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ASTRONOMERS started to puzzle over Chiron as soon as it was discovered.
What was this small, strange object that lay far out in the Solar System,
between Saturn and Uranus? Was it an asteroid or a comet? Chiron was much
farther from the Sun than any known asteroid, yet it did not seem to be
a comet, for it had no tail. But astronomers now believe that Chiron is
indeed a comet, albeit a most unusual one. Since its discovery, a huge coma
of gas and dust has formed around Chiron as it has moved closer to the Sun
and its ice has begun to vaporise.

Astronomer Charles Kowal, of the Hale Observatories in California, discovered
Chiron on 1 November 1977 as he was searching for peculiar objects in the
Solar System. Previously he had discovered the 13th moon of Jupiter. In
his search for new worlds, Kowal photographed the sky with the 1.2-metre
Schmidt telescope on Palomar Mountain in California. He then examined the
resulting photographic plates on a blink comparator, a machine that holds
two plates and alternates them rapidly, like two successive frames in a
movie. If you use plates of the same region of sky taken at different times,
any moving object – comet, asteroid or planet – appears to jump back and
forth from one plate to the other. Stars, which are fixed, hold steady as
the plates alternate. In 1930, Clyde Tombaugh used such a blink comparator
to find Pluto.

As part of his survey, Kowal took photographic plates of the sky in
the constellation Aries on 18 and 19 October 1977. He examined these plates
with the blink comparator on 1 November and noticed a faint object that
had moved slightly through southwestern Aries. The slow apparent motion
meant the object was far away, for distant objects move slowly from night
to night, in the same way that far-off trees and mountains appear to move
slowly past a motorist. Closer objects, such as nearby street lights and
road signs, speed by. The new body turned out to be almost as far away as
Uranus. Since Kowal’s new world was faint, it was probably a few hundred
kilometres across – much smaller than a planet, but quite big for an asteroid
or comet.

An astronomer at the University of Arizona, Tom Gehrels, then located
the strange object on plates he had taken with the same telescope on 11
and 12 October that year. On 3 and 4 November, Richard Green of the California
Institute of Technology took new photographs of the distant body, further
confirming its existence, and Kowal announced his discovery to the world.

Meanwhile, astronomers searched through old photographs to see if this
object appeared on any. It did: the oldest images came from 1895. Aided
by these observations, Brian Marsden of the Harvard-Smithsonian Center for
Astrophysics in Cambridge, Massachusetts, worked out that each orbit of
the new world took 50.7 years, roughly mid-way between the time it takes
Saturn to make an orbit, 29.5 years, and the 84 years Uranus needs. The
body’s mean distance from the Sun is 13.7 astronomical units (one AU is
the distance from the Sun to the Earth). This distance again falls between
Saturn’s distance of 9.6 AU and Uranus’s 19.2 AU.

But Marsden found that the orbit was elliptical, so that the distance
of the object from the Sun varied. At perihelion, the point closest to the
Sun, the distance is just 8.5 AU – less than that of Saturn. At its farthest,
however, the object ventures 18.9 AU from the Sun, almost to the orbit of
Uranus. The object was last at this extreme distance in 1970. When Kowal
discovered it in 1977, it was still far away, at a distance of 18 AU from
the Sun. Since then, the object has drawn closer to the Sun and now lies
11 AU away from it. In 1992, it will cross the orbit of Saturn at 9.6 AU,
and it will come closest to the Sun in 1996.

If it were a comet, the object would have been called Comet Kowal. But
the new world lacked the fuzzy appearance of a typical comet, so astronomers
catalogued it instead as asteroid 2060. Unlike a comet, an asteroid is not
named for its discoverer, but the discoverer picks the name. Kowal chose
Chiron, perfect for the strange world he had found. In Greek mythology,
Chiron was the son of Cronus (Saturn to the Romans) and the grandson of
Uranus, so the name reflected the object’s unique location in the Solar
System. The name also described the object’s contradictory appearance, for
the mythological Chiron was a centaur, a cross between a man and a horse,
just as the astronomical Chiron seemed to be a cross between an asteroid
and a comet.

