Asteroid orbit evolution due to thermal drag

Thermal drag, a variant of the Yarkovsky effect, may act on small asteroids with sizes from a few meters to a few tens of meters. Yarkovsky thermal drag comes from an asteroid's absorbing sunlight in the visible and reradiating it in the infrared. Since the infrared photons have momentum, by action-reaction, they kick the asteroid when they leave its surface. The reradiation, which is asymmetric in latitude over the asteroid, gives a net force along the asteroid's pole. Due to the asteroid's thermal inertia, averaging this force over one orbital period produces a net drag if the spin axis has a component in the orbital plane. A regolith-free basaltic asteroid 60 m in radius can shrink its semimajor axis by 2 AU (the distance from the asteroid belt to the Earth) over the age of the solar system. Regolith-free iron asteroids evolve at about half the rate of basaltic ones. These calculations ignore planetary perturbations, collisions, erosion, etc. The rate of evolution varies inversely with the asteroid's radius for the size range considered here, so that smaller objects evolve faster than larger ones. The rate-radius relation fails for objects smaller than a few meters because the thermal skin depth becomes comparable to the size of the asteroid. Basaltic asteroids covered by regoliths more than a few centimeters deep evolve much more slowly than regolith-free ones. Thermal drag tends to circularize orbits. It can increase or decrease orbital inclinations. An object whose spin axis points in random directions over its lifetime displays little change in orbital inclination. Thermal drag appears to have little to do with the delivery of chondrites from the asteroid belt; the thermal drag timescale (108 years for meter-sized 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 discovered 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 3×107 years, removing it from an Earth-crossing orbit. The near-Earth asteroids also tend to have small to moderate orbital eccentricities, as expected for highly evolved thermal drag objects. However, the time needed to bring them in from the asteroid belt (about 109 years) is long compared with the collisional and dynamical lifetimes (both about 108 years) for Earth-crossing objects, arguing against their emplacement by thermal drag.