Comet Wild 2: Right place, right time, right snowball
Posted: December 30, 2003

Comet 81P/Wild 2 is a fresh periodic comet -- meaning that it moves about the Sun in an elliptic orbit. In Wild 2's case that is once every 6.39 years. Its nucleus is thought to be of low density, with a diameter of about 5.4 kilometers (3.3 miles).

Until September 10, 1974, comet Wild 2's orbit lay between Jupiter and a point near Uranus. But on that date nearly 30 years ago, the comet passed within 897,500 kilometers (557,735 miles) of the solar system's biggest planet, Jupiter. That encounter with Jupiter forever altered the comet's orbit, carrying it for the first time into the inner solar system. The new flight path carried it as close to the Sun as just beyond the distance of Mars and far from the Sun as about Jupiter. On January 6, 1978, astronomer Paul Wild (pronounced "Vilt") discovered the comet during its first passage relatively near to the Earth -- passing within 181,014,000 kilometers (112,476,679 miles).

When a comet comes close enough to the Sun to get heated up, it loses some of its material through a process called sublimation. This happens when a solid becomes a vapor without first melting into a liquid. After about 1,000 trips past the Sun, a comet loses most of its volatile materials and no longer generates a coma, which is made up of the gases that escape off its surface. Since it is the escaping gases that drive the dust particles from the nucleus -- the solid part of the comet -- the comet no longer creates the long beautiful dust tail that we can sometimes see in the night sky.

An important aspect of Stardust's exploration of comet Wild 2 is that by the time Stardust encounters it, the comet will have made only five trips around the Sun in its new orbit. By contrast, Comet Halley has passed close to the Sun more than 100 times, coming close enough to have been greatly altered from its original condition.

Another important aspect is the comet's orbit. Stardust navigators were able to plot a flight path that allowed the spacecraft to encounter the comet at a relatively sedate closing speed. Because of this low-velocity meeting (passing each other at 6.1 kilometers per second, or about 13,600 miles per hour), the spacecraft can capture comet dust, rather than having it blow right through the collectors. The dust samples can then be brought back to the Earth to be analyzed.