Meet the asteroids

Are they gonna hurt us?

Will they obliterate the earth?

How are asteroids found?

Did asteroids deliver life?

What are they made of?

What about comets?

Asteroids in orbit

Orbital oddities
At one time, the distinctions seemed so neat. Asteroids were rock, and comets were dirty snowballs. But a gathering glut of data is murking up the categories. David Lien, a visiting assistant professor of physics at Washington and Jefferson College, has recently found a class of objects -- (well, at least three), that were originally called comets but may actually be asteroids.

1998T3model2 T O P: a negative image of Comet Lagerqvist-Carsenty. The length of the dust tail and its slow expansion are highly unusual for comet tails. Courtesy Hermann Boehnhardt and Olivier Hainaut at the European Southern Observatory.
B O T T O M: a computer simulation showing how the object would look if all dust particles had been emitted by a collision one year earlier. So this "comet" is probably an asteroid surrounded by dust emitted by an impact one year before the top image was taken. Courtesy David Lien.

Like all comets, these odd things have a tail. But they lack a coma -- an expanding halo of gas and dust that forms when the ice vaporizes as the comet approaches the sun. (In a coma on comets? Check our coverage.)

Using software developed to analyze comet's tails, Lien found that the tail seemed to start at a single moment in time -- which he could calculate. To Lien, this shows that the body had been blasted by an impact which shook loose some dust that slowly expanded to become the "tail."

In other words, the "comets" were actually asteroids, as David Balam at the University of Victoria (British Columbia, Canada) had first suggested. But asteroids with tails, you understand...

The results are evidence that asteroids suffer not only the catastrophic collisions that form "families" of smaller asteroids in the asteroid belt, but also cratering collisions that gouge their surfaces and stir up some dust and debris without necessarily busting them apart. The findings hint toward a zoolike diversity for comets and asteroids. According to asteroid expert Donald Yeomans, these sub-planetary thrillers run the gamut from "fluffballs to rocks to slabs of stainless steel."

But how do asteroids get here in the first place?

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The Why Files
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