91av

In 1908 an asteroid exploded over the Tunguska river in Siberia, devastating
trees over an area of some 2200 square kilometres. A much larger object,
which hit the Yucatan peninsula 65 million years ago, is widely believed
to have triggered the mass extinctions that wiped out the dinosaurs. We
now know that hordes of asteroids pass close to our planet, some closer
than the Moon itself. But, fortunately, most asteroids reside farther from
the Sun, in a belt between Mars and Jupiter, and it is these ordinary asteroids
that are currently causing excitement. For astronomers now realise they
hold clues to the origins of the Solar System.

Asteroids have not always attracted such attention. They were once known
as the ‘vermin of the skies’ because they ruined astronomical photographs
as they passed in front of the object of interest. In the past three decades
a whole armada of spacecraft has glided through the Solar System, exploring
every planet from Mercury to Neptune, as well as their largest moons. But
until recently the rocky debris that lies between the orbits of Mars and
Jupiter remained unvisited.

All that has now changed. In 1991 the spacecraft Galileo, which is bound
for Jupiter, snapped the first close-up pictures of a small asteroid named
Gaspra. Last year, the same spacecraft made a rendezvous with a larger asteroid,
Ida. The pictures from this meeting, the last of which was only transmitted
to Earth in June, confirmed some things that astronomers had expected,
but they also sprang a few sur-prises. One of these was the discovery of
a moon around Ida. These encounters have stimulated interest in investigating
the asteroids, with a major NASA mission planned for 1999.

Unlike the planets, asteroids are small and the material in most of
them has changed little since their birth, 4.6 billion years ago. ‘Each
of the planets is thought to have been made by the gathering together of
countless smaller bodies,’ says Clark Chapman, a member of the Galileo imaging
team. ‘Just as the comets are pristine remnants from the formation of the
outer planets, asteroids are thought to be leftover building blocks of the
inner planets.’

The space between the orbits of Mars and Jupiter is not completely filled
with asteroids. Mars lies 1.5 astronomical units from the Sun and Jupiter
5.2. (One au, or astronomical unit, is the Earth’s distance from the Sun.)
Most known asteroids congregate in the asteroid belt and have mean distances
from the Sun of between 2.1 and 3.3 au, closer to Mars than to Jupiter.
Because the asteroids have elliptical orbits, at any particular time many
of them lie outside this range. And the asteroid belt is doughnut shaped,
not flat, because the orbits are also tilted with respect to the plane of
the Solar System, so different asteroids are a different distance above
and below this plane.

Asteroids owe their existence to Jupiter, the largest planet. As the
Solar System was forming, Jupiter’s gravitational force stirred up the objects
inside its orbit and prevented them from gathering to form a world of their
own. As a result, all that exists there is the debris of the planet that
might have been. Today, every asteroid still feels Jupiter’s power. The
planet’s gravity perturbs the asteroids and sculpts the belt they inhabit.
In places Jupiter’s gravity has removed them altogether, leaving ‘Kirkwood
gaps’, named after the 19th-century American astronomer who discovered
them. He found them after comparing the mean distances of known asteroids
from the Sun. An asteroid with a mean distance of 3.28 au from the Sun,
for example, would revolve around the Sun with exactly half Jupiter’s orbital
period and receive a gravitational tug every time it approached the planet,
which would eventually send it into another orbit. The Kirkwood gaps are
not in fact free of asteroids because their elliptical orbits ensure that
at any time there are some at a distance of 3.28 au. But few have this as
their mean distance from the Sun.

CHANCE MEETINGS

The Galileo spacecraft was the first to visit the asteroids. When the
spacecraft was launched by the space shuttle in late 1989, it was not given
enough energy to reach its target, Jupiter. So to steal some orbital energy
to help it on its way, in early 1990 it flew past Venus, then later that
year past Earth, and in 1992 past Earth again. During this roundabout journey,
Galileo navigated the asteroid belt twice. Surely, astronomers thought,
there would be some passing asteroid that the craft might meet? As it turned
out, this was quite a challenge. Science-fiction films might portray the
asteroid belt as a zone choked with deadly debris, but Galileo’s problem
was just the opposite: getting to meet an asteroid. The asteroid belt is
almost empty – millions of kilometres separate the average asteroid from
its nearest neighbour.

Fortunately, Galileo’s controllers found that its trajectory would take
it near to two asteroids. So they altered its path only slightly to make
the spacecraft come very close to both. On 29 October 1991, as Galileo was
skimming the inner edge of the asteroid belt, it passed just 1600 kilometres
from Gaspra. This asteroid lies 2.21 au from the Sun and orbits it once
every 3.3 years. Gaspra was discovered in 1916 by the Russian astronomer
Grigoriy Neujmin, who named it after a popular retreat on the Black Sea.

