Monday, March 12, 2007

Spinning up asteroids

This weekend I read this article about how scientists have measured an asteroid's rate of rotation (spin rate) increasing due to sunlight. This effect is called the YORP effect, after the scientists who first described it: Yarkovsky, O'Keefe, Radzievskii, and Paddack.

The effect is very subtle. (When astronomers say "subtle," we often mean "complicated and hard to wrap your brain around.") When sunlight hits an asteroid (or any planet), much of that sunlight is absorbed and converted to heat. This is why the temperature warms up in the daytime! That heat is then radiated back into space as infrared photons.

Photons carry momentum. They carry very small amounts of momentum, but when you have gazillions of photons, that momentum can start to add up, especially for small things. Particles of dust in space near the earth spiral into the sun in less than 3000 years due to momentum from this infrared radiation.

In order to change the spin of an asteroid, it has to be odd-shaped. Think of this kind of like a windmill. If you put a ball on top of a tower, the wind won't cause it to spin. But it you put up odd-shaped blades, you can make a windmill that spins very rapidly. The effect is similar for asteroids. Round asteroids (and planets) don't get spun up by sunlight, but odd-shaped asteroids can. In fact, many small, funny-shaped asteroids spin very fast; this theory explains why.

So, what is the news here? Until recently, this effect was a hypothesis. We had never measured any asteroid spinning faster and faster. But now Stephen Lowry, an astronomer from Queens University in Belfast, has led a team that measured the spin rates of two asteroids: 2000 PH5 (this asteroid doesn't have a "real" name yet) and Apollo. And the spins of these rocks are increasing at a rate consistent with theory.

What does this mean for you? Another effect similar to the YORP effect, called the Yarkovsky effect, claims that the actual orbits of asteroids could be changed from the momentum of light. If this theory is true, if we find an asteroid that will one day in the future hit the Earth, we could change its orbit by "painting" part of the asteroid with white dust rather than needing to launch a nuclear weapon or a large explosive. However, we need to be sure we understand the effect first, or else such deflection may not work or may even work in reverse, causing a rock to hit us all the sooner.

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