Near‐Earth asteroids and the history of planetary formation

The asteroids are remnant planetesimals from the period of planetary formation ∼4.5 Ga ago. Although most probably experienced some thermal evolution during the earliest chapter of solar system history, few have been heated significantly since then, and hence they are “relics” of that era. Gravitational perturbations between Jupiter and the Sun prevented the largest asteroid, 1 Ceres, from becoming a major planet. A belt of planetesimals, ranging in size from 1000 km in diameter down to the limits of detectability, exists between the orbits of Mars and Jupiter (Figure 1). In addition, there are asteroids whose orbits cross those of the inner solar system planets, including Earth, which we collectively call Near‐Earth Asteroids (NEAs).