Notes from a recent colloquium:

YORP is the acronym from Yarkovsky, O’Keefe, Radzievskii and Paddack, several of the people that described the effects of radiation on the behavior of small celestial bodies. Kepler actually first broached the idea that light from the sun could affect orbits, and the idea has been fleshed out since then. Yarkovsky proposed that the thermal radiation emission could cause a torque on a non-uniform body, and this was later expanded to include radiation pressure effects — the incoming radiation has an effect as well. Radzievskii’s contribution was the idea that albedo changes or differences could contribute, but since absorption and thermal re-emission doesn’t look much different than scattering, from a momentum standpoint, it turns out to be a small effect.

Paddack gave part of the talk, and described the experimental steps taken to investigate this. He took asymmetric rocks and let the fall in a pool — the still water was a proxy for a uniform radiation pressure once the rock had reached terminal velocity — and observed their motion, measuring the increase in rotation rate and deducing an empirical formula for the effect. Later he was able to construct a small-scale satellite that was hung in a vacuum chamber by a thin filament attached to a magnetic bearing (essentially no friction) and observed the angular acceleration when a bright light was shone on the target.

The effect has been observed, with 2000 PH5 getting a lot of press (well, geek press) about a year ago. The effect is more prevalent in smaller bodies — moment of inertia grows faster than surface area as objects of uniform density get larger. Since asteroids are often not held together strongly, increasing the rotation rate can cause them to eject mass, changing their orbit. And it’s not just spin rate; you can get precession and nutation as well, and there is an interplay with the gravitational torques that give rise to some resonances. It is thought that this explains the existence of some binary asteroids.

Nongravitational effects like this are important to know for manmade satellites, such as GPS, whose orbits need to be precisely determined.