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Results of SL9's Impacts Thrill Scientists


From the "JPL Universe"
July 29, 1994

Results of SL9's Impacts Thrill Scientists


The impacts may be over, but the aftermath of comet Shoemaker- Levy 9's (SL9) thrilling collision with Jupiter's atmosphere has left astronomers, scientists and amateurs fairly buzzing with excitement.

It could hardly have been a more spectacular event--one that "far surpassed astronomers' expectations," according to Dr. Paul Chodas, dynamicist in JPL's Solar Systems Dynamics Group in Section 314, and one that has left us with dramatic images and a wide variety of data to be scrutinized.

Indeed, that is SL9's next phase, which the comet's co- discoverer, Eugene Shoemaker, hopes will answer three important questions: How deep into Jupiter's atmosphere did the fragments reach? "We have hints that the fragments deposited their energy high in the atmosphere," he said in a press briefing from Goddard Space Flight Center, July 22, the day the last fragment hit. Knowing this, he said, may help scientists learn what materials comprise comets. "Are they rubble piles or snowballs?"

Secondly, Shoemaker wondered if new chemicals had been revealed via spectroscopic observations of the scars the fragments left after impact.

Finally, is water present on Jupiter?

Shoemaker said that Galileo--which, because of its unique vantage point to the side of Jupiter, was able to make the first direct observations of SL9 impacts--would play a vital role in providing researchers with time histories, including the exact moments of impact.

In fact, some data were already available only two days after the first impact, via the spacecraft's photopolarimeter radiometer instrument--a small telescope-like light meter on the spacecraft's scan platform--according to Dr. Terry Z. Martin, science coordinator for the instrument.

"The instrument observed Jupiter on July 18 and detected infrared light at a wavelength of 945 nm coming from the impact of the H fragment. Impact time was determined to be UT 19:31:59, and intensity was about 3 percent of the brightness of Jupiter itself. The brightening lasted for 25 seconds."

Such a measurement, according to Martin, "will help establish the sequence of events for these impacts, in which an initial meteor flash is followed by a plume of material seen by Earth- based observers. The timing will also permit the Galileo team to pinpoint data being stored on its tape recorder by other instruments, including the camera." This is significant, he explained, "because only a small part of the data recorded can be returned to Earth."

Martin said the team had captured data for fragments B, H, L, Q and S, and saw impact flashes for H, L and Q. The impact times measured by the Galileo instrument and the times at which ground- based observers reported seeing plumes are being used to estimate the actual times of the impacts, which will indicate where on the Galileo tape recorder to start reading out the data.

The impact times predicted by Chodas and Dr. Don Yeomans, senior research scientist in Section 314, were off by only two to 10 minutes.

More cause for amazement has been the scarring of the planet. "The large size and dark color of the impact scars are completely unexpected," Chodas said. "It's spectacular to see the suddenly changed appearance of Jupiter, clearly visible even through small telescopes."

He noted that the scars, some of which are two to three times the size of Earth, "are only gradually fading, and may last for months."

During the collisions, JPL became the nerve center for Internet access to SL9 images pouring in from observatories worldwide. A collection of 470 images on a Lab computer prompted more than 1 million downloads via the World Wide Web access system in the two weeks around the events.

"Ron Baalke of Section 335 deserves kudos for a great deal of time he put into organizing the Shoemaker-Levy access pages on a volunteer basis," said Frank O'Donnell, acting manager of the Public Information Office.

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