Asteroids and Distant Minor Planets: Time Capsules of the Early Solar System

A 12,000-word introduction to the unfairly maligned 'vermin of the sky'.

The minor planets, as their name implies, are small. Even the dozen or so largest, some of which are now ranked as 'dwarf planets', are far smaller than the Moon ? and these account for a thousandth of one per cent of the total known to astronomers. Most are far smaller. Only one, 4 Vesta, is normally ever visible to the naked eye. When well placed, it can attain a magnitude of +5.1. Two others, 1 Ceres and 2 Pallas, are just below the typical naked eye threshold of +6.0, and can only be seen under exceptional conditions and by people with exceptional eyesight. Around 35 main-belt asteroids can attain a magnitude of around +10 at opposition, and are within range of a good pair of 10×50 binoculars. However, before the introduction of adaptive optics in the mid-1990s, even the largest ground-based telescopes could not make out any surface detail and could do little more than determine rotational periods. Space probes have since changed the picture. The first close-up images of a minor planet were returned by the Galileo orbiter in 1991 as it made a flyby of the asteroid 253 Mathilde en route to Jupiter. Several asteroids have now been visited by space probes, and samples have been returned to Earth from the asteroids 25143 Itokawa, 101955 Bennu, and 162173 Ryugu.

Asteroids, once dismissed as "vermin of the skies" and "of very little interest to anybody", are now of great interest, and not just to astronomers. Though we are understandably concerned about the dangers of near-Earth asteroids colliding with our planet, these bodies are also increasingly viewed as a potential source of materials critical for both space exploration and terrestrial use. Metallic (M-type) asteroids may contain high concentrations of iron, nickel, and cobalt, along with significant amounts of gold and platinum-group metals. Stony (S-type) asteroids contain silicates and moderate amounts of metal. Carbonaceous (C-type) asteroids contain water, organics, and volatiles, all of which can be used to support operations in space and reduce dependency on launches from Earth.

The minor planets are the remnants left over from the formation of the planets. When the Solar System formed around 4.57 billion years ago, the newborn Sun was surrounded by a rotating protoplanetary disk of gas and dust. Within this disk, microscopic grains gradually accreted into larger planetesimals. The composition of these planetesimals reflected their distance from the Sun: those closer to the Sun were predominantly silicaceous; those further out were carbonaceous, with traces of volatiles; further out still, ices, volatiles, and organics predominated.

To astronomers, minor planets offer tantalising clues about the early history of the Solar System. They may be likened to time capsules of the solar system because they preserve material from its earliest stages of formation. Unlike planets, which have undergone extensive melting and geological processing, minor planets have often remained largely unchanged. Their compositions serve as a record of the chemical environment of the protoplanetary disk and the processes that operated within it. The sample return missions have provided pristine asteroid material for laboratory study. The material will give planetary scientists insight into how dust and gas condensed into solid bodies, how those bodies differentiated, how water and other volatiles were delivered to the inner planets, and other critical events in the early history of the Solar System.