The European space probe Rosetta has been hanging out with the comet 67/P Churyumov-Gerasimenko since early August. Initially at station-keeping about 100 kilometers away, it’s now dropped down to less than half that. In November it’ll release the lander Philae to set down on the comet, and scientists are deciding now just where to put it.
In the meantime, Rosetta is snapping away, taking a lot of pictures and data. And the comet hasn’t been quiet: A jet of gas has formed coming from the neck region!
That image above was put together by my friend Emily Lakdawalla, and I leave it to her capable keyboarding to describe how she made this image and to give details of the science.
However, I want to add that the jet is very interesting. ALICE, an ultraviolet detector on board Rosetta, has been taking observations and found very little water ice on the surface of the comet. That’s a bit surprising, since we know comets tend to have lots of ice in them. However, ALICE did see oxygen and hydrogen surrounding the comet. That means there’s water ice under the surface, and it’s getting out. I have to say, that jet seems the likely source.
Unlike asteroids, the surfaces of comets constantly change, especially when they near the Sun. It gets warm enough to turn ice into gas, which then blows away in jets and forms the fuzzy coma surrounding the solid nucleus. That’s why the surface of Chu-Ger doesn’t look like an asteroid; the impacts aren’t as obvious when the outer layers get resurfaced all the time. Also, the comet isn’t solid, like a chunk of rock, but more likely crunchy. Impacts won’t leave your more typical-looking craters in that sort of material.
This is all very exciting! We’ve flown missions past comets before, many times, but this is the first time a probe has stuck around. The comet is slowly approaching the Sun in its orbit, and will reach its closest—and therefore warmest—point in its orbit around the Sun next August. Rosetta’s mission isn’t scheduled to end until a few months after that, so it’ll ride the comet down and watch as activity returns to this dirty snowball. And we get to ride along and watch the whole time too.