Ten Things You Don't Know About Comets

By Phil Plait | April 20, 2010 6:00 am


The actual solid part of a comet is tiny.

When you see a picture of a comet it looks huge– and the part you’re seeing really is. The head of the comet — the big fuzzy ball at the front — can be hundreds of thousands of kilometers across, bigger than whole planets. But what you’re seeing is incredibly low-density gas warmed by the Sun, leaking out from the solid part of the comet called the nucleus. In general, the gas is thinner than a laboratory vacuum. It’s very good at reflecting sunlight, so it appears bright, but in reality it’s as ethereal and thin as a politician’s promise.

Amazingly, the solid nucleus of a comet is usually only a few kilometers across, far too small to see in images. The picture here is of Comet Holmes in 2007, taken by Tamas Ladanyi. While Holmes was on its way out, away from the Sun, there was a huge outburst probably caused by some gas leaking out of the nucleus. A vast cloud of gas expanded around it, and even though it was tens of millions of kilometers away from Earth, it was easily visible as a fuzzy patch in the sky. But the nucleus is so small that no telescope on Earth could’ve seen it as more than a tiny dot. In this picture, it is far smaller than the size of a pixel!



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