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Rock Heavy

Week of February 16, 1998

Regular readers know that I have this thing for asteroids. Just run my Bad Astronomy website search engine with the keyword set to "asteroid" to see how many pages have that word in them! But asteroids are inherently interesting: we've known they've existed since 1801, when Ceres was discovered, yet we only have a handful of good images of them, and in many cases we can only guess at their masses.

images of asteroids

At least, until now. James Hilton, at the U.S. Naval Observatory, has calculated the most accurate masses yet for the three biggest asteroids known. He made computer models of the orbits of the asteroids, then compared the plots to actual observations of the rocks (some observations going all the way back to 1801!). By using so many observations, they were able to tell very accurately just how much the asteroids were affecting each other gravitationally. From this he was able to get the masses. For the record: Ceres masses about 8.7 x 10^20 kilograms Pallas is 3.18 x 10^20 kilograms and Vesta is 3.0 x 10^20 kilograms. Does this sound like a lot? Well, that means Ceres is only about 1% the mass of the Earth's Moon, and the others are even smaller!

Still, not much is known about asteroids. The recent NEAR mission revealed that the asteroid Mathilde has enormous craters in it, some of them astonishingly so big that they are actually wider than the radius of Mathilde! Obviously, we need to do some rethinking about these critters. Hopefully, when NEAR goes into a parking orbit around Eros soon, we will learn a lot. Of course, the more we learn, the more questions we learn to ask. One of the things I love about science: it's a never-ending pursuit!

©2008 Phil Plait. All Rights Reserved.

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