YARKOVSKY THERMAL DRAG ON SMALL ASTEROIDS AND MARS-EARTH DELIVERY

There is an optimal size for the delivery of small asteroids from Mars to the Earth by Yarkovsky thermal drag. Basaltic asteroids with radii of about 6 m take on the average 185 million years (Myr) for their se mimajor axes to shrink by 0.52 AU, assuming circular orbits and ignori ng planetary perturbations and collisions. All other sizes take longer . Bigger objects are slower because they are more massive, and smaller objects are slower because they are more isothermal. These results ar e based on treating the asteroids as spheres and solving the heat cond uction equation using spherical Bessel functions. The small near-Earth asteroids show a concentration of sizes in the thermal drag range; th us some of them may come from Mars as survivors of gravitational mecha nisms which eliminate them on the 10 Myr timescale. The possible role of thermal drag in Mars-Earth delivery will remain speculative until i t is included in numerical integrations of the orbits of small asteroi ds.