Asteroids

Near-Earth objects (NEOs) are temporary visitors in the region around the terrestrial planets, and significant work has gone into understanding their dynamical behavior and lifetimes. Gladman et al. (2000) and others found that NEOs have short lifetimes, only about 10 Myr on average. Gravitational perturbations from planets, collisions with the terrestrial planets, or loss into the Sun or ejec-tion of a constantly re-filled steady-state population. Cratering re-cords (Grieve and Shoemaker, 1994; Stöffler and Ryder, 2001) find that the impact flux on the terrestrial planets and the Moon has been relatively constant on Gyr timescales, supporting the idea of a steady-state population. The problem of how to refill NEO space from the seemingly sta-tic Main Asteroid belt was answered with the discovery that aster-oids are quite mobile (Bottke et al., 2000, 2006). Thermal forces work via the Yarkovsky-effect to change asteroid’s semimajor axe