"The Cosmic Event of the Millennium"

Comet Shoemaker-Levy 9 to Crash Into Jupiter

Galileo will have a front-row seat in an extraordinary galactic performance beginning July 16, when Comet Shoemaker-Levy 9 (SL9) collides with the largest planet in our Solar System. The performance will last six days, as some 22 massive chunks of the comet crash into the atmosphere of Jupiter at the average rate of one every six hours. The comet broke into pieces two years ago when its orbit brought it close to Jupiter. This time, scientists anticipate that the impacts will result in large explosions, whose flashes may be visible directly to Galileo and which may be bright enough to be seen by Earth-based observers by reflection off Jupiter's closest moons. The largest piece will collide with the planet on July 20, coincidentally the twenty-fifth anniversary of the Apollo 11 landing on the Moon.

None of this was anticipated when Galileo was launched in October 1989. In fact, SL9 was discovered just over a year ago in March 1993, when astronomers David Levy and Gene and Carolyn Shoemaker spotted the comet from the Mount Palomar Observatory in California. Their discovery sparked the interest of others, and soon many telescopes were tracking the comet and computers were calculating its trajectory. These orbital investigations led to the surprising realization that SL9 was on a collision course with Jupiter.

As luck would have it, Galileo will be well poised to view the celestial fireworks, which will occur on the far side of Jupiter as viewed from Earth and on its nightside. No other spacecraft (except Voyager 2, which is currently over 6 billion kilometers from Jupiter), including the Hubble Space Telescope, nor any Earth-based telescope will have that viewing advantage. Those telescopes will have to settle for observing indirect effects, such as the previously mentioned reflections or the aftermath of the damage as the impact areas rotate into view of the Earth. It is important to realize, however, that Galileo is still 240 million kilometers away from Jupiter, and the impacts--if they are visible at all--will be only a small dot of light in the Galileo images.

The Challenge and Payoff

Capturing the images and other science data is complicated by several factors, including the limited tape recorder space, the reduced downlink capability, the uncertainty in actual effects of the comet impacts (due to our poor understanding of its size), and the several-minute uncertainty in the actual impact time of each fragment. The science teams have adopted a strategy of using different observing schemes on different impacts so as to cover much of the uncertainty space. For example, the Solid-State Imaging camera will utilize four different schemes to measure light intensity from Jupiter. Scientists hope these data will reveal more about the composition of Jupiter's atmosphere, the origin of the comet, and the implications of a similar impact to our planet.

Some believe the collision will create giant bubbles of hot gas that will bring materials from far below Jupiter's thick cloud cover to the top, where those materials can be analyzed. Others think the impact could even cause changes in the planet's distinctive banded appearance or cause the birth of new spots.

How can a tiny comet have such an impact on the giant planet? "Comparing the mass of SL9 fragments to Jupiter is a bit like comparing a gnat with an elephant," writes Clark Chapman, a senior scientist at the Planetary Science Institute in an article for New Scientist (March 5, 1994). "But, just as . . . dust ejecta arising from an impact on Earth changes global climate, so the impacts on Jupiter should profoundly affect that planet's stratosphere." Scientists point out that even a few cubic kilometers of comet ices can overwhelm the dynamics and chemistry of Jupiter's atmosphere locally.

Scientists also hope to learn more about the origin of SL9. From observations of the fragments' brightness and arrangement along their orbits, astronomers believe the comet must have measured between 3 and 9 kilometers in diameter before it fragmented. They also believe that SL9 has been in orbit around Jupiter for 20 to 100Êyears. Since most comets orbit the Sun, this intriguing finding led to speculation that SL9 began life as either a small satellite of Jupiter, a so-called Trojan asteroid locked into Jupiter's orbit, or a comet captured by the planet's strong gravitational pull.

Ironically, if SL9 had not fragmented, astronomers would never have known it existed; it was too small, dark, and faint. But after its breakup, the long string of fragments and the dust associated with them presented a vastly larger surface area off which to reflect sunlight. When Carolyn Shoemaker first spotted the odd-shaped SL9, she thought it was a "squashed comet." Her team immediately notified colleague Jim Scotti in Arizona, who used a larger telescope equipped with sensitive charge-coupled devices and was able to determine that the object was actually a string of comet fragments.

Finally, SL9's crash will give astronomers a chance to actually witness a comet shaping a planet's future, a process that has been going on since the beginning of our Solar System. Nearly 200 large impact craters still exist on Earth, despite erosion and extensive plate tectonics that have erased many of them. Scientists say that every million years or so an asteroid or comet strikes our planet, which can drastically change our global climate. Every hundred million years, one strikes with a size and force that can actually change the course of evolution, like the 10kilometerwide fragment that struck the Earth 65 million years ago and is believed to have wiped out the dinosaurs. What SL9 does to Jupiter will give scientists a better idea of what happened on Earth in the past.

Theories aside, "we've never seen anything hit a planet that is even as big as a house," according to Steve Marain, a spokesman for the American Astronomical Society, who was quoted in the April issue of Aerospace America. Several SL9 chunks may be as large as small mountains.

Worldwide Interest

Astronomers are comparing this event to the excitement generated by Comet Halley, which captured the attention of millions of people around the world in 1986. The Planetary Society is setting up a worldwide network of observers to strive for constant coverage from ground-based telescopes during the string of impacts. Under the title "Jupiter Watch," the Planetary Society will publish guides, provide video links from telescopes to classrooms and television stations, and organize Jupiter Watch parties in order to educate the public about astronomy, space, and the origin of life.

All this attention cannot fail to put the spotlight on Galileo, with its unparalleled view of the action and its ability to detect the effects of the SL9 impacts through several of its instruments. It will also focus attention on Galileo's arrival at the Jovian system on December 7, 1995, some 17 months after the collision.

(Updates on Jupiter Watch events can be obtained by calling 1-800-9-WORLDS.)

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