Solar System
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"Asteroids." Space, Solar System, Q-files Encyclopedia, 25 Sep. 2023.
https://www.q-files.com/space/solar-system/asteroids.
Accessed 19 Mar. 2024.
Asteroids 2023. Space, Solar System. Retrieved 19 March 2024, from
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Space, Solar System, s.v. "Asteroids," accessed March 19, 2024.
https://www.q-files.com/space/solar-system/asteroids
Asteroids
Circling the Sun, mainly between the orbits of Mars and Jupiter, are millions of small, planet-like objects called asteroids or minor planets. The largest, Ceres, discovered in 1801—and, since 2006, upgraded to the status of "dwarf planet"—measures just over 1000 kilometres (600 miles) across, but only a few asteroids (around 200) have diameters greater than 100 kilometres (60 miles). It is thought there may be at least a million objects with a diameter of 1 kilometre or more. About 800,000 asteroids have so far been discovered, of which about 22,000 have been named. Many more are too small to be identified.
Updated 25th September 2023
Orbital paths
Most asteroids lie in bands between Mars and Jupiter, although some do not. The Trojan asteroids, for example, share Jupiter’s orbital path, while Apollo, Icarus and Adonis actually pass inside Earth’s orbit. In fact, more than 7000 near-Earth asteroids have been identified, of which more than 500 are estimated to measure over 1 kilometre (0.6 miles) in diameter.
Ceres alone contains about a third of all the matter in the Asteroid Belt. In fact, if all the asteroids were put together they would still be much smaller than Earth’s Moon.
Formation
Asteroids are small, irregular bodies, covered with craters. Most are rocky, but a few are made of ironand nickel. Astronomers believe that, during the formation of the Solar System, Jupiter’s strong gravitational pull caused nearby planetesimals to smash into one another rather than build up into another planet. This left the belt of rock fragments we call the asteroids. They have continued to collide with one another ever since, producing smaller fragments called meteoroids (although most meteoroids are have broken off from comets), some of which fall to Earth as meteorites.
Studying asteroids
From the study of meteorites that have come from asteroids, scientists have found valuable evidence on how the planets were formed. The fragments have been dated to 4.6 billion years ago, indicating that this is the age of the Solar System itself.
Space probes have been launched to study asteroids at close quarters. The first Near-Earth Asteroid Rendezvous (NEAR) craft, Shoemaker, flew by Mathilde, which has an orbit close to that of Earth’s, in June 1997. It landed on the surface of Eros in 2001. In July 2011, space probe Dawn started orbiting Vesta, and flew on to Ceres in 2015.
Ceres
In 2015 the Dawn space probe arrived at Ceres, the largest object in the asteroid belt. It circled the dwarf planet for several months, mapping its surface. Ceres was the first asteroid to be discovered, in 1801 by the Italian monk Giuseppe Piazzi. It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered. Ceres officially became a “dwarf planet” in 2006, along with Pluto.
Dawn discovered a number of bright spots on the dwarf planet's surface. From the close study of one of them, located inside a giant crater called Occator, scientists found that it was dome structure made of salts which are formed in the presence of liquid water. It is therefore probable that the dome is the remains of an ice volcano (cryovolcano), through which liquid salty water from beneath Ceres's crust has erupted at the surface.
Ceres is thought to have a rocky core surrounded by a 100-kilometre (60-mile) layer of ice—about a quarter of its mass. From the evidence of the ice volcanoes, scientists believe that some of Ceres' water is in liquid form, as a very salty solution, below its surface.
Bennu
Bennu is a near-Earth asteroid measuring about 490 metres (1610 feet) across. Discovered in September 1999, it is a carbonaceous, or C-type, asteroid, so-called because it contains large amounts of carbon. Astronomers have detected the presence of water contained within its rocks. There is a 1-in-1750 chance of Bennu crashing into Earth between 2178 and 2290 (the greatest risk of it doing so is 24th September 2182).
The asteroid was the target of the OSIRIS-REx space probe mission, launched in December 2018. In October 2020, the probe landed on Bennu, collected a sample of the surface rock, before departing in May 2021. A capsule containing the sample landed on Earth in September 2023.
Asteroid impact
Occasionally, asteroids fall to Earth. Surviving the searing heat as they plunge through the atmosphere, they range in size from a few centimetres to huge boulders measuring many metres across. The larger ones punch huge craters in Earth’s surface. One day, far into the future, a large asteroid will crash to Earth, causing widespread devastation and dramatic changes to climates worldwide. A major impact almost certainly contributed to the extinction of the dinosaurs 66 million years ago.
Consultant: Mike Goldsmith