Two hundred scientists from around the world gathered in Baltimore last week to pore over data gathered during Comet Shoemaker-Levy 9's collision with Jupiter. The researchers debated a host of unresolved questions about the comet's composition, what actually happened during the collisions, and why Jupiter was affected so dramatically. For example, the huge dark stains that surrounded many impact sites may have been a thin veneer of dusty debris created from the remains of each comet fragment, or they may represent an entirely new set of compounds synthesized when giant fireballs cooled and fell back into Jupiter's atmosphere with an energetic, high- speed "splat." There was more debate on the size of the comet before its disruption, and on where it might have come from. Based on several lines of evidence, most investigators feel the progenitor was probably in the range of 1.5 to 2 kilometers across, though 4 or 5 km is still favored by some.
Some pulsars have been observed to be traveling through the Milky Way at several hundred kilometers per second. While a lopsided supernova explosion could send these stars hurtling away at such extraordinary speeds, the recent discovery of a pulsar with a powerful jet provides an intriguing alternative. University of Wisconsin astronomers Craig Markwardt and Hakki Ogelman studied observations by the Rosat X-ray satellite of the Vela pulsar. Images reveal a 20-light-year-long jet extending from the spinning star. Although its axis is not aligned with the pulsar's observed proper motion, the Vela pulsar's jet might still have had a hand in driving the star from its birthplace. Their report, in the May 4th issue of the journal NATURE, also notes that the jet may also explain the gradual slowdown seen in the pulsar's spin rate.
Can you still see the rings of Saturn? Soon its majestic rings will be turned edge-on to us for the first time since 1980 and thus rendered virtually invisible. The first of the three disappearances occurs May 21st, and right now the rings are tipped less than 1/2 deg. to our line of sight. So if you observe Saturn right now you're not likely to see much except the planet's distinctly flattened ball. Saturn rises only a few hours before dawn, so it's not particularly well placed for viewing. If you have a good-size telescope you might try to watch the planet's larger satellites play tag. Tethys, Dione, Rhea, and Titan are undergoing eclipses and occultations by Saturn and one another. A complete observing guide begins on page 68 of SKY & TELESCOPE's May issue.
Mercury is in the midst of its best apparition of the year, but don't wait too much longer to see it. The illusive planet can be spotted about 10 deg. up in the west-northwest; don't confuse it with ruddy Betelgeuse, which is slightly higher and due west. The planet reached its greatest eastern elongation from the Sun on May 12th.