Points of impact: observers get a break.

Points of impact: Observers get a break

When the roughly 20 fragments of Comet Shoemaker-Levy 9 plunge one by one into Jupiter next July, each will hit the planet's backside: None of the explosive impacts will be visible from Earth. But new calculations, based on additional observations of the train of fragments, indicate that the pieces will strike closer to the limb, or outer edge, of Jupiter than originally thought.

Each impact site will thus take less time to rotate into view, giving astronomers some hope of directly observing more of the short-lived atmospheric disturbances triggered by each collision.

Paul Chodas and Donald K. Yeomans of NASA's Jet Propulsion Laboratory in Pasadena, Calif., had previously calculated that the collisions would all occur about 30 degrees behind the limb. At that location, it would take the points of impact roughly 75 minutes to rotate past the limb and another 16 minutes to become clearly visible from Earth. In contrast, the revised calculations show that the collisions will occur only 6 degrees behind the limb. Thus each impact site will cross the limb in just 18 minutes and come into full view 16 minutes later.

Data collected since December, when the fragments moved far enough from the sun's glare that ground-based telescopes could once again detect them, helped improve the accuracy of the calculations, Chodas says. For the Galileo spacecraft, which will be the closest craft to Jupiter in July, the revised location means the difference between just barely observing the impacts and detecting them clearly.

"lt's now clear that Galileo will have a direct view of the impact sites," says Clark R. Chapman of the Planetary Science Institute in Tucson, Ariz. He notes, however, that astronomers will have to plan observations with Galileo carefully, since its crippled high-gain antenna will make it difficult to send images.

Though excited about the prospect of viewing the aftermath of the collisions sooner, Heidi B. Hammel of the Massachusetts Institute of Technology notes one disadvantage of the newly calculated location. Astronomers are hoping to indirectly observe the fireballs expected to erupt seconds after each impact by recording the flashes of light reflected off the surface of three of Jupiter's moons - Io, Europa, and Callinto. In the revised location, the collisions will still be reflected, but they won't appear as bright as previous calculations had indicated, Hammel says.