Asteroids are small, rocky objects that orbit the Sun. They are much smaller than planets.
Although most of them reside in the main asteroid belt between the orbits of Mars and Jupiter, some are found in the orbital path of planets. NASA currently counts more than one million of them.
Asteroids are left over from the formation of the solar system around 4.6 billion years ago. All asteroids are different as they formed in different locations and different distances from the Sun. They have irregular shapes and sizes, from hundreds of miles in diameter to the size of pebbles, and they have different compositions, from rock to clay or metals such as nickel and iron. Many larger ones have one or more small moons, eg. Didymos, a half mile wide asteroid orbited by the moon Dimorphos which is only 160 metres (525 feet) across. The largest is Vesta at 330 miles (530km) across.
Asteroids are larger than meteoroids, with the latter being a fragment chipped off an asteroid or comet.
The sunlight reflected from an asteroid's surface is affected by its composition, so analysis of the spectra they produce can tell us about their surface compositions. The spectra have been classified into the following types: A, B, C, D, E, F, G, M, P, Q, R, S, T and V, with types S, C and M being the most common and corresponding to the spectra of common meteorites that arrive on Earth. The majority of asteroids are small, irregularly shaped rocky bodies, with varying amounts of metals.
We know of over 200 asteroids that are larger than 100 km in diameter, and up to 1.7 million asteroids that are larger than 1 km in diameter, however most are potato or peanut-shaped lumps of rock. Some are double, or multiple, systems orbiting around their barycentres. Most have orbital radii of 2.5 to 3.5 AU - the Main Belt asteroids. Many of the Main Belt asteroids have very elliptical and highly inclined orbits and occasionally one can pass near the Earth, such as Hermes in 1937 which came within 800,000 km of our planet. Analysis of the orbits of the Main Belt asteroids has led to groupings such as the Hungaria, Flora, Phocaea, Karonis, Eos, Themis, Cybele and Hilda groupings. The members of each group also share characteristics, such as surface composition and orbital inclination, which has suggested that each group resulted from the break up of a former small parent body. Each group is divided from other groups by a Kirkwood Gap. These are zones of avoidance, mostly at radii from the Sun where any object would have an orbital period that is a simple ratio of Jupiter's. The tidal tugging of Jupiter on any asteroid tends to pull it out of orbit. Close to Jupiter this is reversed and asteroids are concentrated in orbits that are simple ratios of Jupiter's orbital period. It is thought that Jupiter was also responsible for inhibiting the formation of a planet-sized body, but several smaller (Ceres-sized) planetoids were formed, along with a lot of left-over debris. Multiple collisions then broke up these planetoids to form the asteroids we see today, with the gravitational resonance effect of Jupiter clearing out the lanes of avoidance. Collisions are still occurring, supplying fresh dust to the Solar System, with some impacting on planet surfaces or burning up in their atmospheres as shooting stars. Other families of asteroids are classified in terms of their orbits, eg.
• Near Earth Asteroids (NEAs) - their orbits bring them close to Earth;
• Trojans - 2 small groups of asteroids trapped in the orbit of Jupiter. They occupy two regions of gravitational stability, are 60° in front and the other 60° behind Jupiter itself; and
• Centaurs - a group near the edges of the planetary system.
In recent years several asteroids have been visited by passing spacecraft: On 12 February 2001 NASA's NEAR Shoemaker spacecraft landed on the asteroid Eros. NEAR began orbiting Eros a year earlier, taking close up photographs and measuring the size and shape of Eros before landing. The Dawn spacecraft then flew by the asteroid Vesta and the dwarf planet Ceres. Japan also launched a spacecraft named Hayabusa2, which was launched in 2014 on a mission to survey asteroid Ryugu, as well as returning samples to Earth in 2020 .
Another example of a spacecraft visiting an asteroid is NASA's OSIRIS-REx spacecraft which traveled to the near-Earth asteroid Bennu and brought a small sample back to Earth for a study in September 2023. The mission launched in 2016 from Cape Canaveral Air Force Station, and reached Bennu in 2018.
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