Comet Shapes

I don’t know about you, but until recently, I’ve never really thought about the shape of a comet. I’m not talking about the overall shape, with the bright coma and long, beautiful tail. No, I’m talking about the nucleus – the center or core of a comet.

We all know that comets are small icy bodies that on rare occasions enter the inner part of the Solar System and put on a stunning show as the ice vaporizes and becomes visible to us on Earth. But I hadn’t given much thought to what those small, icy bodies look like without their gaseous envelopes until NASA’s EPOXI mission spacecraft had a close encounter with comet Hartley 2 earlier this month.

I had always assumed that a comet looked like…well, like a dirty, icy snowball! I couldn’t have been farther from the truth. Take a look at the image to the right. As you can see, comet Hartley 2 is not spherical at all. In fact, it is a pretty bizarre shape!

So I started doing a little research. Turns out, comets are made of rock, frozen gases (CO2, methane, ammonia), water ice, and dust. Meaning….a lot of miscellaneous stuff – think of the pile of dust that comes out of your vacuum cleaner all clumped together when you empty it. Since they are relatively small – between 100 meters to 40 kilometers across – they have relatively little mass. This means that they don’t have enough gravity to pack all that stuff into a neat round ball. So, they take on a variety of shapes including the bowling pin shape of comet Hartley 2. For more pictures of comet Hartley 2, visit NASA’s EPOXI mission.