From a "new star" to the asteroid belt
The 18th-century Titius–Bode rule suggested a missing planet between Mars and Jupiter. On 1 January 1801 Giuseppe Piazzi in Palermo found a moving object there, Ceres, but soon lost it in the Sun's glare. The young Carl Friedrich Gauss devised a new way to compute an orbit from few observations, allowing Ceres to be recovered the following year; he published the method in 1809. As Pallas, Juno and Vesta followed, William Herschel called them "asteroids" — star-like — because they looked like points. In 1867 Daniel Kirkwood explained gaps in the belt (Kirkwood gaps) caused by resonances with Jupiter's gravity. More than a million asteroids are now known, yet together they total only a few per cent of the Moon's mass.
The impact that ended the dinosaurs
In 1980 physicist Luis Alvarez, his geologist son Walter and colleagues found abnormally high iridium — rare in Earth's crust — at the rock boundary from about 66 million years ago, and proposed that an asteroid roughly 10 km wide caused the mass extinction that ended the non-avian dinosaurs. Identification of the roughly 180 km Chicxulub crater in Mexico's Yucatán in 1991 strongly supported the idea, though how much the Deccan volcanic eruptions in India contributed is still debated. In 1908 an object tens of metres across exploded over Tunguska, Siberia, flattening vast forest, and in 2013 the Chelyabinsk meteor's shock wave injured more than 1,000 people in Russia.
Exploration and planetary defence
Asteroids preserve nearly unchanged material from the birth of the Solar System, making them time capsules. Japan's Hayabusa showed in 2005 that Itokawa is a loose rubble pile and in 2010 returned the first asteroid sample to Earth. Hayabusa2 reached Ryugu in 2019 and delivered samples in 2020, in which amino acids and other organics were found. NASA's OSIRIS-REx found Bennu's surface far rougher than expected and returned a sample in 2023. In 2022 NASA's DART spacecraft deliberately struck the asteroid Dimorphos, shortening its orbit by about 33 minutes — the first demonstration that a spacecraft can change an asteroid's path. No large asteroid is currently known to be on course to hit Earth, and agencies keep surveying near-Earth objects. Asteroids differ from comets in having little ice and no tails.
FAQ
- What is the difference between an asteroid, a meteor and a meteorite?
- An asteroid orbits in space, a meteor is the streak of light as a small fragment burns in the atmosphere, and a meteorite is a piece that reaches the ground. Most meteorites are fragments of asteroids.
- Should we worry about an asteroid hitting Earth?
- Most asteroids larger than 1 km have been found and tracked, and none is known to threaten Earth in the next century. Small objects fall often but mostly burn up in the atmosphere.
Sources reviewed: 2026-10-26
References
- Piazzi, G. (1801). Risultati delle osservazioni della nuova stella scoperta il dì 1 gennajo 1801 all'Osservatorio di Palermo. Palermo.
- Gauss, C. F. (1809). Theoria motus corporum coelestium in sectionibus conicis solem ambientium. Hamburg: Perthes & Besser.
- Kirkwood, D. (1867). Meteoric Astronomy: A Treatise on Shooting-Stars, Fire-Balls, and Aerolites. Philadelphia: Lippincott.
- Alvarez, L. W., Alvarez, W., Asaro, F., & Michel, H. V. (1980). Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science, 208(4448), 1095–1108.
- Hildebrand, A. R., et al. (1991). Chicxulub crater: A possible Cretaceous/Tertiary boundary impact crater on the Yucatán Peninsula, Mexico. Geology, 19(9), 867–871.
- Chapman, C. R., & Morrison, D. (1994). Impacts on the Earth by asteroids and comets: Assessing the hazard. Nature, 367(6458), 33–40.
- Fujiwara, A., et al. (2006). The rubble-pile asteroid Itokawa as observed by Hayabusa. Science, 312(5778), 1330–1334.
- Watanabe, S., et al. (2019). Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu — A spinning top–shaped rubble pile. Science, 364(6437), 268–272.
- Lauretta, D. S., et al. (2019). The unexpected surface of asteroid (101955) Bennu. Nature, 568(7750), 55–60.
- Daly, R. T., et al. (2023). Successful kinetic impact into an asteroid for planetary defence. Nature, 616(7957), 443–447.