ASTEROID ORBIT EVOLUTION DUE TO THERMAL DRAG

Thermal drag, a variant of the Yarkovsky effect, may act on small aste roids with sizes from a few meters to a few tens of meters. Yarkovsky thermal drag comes from an asteroid's absorbing sunlight in the visibl e and reradiating it in the infrared. Since the infrared photons have momentum, by action-reaction, they kick the asteroid when they leave i ts surface. The reradiation, which is asymmetric in latitude over the asteroid, gives a net force along the asteroid's pole. Due to the aste roid's thermal inertia, averaging this force over one orbital period p roduces a net drag if the spin axis has a component in the orbital pla ne. A regolith-free basaltic asteroid 60 m in radius can shrink its se mimajor axis by 2 AU (the distance from the asteroid belt to the Earth ) over the age of the solar system. Regolith-free iron asteroids evolv e at about half the rate of basaltic ones. These calculations ignore p lanetary perturbations, collisions, erosion, etc. The rate of evolutio n varies inversely with the asteroid's radius for the size range consi dered here, so that smaller objects evolve faster than larger ones. Th e rate-radius relation fails for objects smaller than a few meters bec ause the thermal skin depth becomes comparable to the size of the aste roid. Basaltic asteroids covered by regoliths more than a few centimet ers deep evolve much more slowly than regolith-free ones. Thermal drag tends to circularize orbits. It can increase or decrease orbital incl inations. An object whose spin axis points in random directions over i ts lifetime displays little change in orbital inclination. Thermal dra g appears to have little to do with the delivery of chondrites from th e asteroid belt; the thermal drag timescale (10(8) years for meter-siz ed objects) is long compared with their cosmic ray exposure ages, and aphelia in the asteroid belt are not expected for mature thermal drag orbits. However, Yarkovsky thermal drag may act on the recently discov ered near-Earth asteroids, which have radii of 10-30 m. Asteroid 1992 DA, for instance, might have its orbit shrunk by 0.1 AU in 3 x 10(7) y ears, removing it from an Earth-crossing orbit. The near-Earth asteroi ds also tend to have small to moderate orbital eccentricities, as expe cted for highly evolved thermal drag objects. However, the time needed to bring them in from the asteroid belt (about 10(9) years) is long c ompared with the collisional and dynamical lifetimes (both about 10(8) years) for Earth-crossing objects, arguing against their emplacement by thermal drag.