A dead comet for SOHO

Contributed by
Sep 26, 2007
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Sometimes astronomy news isn't spectacular, but it is still so way cool.

SOHO is an observatory that is parked about a million miles towards the Sun, where it can stare at the nearest star all the time. One instrument, LASCO (the Large Angle and Spectrometric Coronagraph), has a little paddle that blocks the fierce light of the Sun itself so that it can see the fainter hiccups and belches of billions of tons of plasma erupting from the surface.

But it can also see stars in the background, and planets, and, sometimes, comets. They are really amazing to watch in the animations as they streak in toward the Sun. Most -- and over a thousand have been seen -- evaporate away and are never seen again.

But a sharp-eyed observer named Sebastian Hoenig, a German PhD student, found one that came back! Although it's not clear from the press release, I assume he was tracking comets and calculating the orbits of them. In 2005 he realized that two comets may be one and the same because the mathematics of their orbits (shape, size, period, etc.) were so similar. Assuming he had seen two apparitions of the same object, he predicted it would return in early September, 2007. Bang! There it was, right on schedule.

Now called, P/2007 R5 (SOHO), it's a weird one. Comets usually get a tail, especially when close to the Sun. This one doesn't. But it does brighten a lot when it gets close to the Sun, like comets do. It's probably an extinct (well, almost extinct) comet nucleus. It used to be rock and ice, but now it's almost all rock, with some ice still buried inside that only sublimates (turns into a gas) right when the comet is closest to the Sun.

It's tiny, maybe 200 meters across, so it's basically invisible to Earth-based telescopes, and only becomes visible when it's near the Sun. Over time, any comet could become one of these weirdos, so it may not be all that weird-- there could be thousands more like it on short orbits (this one has a four year orbit).

If you've never done it before, go poke around the SOHO website (linked above). It's an amazing success story from NASA and ESA, with tons of incredible information, pictures, and animations. It's time well spent.