Hubble Discovers Missing Pieces of Comet Linear

1. What do the pictures show?

Hubble's close-up view [lower right] shows that the comet nucleus has been reduced to a shower of glowing mini-comets, which resemble the fiery fragments from an exploding aerial firework. This is the first time astronomers have gotten a close-up look at what may be the smallest building blocks of comets, the icy, solid pieces called cometesimals, which are thought to be less than 100 feet across. The farthest fragment [to the left], which is now very faint, may be the remains of the parent nucleus that fragmented into the cluster of smaller pieces to the right. The comet broke apart around July 26, when it made its closest approach to the Sun. The Hubble telescope photographed the comet on Aug. 5.

A wider view of the comet can be seen in the picture at upper left. A ground-based telescope snapped this image of the comet at nearly the same time as the Hubble observations. The comet appears as a diffuse, elongated cloud of debris without any visible nucleus. The small box to the left outlines Hubble's view.

2. What are comets?

Since the 1950s, comets have been commonly assumed to be loose agglomerations of ice and dust -- "dirty snowballs" -- weakly held together by gravity. Solar heat causes the ices to sublimate [change from ice to gas] and violently release gas as explosions and garden-hose-style jets. Solar radiation pressure blows away particles like debris caught in a gale. Comets spend most of their lives in the frosty outer limits of our solar system, and only periodically visit the inner solar system, making passes around our Sun.

In 1992, comet Shoemaker-Levy 9's nucleus was broken up into about 20 mountain-sized pieces after a close encounter with Jupiter. In 1994 these pieces eventually collided with Jupiter in a stunning string of explosions. But that comet was too far from the Earth to be studied in the same kind of detail as Comet LINEAR. Some astronomers think that the fragments now being seen in Comet LINEAR may be the primordial building blocks of the original nucleus. By investigating how this comet is coming apart, astronomers hope to learn how it was put together in the first place, roughly 4.6 billion years ago.

3. Why did the nucleus break apart?

Astronomers speculate that the comet was so fragile that it couldn't take the Sun's heat and came unglued. Several observatories recorded the comet brightening as made its closest approach to the Sun. Astronomers interpreted the brightening as the nucleus completely falling apart.

Some astronomers believe that this was Comet LINEAR's maiden voyage to the solar system, after traveling for nearly the distance of a light-year from the vast comet storehouse called the Oort cloud. Researchers estimate that 20 to 30 percent of comet visitors are so fragile they completely disintegrate when they meet the Sun up close. Other comets can survive for as many as a thousand orbits around the Sun -- one of the most celebrated being Halley's Comet, which last appeared in 1986.

Other astronomers suggest that Comet LINEAR may have been a fragile piece of a larger comet, breaking away from its parent during a visit to our solar system more than 10 million years ago. Straggling behind its parent comet for millions of years, LINEAR at last returned to our solar system for one last circuit around the Sun.