Feb. 17, 1994
Vol. 13, No. 12

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    Comet collision with Jupiter destined to make cosmic wave

    Comet Shoemaker-Levy 9 is destined to make waves in the solar system this summer when it smashes into Jupiter during the week of July 17. For Mordecai-Mark Mac Low, Research Associate in Astronomy & Astrophysics, the cosmic accident is presenting a once-in-a-lifetime research opportunity.

    "This is a unique opportunity," said Mac Low. "It's the largest impact in our solar system that we've been able to predict and then observe. We've never seen anything like this before, and we probably won't see another in our lifetimes."

    Shortly after the comet was discovered nearly a year ago, Mac Low, an expert in astrophysical gas dynamics, joined forces with Kevin Zahnle, a research scientist at NASA Ames Research Center, to study what may happen when the comet -- which is now fragmented into 21 pieces -- hurtles into Jupiter's gaseous interior. Enveloped in a heavy cloud cover, Jupiter is a ball of primarily hydrogen gas with an inner core of solid, metallic hydrogen.

    Mac Low and Zahnle, who studies planetary impacts, have combined their expertise to devise a computer model to predict what will happen when the comet hits Jupiter. Although it will be several months before they are proved right or wrong, Mac Low said he thinks they already have a good understanding of the problem.

    "I was already working on problems in interstellar gas dynamics with very similar physics, so we were able to move quickly," Mac Low said.

    "We predict that the comet will explode. The comet will fragment, vaporize and release its tremendous kinetic energy relatively quickly. The resulting explosion will be as energetic as 200,000 to a million megatons of TNT -- larger than the world's combined nuclear arsenal," he said.

    For scientists on Earth who will be watching for the impact, the explosion should produce a plume detectable above Jupiter's cloud layer, he said.

    There's some uncertainty concerning the amount of energy that will be released because scientists are uncertain about the size of the comet's fragments. Estimates vary from half a kilometer to 5 kilometers across. Because a factor of 10 in diameter is a factor of 1,000 in mass, and because energy released is directly proportional to the mass of an object, a rock 10 kilometers in diameter would release 1,000 times more energy than a rock one kilometer across.

    No one can predict whether there will be any long-lasting effects on Jupiter itself, but because the planet is so large -- 318 times as massive as the Earth -- scientists think the long-term effects will be negligible. Something of the same magnitude on Earth, Mac Low said, "would be a continent-killer."

    Scientists estimate that the comet has been orbiting Jupiter since the 1950s. It is surmised that the comet was pulled apart into the many fragments we see today when it made a pass close to the planet in July 1992 and was ripped apart by Jupiter's gravitational forces.

    The comet was discovered in March 1993 by Carolyn and Eugene Shoemaker and David Levy, who were actually looking for asteroids that might be crossing Earth's orbit. Shortly thereafter, scientists calculated that the comet's orbit would take it directly into Jupiter's path this summer.

    Mac Low's brightly colored computer animation brings the impact to life. The computer model shows the comet entering the Jovian atmosphere, traveling to about 50 miles below the cloud layer and then exploding, driving a fireball back up through the clouds.

    Unfortunately, the comet will hit the far side of Jupiter, so the only direct observers of the event will be NASA's space probes Galileo and Ulysses. Galileo has a visual telescope, but a broken main antenna, so it is communicating at only 10 bits per second and will take nearly a week to transmit an image. Ulysses has only a radio telescope, and predictions of radio emission from the impact remain uncertain.

    The comet's fragments will hit Jupiter over a period of five days, beginning July 17. Eighteen minutes after each impact, the impact site will rotate to face Earth, providing scientists around the world an opportunity to view the aftermath.

    Mac Low said the impact will provide a rare opportunity for scientists to learn more about Jupiter. The comet will act as a probe of the Jovian atmosphere, said Mac Low, lifting material from the deep atmosphere above the clouds, where scientists may be able to observe and characterize it for the first time. The seismic waves from the impact may also give clues about the deep interior of the planet -- much the way earthquakes tell us about the Earth's core -- and the dust and ice crystals remaining in the stratosphere after the impact may trace the winds in the transparent layer above the clouds.

    If Mac Low and Zahnle's model proves effective in predicting what happens on Jupiter this July, the model could be used to make theoretical predictions about any asteroids that may be heading toward Earth. With some modifications to account for the difference in gravity and atmospheric composition, Mac Low said, "Our model could tell us how big a rock has to be before we have to worry about it."

    But that's not something we'll need to be concerned about in the near future, he added. Even if scientists do locate an asteroid or a comet that's headed for a collision with Earth, he said, we'll probably have at least a hundred years' warning.

    -- Diana Steele