ASTEROIDS

Beyond the orbit of Mars lies the main belt of asteroids or minor planets. Only one (Ceres) is as much as 900 kilometres (560 miles) in diameter, and only one (Vesta) is ever visible with the naked eye; most of the members of the swarm are very small indeed, and there are fewer than 20 main-belt asteroids which are as much as 250 kilometres (150 miles) across.

Ceres, the largest member of the swarm, was discovered on 1 January 1801 – the first day of the new century – by G. Piazzi at the Palermo Observatory. He was not looking for anything of the sort; he was compiling a new star catalog when he came across a star-like object which moved appreciably from night to night.

This was somewhat ironical in view of the fact that a planet hunt had been organized by a team of astronomers who called themselves the ‘Celestial Police’.

A mathematical relationship linking the distances of the known planets from the Sun had led to the belief that there ought to be an extra planet between the paths of Mars and Jupiter, and the ‘Police’ had started work before Piazzi’s fortuitous discovery.

They did locate three more asteroids – Pallas, Juno and Vesta – between 1801 and 1808, but the next discovery, that of Astraea, was delayed until 1845, long after the ‘Police’ had disbanded. Since 1847 no year has passed without new discoveries, and the current total of asteroids whose paths have been properly worked out is considerably more than 40,000.

Some small bodies have been found, lost and subsequently rediscovered; thus 878 Mildred, originally identified in 1916, ‘went missing’ until its rediscovery in 1990.

The asteroids are not all alike. The largest members of the swarm are fairly regular in shape, though No. 2, Pallas, is triaxial, measuring 580 530 470 kilometres (360 330 290 miles), and smaller asteroids are certainly quite irregular in outline; collisions must have been – and still are – relatively frequent.

Neither are the compositions the same; some asteroids are carbonaceous, others siliceous, and others metal-rich. No. 3, Vesta, has a surface covered with igneous rock; 16 Psyche is iron-rich; 246 Asporina and 446 Aeternitas seem to be almost pure olivine, while in 1990 it was found that there are indications of organic compounds on the surfaces of a few asteroids, including the unusually remote 279 Thule.

Some asteroids are fairly reflective, while others, such as 95 Arethusa, are blacker than a blackboard. Obviously, no surface details can be seen from Earth, and almost all our information has been obtained spectroscopically.

No asteroid has an escape velocity high enough to retain atmosphere. The three largest members (Ceres, Pallas and Vesta) account for 55 per cent of the total mass of the main-belt bodies.

Two asteroids, 951 Gaspra and 243 Ida, have been surveyed from close range by the Galileo spacecraft, which passed through the main zone during its journey to Jupiter; in 1997 another asteroid, Mathilde, was imaged by the NEAR spacecraft on its way to rendezvous with the asteroid Eros in December 1998. Asteroids appear so small that to record surface detail in them is far from easy.

However, in 1994 Vesta was imaged by the Hubble Space Telescope; the asteroid’s apparent diameter was a mere 0.42 of an arc second.

There are bright and dark features; part of the surface seems to be covered with quenched lava flows, while another part indicates molten rock which cooled and solidified underground, to be later exposed by impacts on the surface.

Most main-belt asteroids have reasonably circular orbits, though some are highly inclined by 34 degrees in the case of Pallas, for example. They tend to group in ‘families’, with definite regions which are less populated.

This is due to the powerful gravitational pull of Jupiter, and it seems certain that it was Jupiter’s disruptive influence which prevented a larger planet from forming.

READ IT OUT

▼ The Celestial Police. This is an old picture of the observatory at Lilienthal, owned by Johann Hieronymus Schröter. It was here that the ‘Celestial Police’ met to work out the way in which to search for the missing planet moving between the orbits of Mars and Jupiter. Schröter’s main telescope was a 48-cm (19-inch) reflector, but he also used telescopes made by William Herschel.

▼Gaspra. This was the first close-range picture of a main-belt asteroid, obtained by the Galileo probe on 13 November 1991 from a range of 16,000 km (less than 10,000 miles). Gaspra (asteroid 951) proved to be wedge-shaped, with a darkish, crater-scarred surface. Gaspra is irregular in shape; it is 16 km (10 miles) long by 12 km (7.5 miles); and the smallest features recorded are only 55 metres (180 feet) across.