Wednesday, May 07, 2008

2008 DDA meeting, Part 2

Monday, April 28, 2008
Session 2: The Yarkovsky and YORP Effects
Invited Talk: Analytic Theory of the YORP Effect for Near-Spherical Objects

David Nesvorn√Ĺ, Southwest Research Institute

SwRI's own David gave a nice invited talk on YORP, a thermal effect that changes the spins of small bodies. The YORP effect was confirmed for two Near-Earth Asteroids (NEAs) in 2007, resulting in fame, fortune, and papers in Nature and Science for two groups. YORP may be the main mechanism that forms binaries among the NEAs. Most modeling of YORP has been done numerically, but David and David derived an expansion of the YORP torque in spherical harmonics that agrees well with numerical results for reasonably round bodies such as 1998 KY26.

In the same session, Steve Chesley of JPL gave a talk describing his work with David on a clever approach to look for the Yarkovsky effect in NEAs by modeling it as a transverse acceleration (at), just as he does to model nongravitational forces in comets. The model value of at translates into a drift rate in semi-major axis, which is the key effect he's looking for. As I recall, he has tentative detections for over a dozen NEAs. It just goes to show how prescient I was a decade ago when I got a proposal to review on how Yarkovsky might affect the orbits of asteroids. Many years before, I had learned of the Yarkovsky effect in a course taught by Joe Burns while he was on sabbatical in Berkeley. I had Yarkovsky pigeonholed as one of those strange radiation effects like Poynting-Robertson that applies to dust and rocks, but certainly not to mountains in space. Now there's a veritable Yarkovsky industry. If you want to read all about it, here's a review paper by Bill Bottke and the Davids. As Bill likes to say, with two Davids we can Czech and re-Czech our results.