According to current scientific knowledge, asteroids (and also comets) are small bodies left over from the time that the planets formed 4.5 billion years ago. The four rock- and metal-rich inner planets and the four outer gas giants in the Solar System were formed from the gas and dust surrounding the nascent Sun.
During this process, planetesimals – the growing but not fully-formed planets – were initially created from dust particles bonding together. These are the building blocks of planets. Impacts between planetesimals led to further growth of these bodies. Some agglomerated and formed small planet-like bodies. However, the Solar System was very much more dynamic in its early stages than it is today, and the bodies in the inner part of the system collided at very high speeds. In many cases, this led to the small bodies shattering and the resulting fragments are today known as asteroids. These contain not only information about the origin and development of the planets, but also about their original composition.
Asteroids are bodies composed of rock and metal, and sometimes ice, which circle the Sun between the orbits of Mars and Jupiter and vary greatly in size. Due to their orbital dynamics and physical characteristics – such as the lack of a permanent atmosphere and a weak gravitational field – these small objects form a special class of celestial body.
Due to their different sizes, some asteroids are also referred to as 'minor planets'. The first asteroid to be discovered was Ceres, the largest object in the Asteroid Belt. Due to its size (a diameter of approximately 945 kilometres) and its associated balanced, nearly spherical shape, Ceres is now classified as a dwarf planet.
Asteroids are generally much smaller than planets. Some are orbited by smaller 'moons' or another asteroid; these are referred to as double asteroids. There are millions of asteroids in the Solar System and new ones are being discovered daily.
Asteroids are mainly located in the Asteroid Belt, a region located between the orbits of Mars and Jupiter. Due to collisions and gravitational interactions with planets (principally Jupiter) some asteroids, however, do sometimes cross the orbits of Mars and Earth. An asteroid whose orbit can bring it close to Earth (in astronomical terms) is referred to as a 'Near Earth Object' (NEOs). When these NEOs are close to Earth they can be observed relatively easily using both Earth-based and space telescopes. The aim of such observation campaigns is to determine important statistics such as size, brightness (albedo), surface properties and composition. This knowledge of NEOs is important for gaining information about their origin, and their relationship with the asteroids in the asteroid belt and with meteorites.
What does an asteroid consist of?
The physical properties of asteroids vary. We know that they often consist of rock and metal, and that they can contain traces of amino acids and other organic compounds, but there is still a lot of research to be done.
There are several types of asteroids. The three most important ones are:
- C type – this group comprises asteroids that contain carbon and is the most abundant asteroid type
- S type – these asteroids are composed of silicate rock and are the second most frequent type
- M type – these asteroids largely consist of metal and form the third most frequent type
Comet, asteroid, meteorite – what is the difference?
In contrast to comets, asteroids do not show active comet behaviour such as outgassing, a coma or a tail. Comets are much more abundant than volatile asteroids, which vaporise when heated. Meteorites are asteroids that have landed on Earth having not completely burned up in the atmosphere.
In spite of their difference to comets – which consist of clumps of dust and ice instead of metal and rock – asteroids and comets alike fascinate scientists. As remnants of the origins of the Solar System, it is thought that both classes of bodies contain the oldest organic materials in the Solar System. The development of terrestrial (Earth-like) planets has been significantly influenced by impacts from small bodies like these. It is possible that they helped to create the conditions necessary for the development of life – perhaps by bringing the basic building blocks of life to Earth, or delivering a large proportion of the waters in Earth's oceans. How can this be verified? The simplest way is to examine different asteroid types. The Hayabusa2/MASCOT mission will do this at the C class asteroid Ryugu.