Would it turn out to be such a small and ordinary asteroid after all?
When astronomers began to turn their telescopes on the small world they
found that it brightened and dimmed by a factor of two as it rotated. This
large variation suggested that Gaspra was elongated: it would then appear
dimmest when it faced Earth end on. The period of the light variation also
meant that the asteroid spun on its axis once every 7.04 hours, which is
faster than any planet but normal for an asteroid. From its small size,
astronomers deduced that Gaspra had not been around for long. They concluded
that it was the fragment of a larger asteroid that had shattered. They
put its age at no more than a few hundred million years, a fraction of
the Solar System’s age of 4.6 billion years.

MARTIAN MATCH

Galileo took its time to confirm these deductions. The spacecraft’s
antenna has not unfurled, so it could not easily transmit data to Earth.
It was only in late 1992 – more than a year after the encounter – that scientists
received all the Gaspra data. When at last the data arrived, the spacecraft’s
images revealed that Gaspra is indeed elongated, with axes (solid elliptical
objects have three) measuring 18.2, 10.5, and 8.9 kilometres. Its mean diameter
is 12.2 kilometres, which makes Gaspra the same size as Deimos, the smaller
moon of Mars which some astronomers believe was once an asteroid itself.

As expected, Gaspra is grey and cratered. It belongs to the class of
asteroids known as the S type, one of two main types of asteroids that astronomers
identified in the 1970s. S-type asteroids are grey objects that reflect
about 20 per cent of the sunlight that strikes them. They dominate the inner
asteroid belt, where Gaspra lies. The other group, the C type asteroids,
are black and reflect less than 5 per cent of the sunlight that falls on
them. They were formed further from the Sun than S-type asteroids and dominate
the outer belt.

The number of craters on Gaspra reveals how long the asteroid has existed
in its present state. The longer it has been exposed to impacts from other
asteroids, the more craters it should have. Gaspra has relatively few craters,
which implies that the asteroid is roughly 200 million years old – fairly
young for a planetary body.

But Galileo also turned up some surprises. It discovered parallel and
crisscrossing depressions running across Gaspra’s surface for over a kilometre.
Similar grooves have been found on Phobos, the larger Martian moon. They
may have formed when Gaspra was hit by something that fractured the asteroid.

Galileo’s magnetometer also found evidence of a magnetic field on Gaspra
– a surprise, since small bodies were thought not to have such fields. ‘That
was not predicted,’ says Chapman. ‘I was there in the room some years ago
when we decided to give a little bit of space on the tape recorder for the
magnetic data, and the magnetometer experimenters simply wanted to do it
for reasons of serendipity.’ The origin of Gaspra’s magnetic field is still
a mystery.

Peter Thomas, a planetary scientist at Cornell University in Ithaca,
New York, also notes that Gaspra’s loose fragmented surface material, or
regolith, seems quite deep. ‘The thing that caught a lot of people’s eyes,’
says Thomas, ‘is that something so small as Gaspra would have evidence of
regolith’. Before the Galileo flyby, scientists believed that an object
with such weak gravity would not be able to retain much regolith because
impacts from other asteroids would blast it into space. But Gaspra’s regolith
may be tens of metres deep, so the asteroid has somehow held on to this
material.

After Galileo flew past Gaspra, it headed back to Earth, which gave
it the final shot of energy to send the spacecraft through the entire asteroid
belt. This time Galileo encountered what promised to be a more interesting
prospect still: Ida, a much bigger asteroid that had been discovered in
1884. With a mean distance of 2.86 au from the Sun, Ida resides in the outer
asteroid belt and completes an orbit every 4.8 years. Nevertheless, like
Gaspra, it is a grey, S-type asteroid. As well as being bigger, Ida also
spins fast, rotating once every 4.63 hours. Moreover, it belongs to what
astronomers call a family, whereas Gaspra does not. Members of an asteroid
family share similar mean distances, orbital eccentricities (the measure
of how elliptical the orbit is) and inclinations (how tilted the orbit
is). It is thought that these similarities arise because the members of
a family were once part of a larger asteroid that was smashed to bits in
a collision.

Before the Galileo flyby, astronomers believed that the Koronis family,
to which Ida belongs, was young. This belief was based in part on observations
of the spins of the member asteroids. When a family first forms, many of
the asteroids should be spinning more or less in the same direction. Over
time, though, collisions disturb these spins and make them more random.
But the spins of Koronis members were not random, which suggested that the
family was no more than a few hundred million years old.

Galileo challenged this and other expectations. The spacecraft flew
2400 kilometres from Ida on 28 August 1993 but did not deliver most of the
data until this past spring. Like Gaspra, Ida turned out to be elongated,
as astronomers had expected from variations in its brightness. The asteroid’s
longest axis measures about 58 kilometres, three times that of Gaspra’s.
Like Gaspra, Ida has grooves, a deep regolith, and a possible magnetic field.

HOW OLD IS IDA?

