Every asteroid in the main belt combined weighs just 3 percent of our Moon
Roughly a million asteroids are known, most circling between Mars and Jupiter, yet pile them all together and they would make only 3 percent of the Moon. The first was found by accident on New Year's Day 1801, by a priest in Palermo who thought he had spotted a comet.
Astronomers were half expecting it. In 1772 Johann Elert Bode, drawing on Johann Daniel Titius, published a number sequence that seemed to predict planetary distances, apart from a suspicious gap near 2.8 astronomical units. When William Herschel found Uranus close to where the pattern said a planet beyond Saturn should be, the idea gained credibility. In 1800 Franz Xaver von Zach recruited 24 observers, nicknamed the celestial police, to hunt for the missing world. Before his invitation even arrived, Giuseppe Piazzi noticed a faint point near a catalogued star and watched it shift night after night. He had found Ceres. The formula is now discredited, but the police went on to find Pallas, Juno and Vesta.
Early telescopes showed these bodies as starlike dots that moved, so William Herschel coined asteroid, from Greek for star-like. For decades the word was used interchangeably with planet. Today the International Astronomical Union prefers small Solar System body, and since 2006 Ceres, about 975 kilometres wide, counts as a dwarf planet. Asteroids come in carbon-rich, metallic and stony types, from rubble piles under a kilometre across upwards. Only Vesta, with its brighter surface, can normally be seen without a telescope.
The line between asteroid and comet is blurrier than it looks. Comets grow a glowing coma as sunlight boils off their ices, but some objects first labelled minor planets later showed such activity, and old comets may exhaust their ices and turn asteroid-like. Dust returned by the Stardust probe pushed scientists towards seeing a continuum rather than a clean divide.
Close-up exploration began with the Galileo spacecraft. Japan's Hayabusa missions brought back pieces of Itokawa and Ryugu, OSIRIS-REx delivered a sample of Bennu in 2023, and in September 2022 NASA's DART deliberately rammed the harmless Dimorphos to test planetary defence. The stakes are real: the Chicxulub impact is widely blamed for the extinction that ended the Cretaceous.
Source: Asteroid
