Aug. 18, 1994
Vol. 14, No. 2

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    University scientists have Earth's best view of comet crash


    At 6:30 p.m. on Saturday, July 16, astronomer Mark Hereld was supposed to be at a dinner party. But before leaving his lab that evening, he decided to check the stream of data coming in from the near-infrared telescope stationed at the South Pole. What he saw not only made him cancel his dinner plans, but captivated the world's attention for nearly a week.

    Hereld, Senior Research Associate in Astronomy & Astrophysics, led the team that designed and built the South Pole Infrared Explorer (SPIREX) telescope, which eyes the sky from atop a 2-mile-thick sheet of ice capping the South Pole. Hereld had asked Research Associate Hien Nguyen -- stationed at the South Pole -- to aim SPIREX at Jupiter to capture images of the planet as Comet Shoemaker-Levy 9 hurtled to its demise in Jupiter's gaseous depths. Hereld thought it would be a good exercise, but he had doubts that it would yield any important data.

    "The most recent predictions had indicated that it might be incredibly difficult to see anything at all with our instrumentation," Hereld said. "But we were obligated to give it a shot and plow through the data, even though we thought it very unlikely that anything would be visible."

    To his surprise and delight, the aftermath of the comet's impact proved to be not only visible, but spectacular. A glowing disk, brighter than one of Jupiter's moons, marked the spot where the first comet fragment had disintegrated and exploded when it burned up in the friction of Jupiter's atmosphere.

    "It took a little bit of convincing to make sure that what we were seeing was real, but when we started getting reports from other observatories around the world, saying they had seen something, too, I knew we had something," he said. "From that moment on, we were frantic."

    SPIREX's unique location allowed for an around-the-clock vigil of Jupiter. From other Earth-based observatories, Jupiter is visible for only part of the night and then sets below the horizon. Viewed from the South Pole, Jupiter appears simply to migrate around the horizon and never sets during the constant darkness of the six-month-long Antarctic winter night. SPIREX captured images from the aftermath of most of the 21 fragment impacts, except when blowing snow and clouds obscured Nguyen's view of the sky.

    After that first astonishing picture, the images became only more dramatic and more complex, as the glowing spots near Jupiter's south pole continued to be visible for days and spots from new impact sites began to overlap those from old ones.

    Hereld and his team at Chicago and at the Pole spent the next week working virtually around the clock. The pace was hectic as astronomers from observatories around the world raced to be the first to report data or produce pictures of Jupiter's impact sites. The Chicago scientists synchronized their waking hours with the times they could maintain contact with the Pole via satellite, the means by which they communicated and transmitted data. They worked through the night and took what amounted to a long nap in the middle of each day when the satellite link was down. At the Pole, Nguyen had help from two other astronomers, who worked in shifts to keep the telescope operating continuously.

    The 24-hour-a-day, six-day record of Jupiter's appearance as it changed throughout the week will prove invaluable as scientists begin to analyze all the data collected during the comet's impact.

    With SPIREX's near-infrared detector and camera, Hereld saw what he now thinks is hot dust -- "pulverized comet junk" -- that spewed above Jupiter's cloud tops. "It behaves as if it were embers or charcoal," Hereld said. "It's not burning, but it's glowing with residual heat from the impact."

    SPIREX's camera "sees" what radiates from the planet as heat. Hereld said he will continue to use SPIREX to take images of the planet to see how long it takes the hot spots to cool and fade from view.

    "This hot dust wasn't something that anyone was anticipating," Hereld said. And it wasn't the only thing that surprised him about the week.

    "I knew we were going to be busy taking data," he said, "but I had no idea that starting the next day we would be in the middle of a maelstrom of media attention." Reporters called at all hours for telephone and e-mail interviews, and television crews made once- or twice-daily visits to the lab.

    Although much of the media attention focused on Hereld and Nguyen, Hereld said SPIREX's success built on the efforts of many people. Bernard Rauscher, Research Associate in Astronomy & Astrophysics, designed and built SPIREX's detector, dubbed "GRIM," and Fred Mrozek, Engineer, designed and built the telescope itself. Graduate student Scott Severson helped with the stream of data coming in from the Pole, and Research Specialist James Lloyd, who joined the group only one week before the comet hit Jupiter, manned the Internet connection.

    At the South Pole, the building that houses SPIREX became a gathering spot for the "winter-over" crowd, even though it is located nearly a mile's walk away -- in frigid winter weather -- from the main complex. Chicago's John Briggs, at the Pole working on another project, became one of the two people who helped Nguyen man the telescope, and, with the University of Wisconsin's Joe Spang, at one point "heroically cleared" the telescope of blowing snow in strong winds and temperatures of 76 below zero Fahrenheit.

    The last comet fragment finally smashed into Jupiter early on the morning of July 22, and Chicago's astronomers breathed a sigh of relief as they were able to get their first night's sleep in nearly a week. Looking back after it was all over, Hereld said, "It was a wild ride. I certainly don't expect to go through another week like that again."

    Now the hard work of analyzing the data is left. SPIREX captured more than 2,500 images of Jupiter, by far the most continuous infrared record of the comet's impact. "When it comes to comparing one impact to another, we're going to have some of the best information in the world," Hereld said. But he said the lasting importance of SPIREX's data will emerge when the findings are combined with results from other groups to make a data base that scientists can sift through to learn more about Jupiter and about comet impacts in general.

    -- Diana Steele