Keck's Comet 'Video' Most Detailed

Turned into a Giant Video Camera, the Telescope Snaps Shots of the Shoemaker-Levy Collision. Saturn's Rings Are On Next

by Robert Sanders

In their first detailed analysis of the collision of a large fragment of Comet Shoemaker Levy 9 with Jupiter last July, Berkeley astronomers provide the most complete play-by-play yet for any of the 22-plus fragments that smashed into the planet.

The Berkeley team took a series of pictures every 7.7 seconds to produce a video of the collision that shows more detail than was seen by any other telescope around the globe. The rapid video sequence was possible thanks to the incredible light-gathering power of the 10-meter W. M. Keck Telescope perched atop Mauna Kea in Hawaii--the largest telescope in the world.

The astronomers' report on fragment R's collision with Jupiter on July 21 appears in the March 2 issue of Science, along with the reports of six other teams detailing their observations of the astronomical event of the decade.

Of the dozens of telescopes around the world trained on the planet during the unprecedented collision, only the Keck was large enough--and therefore fast enough--to obtain video of the collision. The 7.7 seconds between frames is several times better than the time-resolution of the other large telescopes that observed the event.

The astronomers recorded in detail the meteor trail left by the fragment, the fireball it created upon exploding, and a huge flare that resulted as all the debris from the explosion fell back toward the planet.

The meteor trail--which seen from the planet would have looked like a shooting star--shone for about 40 seconds as the comet fragment sped through Jupiter's upper atmosphere at 60 kilometers per second (40 miles per second).

About a minute after the peak of this meteor flash, the fireball from the exploding fragment appeared over the limb of the planet, indicating it probably shot 240 km (150 m) above Jupiter's cloud layer, said James Graham, assistant professor of astronomy. The fireball, akin to the mushroom cloud from an atomic bomb explosion, was visible for about three minutes before it faded.

While the meteor trail and fireball were predicted beforehand, Graham said, a third event was not. About six minutes after fragment R shot through the atmosphere, the impact site rotated into view, revealing a bright flare that briefly outshone the entire planet at infrared wavelengths.

"The thing that surprised everybody in their data is the really bright flare that occurred about six minutes after the fist blip," Graham said. "In all the analysis we forgot that what goes up must come down."

The flare, which lasted for 10 minutes and eventually reached 7,500 km (4,700 m) across, probably represented gravitational energy released as atmospheric gases thrown perhaps as high as 3,000 km (2,000 m) above the visible cloud tops fell back toward the planet. A few spikes in the measured brightness probably represented large clumps of gas falling back into the planet, Graham said.

The energy was so great that the flare expanded outward at a speed of about eight kilometers (five miles) per second, based on their calculations.

Team leader Imke de Pater, professor of astronomy, noted that the long meteor flash lasting 40 seconds indicates that fragment R had broken into numerous pieces before entering the atmosphere, probably spreading out over a distance of 1,100 miles.

"The long flash suggests that fragment R likely was pulled apart by Jupiter's tidal forces before impacting the planet," de Pater said.

The images of fragment R were captured by the Keck's Near Infrared Camera, which Graham helped build while at Caltech.

To obtain such rapid video the Berkeley team developed a special computer-software "movie" mode for the Keck Telescope. The team compiled 190 finished images into a video of the fragment R event.

This is the first time the Keck was used as a giant video camera, Graham said.

De Pater, Graham and the infrared camera will take part in another Keck video project later this year. Beginning May 20 the telescope will be trained on Saturn to look at the rings edge-on as the Earth briefly aligns itself with the ring plane. Because the rings are only a kilometer thick, observations must be taken within a window of only several hours around the crossing, de Pater said.

The team hopes to determine more accurately the geometry of the rings, in particular the fainter E ring, which is more dusty and therefore probably newer than the other rings.

They also will accurately time the disappearance of one of Saturn's moons behind the planet so as to pinpoint more precisely the moon's location. The moon and rings interact gravitationally producing density waves in the ring and theoretically causing the moon to slowly spiral outward, de Pater said.

The Berkeley team consisted of faculty members Graham and de Pater; J. Garrett Jernigan, an associate research physicist at Berkeley's Space Sciences Laboratory; and graduate students Michael C. Liu and Michael E. Brown. The work was funded by the National Aeronautics and Space Administration, the National Science Foundation, and the Packard Foundation.


Copyright 1995, The Regents of the University of California.
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