Editor's
Note: In early July, NASA will strike a comet with a table-sized space
probe. The event will be visible
from Earth. This article tells a curious tale of the comet that's about
to be hit.
On the night
of April 3, 1867, at Marseilles Observatory in France, Wilhelm Tempel discovered
a ninth-magnitude comet near the star Zubeneschamali, in the constellation of
Libra, the Scales. Unfortunately, the comet - Tempel's ninth over the previous
eight years - would remain rather dim and unimpressive as it followed a path
through Libra and into nearby Scorpius, the Scorpion.
Despite
its poor showing, however, Tempel's find attracted considerable attention when
calculations showed that it was moving around the Sun in an elliptical orbit,
taking only about 5½ years to complete one trip around the Sun. At that time,
astronomers knew of only eight other comets with short-period orbits (comets
taking less than 200 years to revolve around the Sun). This was also the first
of two periodic comets that would ultimately bear Tempel's name. Today it is
catalogued as Comet 9P/Tempel 1 (the ninth periodic comet discovered and the
first of the two periodic comets discovered by Tempel).
By the end
of August 1867, Comet Tempel 1 had faded away as it moved back out into space.
Near aphelion (that point in its orbit farthest from the Sun), the comet passed
close to Jupiter, and the planet's massive gravitational pull significantly
altered the comet's orbit. In fact, the next expected approach of Comet Tempel
1 to the Sun was delayed by 118 days in early 1873, all because of Jupiter's
interference. As a result, the comet's orbital period was lengthened slightly
to almost exactly six years. The comet was observed again during the spring
of 1879.
But on its
outward journey following its 1879 appearance, Comet Temple 1 had another encounter
with Jupiter in 1881, this time coming much closer to that giant world than
before. So dramatic in fact, were the changes wrought by Jupiter upon the comet's
orbit, that Tempel 1 went completely unobserved during 1885. And with no new
information concerning the comet's altered path through space, astronomers considered
it hopelessly lost.
Interestingly,
however, the game of tug-of-war between Jupiter and Comet Tempel was far from
over. In fact, about every 12 years, the two objects came close to each other
and each time the comet's path through space was slightly altered.
Now fast-forward
to the early 1960s:
Astronomers
had changed their method of computing orbits and were now using electronic computers.
Although the computing technology of more than 40 years ago was stone-age compared
to today's PCs, those early computers did allow for relatively easy study of
comet orbits.
In fact,
before they arrived on the scene, trying to account for the effects of planetary
perturbations would have taken countless hours of calculations, all done manually.
But by the 1960s, these new "electronic brains" could provide details
on a particular comet's orbit in just a fraction of the time it would have taken
by hand.
It was orbital
expert Brian Marsden, who in 1963 initiated an investigation into the loss of
Comet Tempel 1. He and astronomers, J. Schubart and G. Schrutka issued predictions
for possible returns of the comet in 1967 and 1972 - of which the latter apparition
was expected to be very favorable. Although a photograph was taken in June 1967
with the 61-inch telescope at Catalina Observatory of a tiny object near the
comet's predicted position, no confirming picture could be taken. Proof of the
recovery had to await the comet's 1972 return and in January of that year photographs
taken from Steward Observatory in Arizona showed the comet very close to its
predicted positions.
Comet Temple
1 has since been observed on five more returns to the Sun. On New Years' Eve
in 1997, the Hubble Space Telescope was even able to detect the comet nucleus:
a potato-shaped lump of rock coated with icy, volatile gases measuring roughly
8½ miles (14 kilometers) long.
The stage
is now set for mankind's first direct interaction with a comet, with Tempel
1 chosen as the target. NASA will smash a probe into the comet in early July.
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