Even after Chiron received its name, astronomers did not know what to
make of it. Irresponsible reporters immediately proclaimed Chiron the 10th
planet, even though it was far too small to be one. Astronomers, at least,
knew better, but Chiron still posed a problem. Was this far-off object an
asteroid or a comet? If Chiron were an asteroid, it was much farther away
than any other. Most asteroids circle the Sun in a belt between the orbits
of Mars and Jupiter. There are a few exceptions, however: some asteroids
cross the orbit of Earth, a group known as the Trojan asteroids lie at Jupiter’s
distance from the Sun, and one wayward asteroid called Hidalgo pursues a
path that takes it from the asteroid belt to the orbit of Saturn. If it
was an asteroid, Chiron was even more extreme than Hidalgo.

But Chiron presented other problems as a comet. Comets come from a zone
of icy bodies orbiting the Sun beyond Pluto. Every now and then, one falls
towards the Sun and heats up in the inner Solar System. The ice vaporises,
forming the fuzzy coma of gas and dust and long trailing tail that are a
comet’s hallmark. Comets then swing around the Sun and head back beyond
Pluto – unless a planet like Jupiter or Saturn interferes. The gravity of
Jupiter or Saturn can pull a comet into an orbit with a relatively short
period. Sometime in the distant past, this happened to Halley’s Comet. Thousands
of years ago, Halley’s Comet emerged from beyond Pluto and would have returned
there, but it passed close enough to one of the giant planets to alter the
comet’s orbit. As a result, Halley now revolves around the Sun every 76
years in an orbit that takes the comet little farther out than Neptune.

Was Chiron a comet? It did not look like one: it had no coma and it
had no tail. And all the other comets come closer to the Sun than Chiron
does. Halley’s Comet, for example, cuts inside the orbit of Venus, while
Chiron never gets much nearer than Saturn.

During the 1980s, as Chiron gradually approached the Sun, astronomers
tried to find out more about its physical properties. Though far smaller
than any planet, at roughly 200 kilometres across Chiron is quite big for
an asteroid or comet. Only 20 or 30 asteroids are bigger. By comparison,
Ceres, the largest asteroid, measures about 900 kilometres across. If, on
the other hand, Chiron is a comet, then it is the biggest one of all. Halley’s
Comet is just 12 kilometres in diameter.

Through even the largest telescopes Chiron appears as a featureless
speck, which fluctuates regularly in brightness as Chiron rotates. Chiron
must therefore have bright and dark regions on its surface. When the bright
regions turn toward us, Chiron looks brighter; when the dark regions face
us, Chiron seems dimmer. In the mid-1980s, Schelte Bus and Edward Bowell
at Lowell Observatory in Flagstaff, Arizona, Alan Harris of the Jet Propulsion
Laboratory in Pasadena, California, and Anthony Hewitt of the US Naval Observatory
in Flagstaff, Arizona, measured Chiron’s brightness at different times and
found how long it took to rotate. Their result, 5 hours and 55 minutes,
did not simplify the problem of what Chiron is, because a rotation period
of a few hours is typical of both asteroids and comets.

Chiron is a dark body, like most asteroids and comets. It reflects only
10 per cent of the sunlight that strikes it. Ceres reflects the same proportion,
and Halley just 4 per cent. And because it lies so far from the Sun, Chiron
is cold. Its temperature varies from 60 K when Chiron is farthest from the
Sun to 90 K at perihelion.

Chiron becomes a comet

As Chiron crept nearer to the Sun, astronomers searched for signs that
it was behaving as a comet. For years they failed to find anything unusual.
Finally, in February 1988, David Tholen of the University of Hawaii, William
Hartmann of the Planetary Science Institute in Arizona, and Dale Cruikshank
of NASA’s Ames Research Centre in California observed that Chiron was 75
per cent brighter than normal. By late 1988, Chiron was 2.5 times as bright
as it should have been. Observations from late 1987 indicate that Chiron
was brighter at that time as well. From this the astronomers inferred that
Chiron’s ice was beginning to melt in the warmth of the Sun.