The first big surprise came when scientists saw the first pictures of
the asteroid: ‘Ida has a very large number of craters,’ says Chapman. According
to conventional estimates, Ida’s high density of craters implies that it
is at least a billion years old. This is much older than the supposed age
of the Koronis family. But Chapman suspects that the cratering rate does
not indicate Ida’s true age. The huge density of craters could be misleading,
he says, if Ida got hit by a lot of debris during the break-up that formed
the Koronis family.

An even bigger surprise, though, came earlier this year. Scientists
who were analysing the images of Ida noticed a small object nearby. At first,
they thought it might simply be a background star or Galileo’s target, Jupiter.
The new object was small and faint. But further work confirmed that it is
a moon. At 1.5 kilometres across, it is the smallest moon ever found. ‘For
Ida,’ says Thomas, ‘the satellite is the thing that excites everybody the
most. This is a phenomenon that’s been speculated about and hinted at before,
and now we finally see it and have to grapple with figuring out how it formed.’

Astronomers had found previous evidence for moons around asteroids.
In the 1970s some American astronomers had observed asteroids that passed
in front of stars, blocking their light, and noticed a second dimming of
the star’s light that might be caused by a moon. But these claims were,
and still are, controversial because the observations are so difficult to
make. So Ida’s moon is the first definitive example of an asteroid satellite.

The moon is about 100 kilometres from Ida, or twice the length of Ida’s
long axis. ‘The nice bonus that we hope the moon will give us is the mass
of Ida,’ says Thomas, because the faster the moon revolves, the more mass
Ida must have. ‘That’s something we hadn’t even hoped for from Galileo,’
he says. To compute Ida’s mass, however, astronomers will have to study
images of the satellite to determine its orbital period around Ida.

Scientists are puzzled as to how Ida got its moon. One theory is that
it’s literally a chip off the old block: an object hit Ida and knocked a
lump of rock off it that then went into orbit around it. The same collision
may also have given Ida its rapid spin. But Chapman favours another theory,
which invokes Ida’s membership of the Koronis family. ‘Maybe the break-up
of the precursor asteroid that resulted in this family of asteroids created
the satellite,’ he says. If so, both Ida and its moon happened to get thrown
from this cataclysm in the same direction and with the same speed, and Ida’s
gravitational pull captured the tiny rock.

Whether other asteroids have moons is still uncertain. Earth-based observations
that bounce radar echoes off asteroids approaching Earth indicate that some
of these bodies are contact binaries – that is, they consist of two more
or less equal bodies stuck together. But these cases differ from Ida’s,
in which the main asteroid dwarfs its companion.

Scientists need to see more asteroids before they can hazard a guess
as to how typical Ida is, and NASA is now planning to launch a craft equipped
with instruments designed specifically to scrutinise asteroids, unlike Galileo,
whose main goal was always Jupiter. The principal target of the mission,
called NEAR (Near Earth Asteroid Rendezvous), will be Eros, an S-type asteroid
that crosses the orbit of Mars and approaches Earth. This means that the
spacecraft will be able to fly there with relatively little energy, and
will therefore be relatively cheap.

If NEAR receives funding, the spacecraft will leave Earth in 1996 and
reach Eros in 1999. Unlike Galileo’s encounters, which were quick flybys,
NEAR will orbit the asteroid and study it for a year: taking photographs,
determining the surface composition, searching for satellites, and looking
for a magnetic field. The result will be the most thorough examination of
an asteroid ever – and perhaps answers to at least some of the questions
that Galileo has raised.

* * *

The planets that might have been

As long ago as 1596, Johannes Kepler and other astronomers were speculating
that the large gap between Mars and Jupiter might harbour a missing planet.
In 1801 a Sicilian astronomer, Giuseppe Piazzi, thought he had found it.
But this object, which Piazzi named Ceres, was too small to be a planet,
and the following year another astronomer found another such object, Pallas.
In 1804 a third object, Juno, was found, and in 1807 a fourth, Vesta. At
the time astronomers speculated that these small objects, named asteroids
because they looked point-like, or star-like, through a telescope, might
be the remains of a planet that had exploded. Although astronomers hunted
for additional asteroids, decades passed with no further discoveries. The
fifth asteroid was finally found in 1845, and thousands of asteroids have
been discovered since. To date, astronomers have determined precise orbits
for more than 6000 of the bodies, and hundreds more are found each year.

An asteroid with a well-determined orbit receives a number that reflects
the approximate order of its discovery. So Ceres, the first asteroid to
be discovered, is officially known as 1 Ceres. The asteroid’s name is usually
given by the discoverer, and these names run the gamut from the serious
(2001 Einstein) to the whimsical (2309 Mr Spock). Because most of the larger
asteroids were discovered first, generally the lower the asteroid’s number,
the larger it is. The two asteroids that the Galileo spacecraft flew past
obey this rule: 951 Gaspra is only a third the size of 243 Ida.

Ken Croswell is an astronomer in Berkeley, California.