Swimming-pool science

Posted by Daniel Pendick
on Thursday, March 29, 2007

They say that Galileo dropped cannon balls and feathers off the Tower of Pisa to study gravity. It's not true, but the story remains in the collective memory as an example of how to do modern science: You go out and conduct an experiment yourself.

These days, lab experimentation isn't the only game in town. High-performance computers all us to run tests without having to drop things off towers. It's possible to simulate all kinds of physical processes using computer models. Decades ago, computing time was not cheap, and it was not abundant. So to figure out what makes asteroids spin, aerospace engineer Stephen Paddack jumped into a swimming pool to do an experiment.

Paddack is the "P" among the quartet of scientists whose work brought us the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect. I spoke to Paddack the other day to understand the significance of new research on the YORP effect published online March 8 by Science. He had some interesting stories to tell.

First, what is YORP? It's a process that acts on asteroids to alter their spin. When sunlight strikes an asteroid, it exerts a miniscule pressure, like wind hitting a sail. As the heated side of the asteroid turns away from the Sun and cools, it emits photons of infrared light. The photons act like the exhaust of a rocket engine, exerting a miniscule reaction force opposite to the direction they are moving. Depending on the asteroid's tilt with respect to the Sun, this pair of forces either increases or decreases the asteroid's spin.

Now, back to the swimming pool. In the ‘60s and ‘70s, Paddack investigated whether light alone could cause a geometric shape to rotate. He used a vacuum chamber to show that it was, in fact, possible to make an object turn by bouncing photons of light off it. It doesn't work if the object is a perfect sphere; the forces exerted by light cancel each other out. But a lumpy asteroid acts like a propeller turning in flowing water. So shape was important.

Paddack also gathered up lots of rocks randomly and watched from underwater as an assistant released the stones. "All but one of them spun," he told me. The other one just fluttered to the pool floor like a falling leaf. These experiments helped him to calculate the forces acting on an actual asteroid in space.

Paddack's work helped lay the foundation for the recent work by astronomers in the United States and Europe, who confirmed that YORP does alter the spin of real asteroids — in this case, a space rock called PH5 2000, which the researchers observed over a period of 4 years. Everyone knew YORP must exist, but nobody had yet caught the effect in the act out in space.

Today, somebody could probably just crank up a desktop supercomputer and figure out the same thing that Paddack did in his experiments. But swimming-pool science sounds like it would be more fun and refreshing.

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