Asteroids
ATLAS OF THE UNIVERSE
B
eyond 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 dis-
covered 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 catalogue 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▼ 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.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 in-
fluence which prevented a larger planet from forming. 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. Mathilde, imaged by the
spacecraft NEAR in 1997. The
albedo is very low, and there
are huge ‘holes’.C Atl of Univ Phil'03stp 2/4/03 3:07 pm Page 90