The History of Tempel 1, the 'Deep Impact' Target

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 5? 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 8? 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.

• Deep Impact Special Report