Exceptional Asteroids
ATLAS OF THE UNIVERSE
N
ot all asteroids are confined to the main swarm. There are
the Trojans, for example, which move in the same orbit as
Jupiter, occupying what are termed Lagrangian points. In
1772, the French mathematician Joseph Lagrange drew atten-
tion to the ‘problem of the three bodies’, which applies when
a massive planet and a small asteroid move round the Sun in
the same plane, with virtually circular orbits and in equal
periods; if they are 60 degrees apart, they will always remain
60 degrees apart. Therefore, the Trojans are in no danger of
being swallowed up, even though they do oscillate to some
extent round the actual Lagrangian points. By asteroidal stan-
dards they are large; 588 Achilles, the first-known member of
the group, is 116 kilometres (72 miles) in diameter and 624
Hektor as much as 232 kilometres (145 miles), though it has
been suggested that Hektor may be double. Because of their
remoteness, the Trojans are very faint; many hundreds are
now known. There are also three known Martian Trojans, of
which the brightest is 5261 Eureka, and no doubt others exist.
Some small asteroids have very eccentric orbits. The
first to be discovered was 944 Hidalgo, with a path which
takes it between 300 million and 870 million kilometres (190
and 540 million miles) from the Sun. 5335 Damocles has an
orbit crossing those of Mars, Jupiter, Saturn and Uranus; theperiod is 40.9 years, and the high inclination (60.9 degrees)
means that at the present epoch there is no fear of collision
with a planet. It is no more than 15 kilometres (9 miles)
across. A few other asteroids with similarly eccentric orbits
are known, and are classed as Damocloids.
Many ‘close-approach’ asteroids have been found, and
have been divided into three groups. Amor asteroids have
orbits which cross that of Mars, but not that of the Earth;
Apollo asteroids do cross the Earth’s orbit and have mean
distances from the Sun greater than one astronomical unit,
while Aten asteroids have paths which lie mainly inside that
of the Earth, so that their periods are less than one year.
One member of the Aten group, 2340 Hathor, is only
500 metres (1600 feet) across. At present the holder of the
‘approach record’ is 1994 XM 1 (yet to be named), which
brushed past us on 9 December 1994 at a mere 112,000 kilo-
metres (70,000 miles). Though it is no more than 7 to 12
metres (23 to 40 feet) across, it would have caused tremen-
dous global devastation if it had collided with the Earth.
All the close-approach asteroids are midgets. One, 4179
Toutatis, was contacted by radar and found to be a contact
binary, with components 4 kilometres (2.5 miles) and 2.5
kilometres (1.5 miles) across touching each other and mov-
ing round their common centre of gravity in 10.5 days.
New close-approach asteroids are being found regularly,
and it seems that they are much commoner than used to be
thought, so occasional impacts cannot be ruled out. There
have been suggestions that we should keep a watch out for
them, so that if we see one homing in on us we can try to
divert it by sending up a nuclear missile. Indeed, in 2001 the
Spaceguard organization in Britain set up an observatory
specially to observe them and to act as an information centre.
Some asteroids, such as Icarus and Phaethon, have paths
which take them inside the orbit of Mercury, so that at peri-
helion they must be red-hot. 3200 Phaethon moves in much
the same orbit as the Geminid meteor stream, and it may
well be an ex-comet which has lost all its volatiles.
It is also true that Asteroid 4015, discovered in 1979, has
been identified with an object seen in 1949 and classed as a
comet (Wilson–Harrington), while a tailed comet, Elst–
Pizarro, moves wholly within the main asteroid belt, and has
been given an asteroid number –7968 – a case of celestial
‘dual nationality’! Evidently the distinction between comets
and asteroids is not nearly so clear-cut as used to be thought.
Small though they may be, the asteroids are fascinating bod-
ies. We no longer regard them as the ‘vermin of the skies’.
The first asteroid found to come well within the orbit of
Mars was 433 Eros, discovered by Witt in 1898. It can
approach Earth to 23 million kilometres (14 million miles).
It is sausage-shaped, with a longest diameter of 23 kilome-
tres (14 miles). It has been surveyed from the spacecraft
NEAR (Near Earth Asteroid Rendezvous, later named after
the American geologist Eugene Shoemaker). NEAR was
launched in March 2000, and entered orbit around Eros; on
12 February 2001 it made a controlled landing there. Eros is
an S-type asteroid, with an escape velocity of 10 metres per
second; the rotation period is 5 hours, and the temperature is
100 degrees C on the day side and 150 degrees C on the
dark side. NEAR approach pictures showed many craters up
to 1000 metres (3300 feet) in diameter, but there were fewer
small craters than expected, and a scarcity of large boulders.
NEAR touched down just south of the saddle-shaped struc-
ture Himeros. Studies of the surface features indicate that
most of the larger rocks were caused by a single meteoritic
impact, perhaps 1000 million years ago.
The last signals from NEAR were received on 28
February 2001. The probe remains on the edge of Himeros,
and there it will stay until some future astronaut collects it! Orbit of Phaethon.
3200 Phaethon, discovered
in 1983, is about 5 km
(3 miles) in diameter. Its
orbit carried it within that of
Mercury; the distance from
the Sun ranges between
21 million km (13 million
miles) and 390 million km
(242 million miles). The
orbital period is 1.43 years,
and the rotation period
4 hours. Phaethon may
well be the ‘parent’ of the
Geminid meteor stream.
Another asteroid known to
cross Mercury’s orbit is
1566 Icarus.JupiterMars
SunVenusMercury PhaethonEarth Asteroid positions.
This diagram shows the
positions of known asteroids
in 1990. The orbits of the
Earth, Mars and Jupiter
are shown. It is clear that
most of the asteroids lie
in the main belt, between
the orbits of Mars and
Jupiter; some depart from
the main swarm, but the
‘close-approach’ asteroids
are all very small, while
the Trojans move in the
same orbit as Jupiter.JupiterMars
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