Repaired Hubble eyes Jupiter-bound comet.

The fragments of Comet Shoemaker-Levy 9 are shifting position, and some may continue to splinter before they crash into Jupiter this July. The new findings come from the first pictures of the comet taken by the Hubble Space Telescope since the instrument's repair.

The Hubble images, recorded in late January and released by NASA last week, show that some of the cometary chunks no longer fit the orderly lineup that astronomers have likened to pearls on a string. The orientation of these out-of-kilter fragments may indicate that they formed after the main breakup 2 years ago of Shoemaker-Levy 9, says Harold A. Weaver of the Space Telescope Science Institute in Baltimore.

The pictures show that 9 of the 20-odd pieces of the comet seem to march to the beat of a different drummer, no longer following the same path along the sky as their companions. Three of these off-line chunks lie near the brightest fragment and were also seen -- though less clearly -- in pictures taken by Hubble last July. before its optical flaw was fixed (SN: 10/23/93, p.260).

Since July, notes Weaver, the three chunks have dramatically shifted position. The chunk nearest the brightest fragment increased from 35 degrees to about 90 degrees the angle it makes with the in-line pieces. Moreover, this piece now lies about 3,700 kilometers away from the brightest, more than triple the separation in July.

Studies indicate that the tidal gravitational force that shattered the comet when it passed near Jupiter in 1992 gave the trail of fragments a unique geometry. They would separate but remain in the same line, as viewed from Earth.

Based on these studies and the new images, Weaver suggests that the off-line chunks were created more recently and by a force other than Jupiter's gravity. He says that rapid rotation of a fragment or the sudden venting of volatile compounds from beneath its surface could split a chunk into smaller pieces. "There's no question that there's further fragmenting," says Donald K. Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The total energy dumped into Jupiter's atmosphere this July will remain the same whether or not fragmenting continues, Weaver says (see page 120). But because smaller chunks make a smaller splash, each impact would be more difficult to detect, he adds. Weaver is now analyzing the Hubble images to get a better estimate of the average diameter of the chunks, the best indicator of the power of each Jovian collision. "That's what everyone is at the edge of their seats waiting for; he says.

That task will require making assumptions about the reflectivity of the fragments since their sizes can't be determined directly. Given the distance of Shoemaker-Levy 9 from Earth, Hubble can only resolve objects 360 km across. Alas, the fragments probably measure no more than 4 km.