocks thrown in. Primarily this was because we could only get a halfway-decent look at them as they got very close, but this meant they were also close to the sun and thus active, spewing out an obscuring haze of vapor. Most meteor showers, however, come from the Earth passing through the orbit of comets and encountering their trail of expelled dust, the solid stuff they left behind on their long elliptical orbits of the sun. Vapor wouldn’t be able to produce meteors, so there had to be at least some solid material, but the extent of the coma and tail led us to believe that there was extensive ice. Recent probes (notably Giotto, Stardust, and Deep Impact) revised this concept, and this image from Rosetta helps confirm it: comets, or at least the ones we’ve managed close examinations of, are far more solid matter than originally theorized.
Will we be able to go out some night and spot comet 67P/Churyumov-Gerasimenko in the sky? Nope – not without a pretty decent telescope, since it estimated to, at best, only get to around magnitude 11. The limit of our vision in good viewing conditions is magnitude 6 or 7 (the smaller the number, the brighter the object – Sirius, the brightest star, achieves magnitude -1.46.) But that’s okay, because the Rosetta/Philae mission will provide plenty of detailed images – esa’s website on the mission is brimming with info already. And there are always a few comets that become faintly visible each year, though they often take some effort. Heavens-Above.com is a great resource for finding items of interest in the night sky, customized to your own location, and Stellarium is an excellent freeware program as well.
I will likely bring some updates in November as Philae drops down to the comet and tries not to bounce off (this is actually a serious consideration, since the comet measures only 4.5 km, or 2.8 mi, in length and thus has such feeble gravity that the lander will effectively weigh a few grams – it is equipped with augers in the feet to drill into the surface and hang on, and they’re designed so that they don’t push the lander away in the attempt.) Keep watching (this) space…