Then, in April 1989, Karen Meech of the University of Hawaii and Michael
Belton of Kitt Peak National Observatory in Tucson, Arizona, discovered
that Chiron had gained a fuzzy coma spanning at least 40 000 kilometres.
By December 1989, the coma’s size had doubled, and by February this year,
tripled. At last report the coma measured more than 130 000 kilometres across.

Together these events – the brightening and the development of a coma
– mean that Chiron is a gigantic comet in an unusual position. Like other
comets, Chiron probably came from beyond Pluto and moved into the region
where the planets orbit the Sun. But unlike other comets, Chiron is stuck
between the orbits of Saturn and Uranus.

Someday, however, it probably will come closer to the Sun. Studies of
Chiron’s orbit indicate that the comet is only temporarily lodged between
Saturn and Uranus. In 1979, Shio Oikawa and Edgar Everhart of the University
of Denver published their investigation of Chiron’s movements. They found
that Chiron’s orbit is unstable. Unlike Earth, which will keep circling
the Sun in its present orbit forever, Chiron will not last for more than
a few million years in its current orbit. Chiron will come close to Saturn
eventually, as the two bodies follow their orbits, and Saturn’s gravity
will then toss Chiron out of its present path.

The comet awaits its fate

Because astronomers are uncertain about the exact shape of Chiron’s
orbit, no one can say when this dramatic event will happen or what the outcome
will be. Instead, Oikawa and Everhart computed 60 slightly different orbits
for Chiron to see what fate is most likely to befall this small world. In
more than 85 per cent of the orbits, Saturn hurls Chiron closer to the Sun,
into Jupiter’s region of the Solar System. In fewer than 15 per cent of
cases does Saturn toss Chiron the other way, out of the Solar System altogether.
These calculations suggest that Chiron’s orbit is evolving inward rather
than outward, and that Chiron is a comet that came in from beyond Pluto
rather than an asteroid that came out from the asteroid belt.

Further evidence that Chiron is moving inward comes from recent work
by Alan Stern of the University of Colorado. Stern has investigated the
ice on Chiron that is now producing the coma and tail characteristic of
a comet. He found that if Chiron had once been closer to the Sun than it
is now – perhaps in the asteroid belt between Mars and Jupiter – then the
warmth of the Sun would have vaporised Chiron’s ice long ago. In fact, Stern
found that even if Chiron had always occupied its present orbit between
Saturn and Uranus, it would also have lost all its ice. According to Stern,
Chiron has been in its current orbit for no more than 300 million years,
much less than the age of the Solar System. Stern raises the possibility
that Chiron entered its present orbit only a few tens or hundreds of thousands
of years ago. Either way, Stern’s calculations mean that Chiron began its
life beyond Uranus, and probably beyond both Neptune and Pluto.

Unusual though it is, Chiron does have a few distant cousins. The periodic
comet Schwassmann-Wachmann I, discovered in 1927, orbits the Sun between
Jupiter and Saturn and is 80 kilometres across. Though it never gets any
closer than Jupiter, Schwassmann-Wachmann I occasionally becomes brighter
as its ice vaporises, just as Chiron is now doing. Chiron also resembles
Phoebe, Saturn’s outermost moon: both bodies are dark, and both are about
the same size. Phoebe circles Saturn in the opposite direction to most of
its moons, so it was probably captured by the planet’s gravity, rather than
forming with the planet. Chiron may be a Phoebe that has so far eluded Saturn’s
gravitational grasp and maintained its independence.

As Chiron heads for its closest point to the Sun in 1996, scientists
will continue to monitor this distant object. But we now know what Chiron
is: a huge comet that probably originated beyond Pluto, is now stuck between
Saturn and Uranus, and is likely to be thrown closer to the Sun by Saturn
someday.

If Chiron enters the inner Solar System, it will provide a tremendous
spectacle for our descendants on Earth. For their sake, though, let us hope
it does not pass too close!

Ken Croswell is a doctoral candidate in astronomy at Harvard University
and a contributor to Time-Life Books’ astronomy series, Voyage Through the
Universe.