They have existed largely unchanged since the beginning of the Solar System. In the past, they were considered harbingers of disaster. What are they, and why has a NASA probe recently visited one?

Just a few weeks ago, the NASA Stardust probe made its closest approach to Comet Wild 2, just 240 km (about 150 miles) from the surface of this ancient object. As it flew past, Stardust extended a collector made of aerogel, designed to capture some of the dust particles that surround the comet's nucleus. The particles collected will be brought home to Earth for study. The landing is scheduled for two years from now, in January, 2006. By the end of its mission, Stardust will have traveled more than 4,000,000,000 km.

Why did NASA send this probe on such a long chase of this comet? The answer deals with the nature of comets, and the differences between these fascinating interplanetary travelers and other Solar System objects like planets and asteroids. When the Solar system began to form, nearly 5,000,000,000 years ago, it started as a giant cloud of gas and dust. As gravity pulled material into the center, the Sun and the planets formed, but not all the material from the original cloud was drawn into these large objects. Some was left as interplanetary dust or smaller objects like asteroids. All the material in the inner part of the Solar System, however, has been heated by the Sun ("cooked") and bombarded by other objects (the same things that left most of the craters we see on the Moon), so none if it is quite the same as it started. Other material was left in the outer reaches of the Solar System, and this leftover collection of "Solar System ingredients" is thought to be essentially unchanged from the time the Solar system began. It is from these distant regions that comets originate, and so comets are expected to be time capsules of what our corner of the Universe was like before the Sun formed. When some disturbance in the outer Solar System sends a comet in towards the Sun, the heating causes parts of these "dirty snowballs" to break free, forming the spectacular tails that have amazed (and alarmed) people throughout history.

In the case of Comet Wild 2, we know that this comet lived in the cold regions beyond the orbit of Jupiter until 1974, when it came close enough to Jupiter to be pulled into a new orbit that sent it toward the inner Solar System. That makes this comet a prime candidate for study, since it is a newcomer. It has only come close to the Sun five times in its new orbit. That collection of particles that the Stardust mission is bringing back to Earth represents a tiny fragment of our distant past - a chance to learn the composition of the pre-Solar gas cloud.

The Stardust Web site, http://stardust.jpl.nasa.gov/index.html , has lots more information about the encounter with Comet Wild 2, including a series of pictures from the flyby.