Ten Things You Don't Know About Comets

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




Comets spend most of their lives looking pretty much like asteroids.


Comets tend to have long, elliptical orbits. The farther they are from the Sun, the slower they travel, so really they might spend 99.9% of their lives far, far from the heat of our nearest star. That in turn means that all that volatile stuff in the nucleus is frozen, and actually it gets so cold that even the water freezes into ice harder than rocks on Earth.

That image above is actually of Comet Halley! It was taken in 1982, about three years before passed us once again. It was out past the orbit of Saturn when the Palomar observatory took this image (it’s a negative, which helps our eyes seen faint objects). The comet is circled, and if it weren’t you’d never notice it. That far from the Sun, there’s no outgassing, no spectacular head or tail. Just a little dot, frozen hard as nails.

In fact, the difference between asteroids and comets may just be how much stuff is in them that doesn’t blow away when they get near the Sun. After all, if we see a rock with just a little bit of gas coming out of it, is it an asteroid or a comet? In fact, many asteroids are in comet-like orbits; maybe some had their orbits affected by Jupiter, or maybe they are simply burned out comets.



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