Asteroids, dwarf planets and comets

Asteroids are rocky or metallic bodies smaller than planets; most lie in the asteroid belt between the orbits of Mars and Jupiter (≈ 2.1–3.3 AU), whose total mass is only a few percent of the Moon’s. The first asteroid, Ceres, was found by Piazzi on 1 January 1801; it is now a dwarf planet ≈ 940 km across (about a third of the belt’s mass). Vesta is ≈ 525 km. Orbits in resonance with Jupiter produce the Kirkwood gaps. A dwarf planet orbits the Sun, is nearly round under its own gravity, but has not cleared its orbit: Ceres, Pluto (visited by New Horizons in 2015), Eris, Haumea, Makemake. Near-Earth asteroids may be hazardous: ≈ 66 million years ago an object ≈ 10 km across made the Chicxulub crater and caused the extinction of the dinosaurs; in 2013 an object ≈ 20 m wide exploded over Chelyabinsk. The asteroid Apophis will pass the Earth at ≈ 32,000 km on 13 April 2029 (no impact risk). In September 2022 NASA’s DART spacecraft hit the asteroid Dimorphos and shortened its orbital period by ≈ 32 minutes – a test of the deflection method.