Comets have long been associated with outbursts of celestial spectacle, but as performers they can sometimes be a bit erratic and hard to predict.
Astronomers weren't making any advance bets on fireworks from Comet LINEAR, a recently discovered comet that hurtled past the Earth in late July. LINEAR disappointed even their dimmest expectations, failing to become bright enough to be visible to the naked eye and leading one Web pundit in early July to label it "the fizzle of 2000."
However, as that report was written, Hopkins astronomers Hal Weaver and Paul Feldman were using NASA's Hubble Space Telescope to watch LINEAR produce a powerful, flashy "bang." As they observed on July 5, 6 and 7, the comet flared to seven times its original brightness and dimmed again.
Several possible explanations for the flare emerged on July 7, when pictures from the Hubble telescope revealed a small chunk of cometary material in the comet's tail, clearly separate from the main body, or nucleus, of the comet. The chunk was slowly sailing down the tail at about 6 miles per hour.
"We lucked out completely," Weaver says. "In one surge of brilliance, this underperforming comet showed us what it could have been. Comet LINEAR generally has not been as bright as we had hoped, but it occasionally does something exciting."
Possible causes behind the explosive outburst include a previously shaded, volatile area of the comet suddenly vaporizing during its first exposure to direct sunlight; or a longer-term buildup under the comet's surface of gas pressure as exposure to the sun changed ice to gas.
The latter scenario would have led to the cometary equivalent of a volcanic eruption, although at temperatures much lower than the term "volcanic" normally suggests. Subsurface gas pressure in the comet, whose core temperatures are normally hundreds of degrees below freezing, could have blown off a chunk of the surface and a large quantity of ice and dust, creating the flare in brightness that researchers observed.
Evidence accumulated over the past decade has supported the idea that comets may be agglomerates of smaller clumps of ice and gas known as cometesimals, and this suggests a third possible explanation. One of the comet's constituent cometesimals may have dropped out of the nucleus. Gas streaming from the surface of the chunk could have produced a jetlike force that forced the chunk to move down the tail, where it eventually broke it into pieces too small to see.
Weaver, a cometary science specialist, observed similar "daughter" fragments when studying Comet Hyakutake in 1996. The new LINEAR observations lead him and others to speculate that such cometary breakups are more common than previously thought.
"Although we feel very lucky to have seen this happen, it's unlikely that we just happened to catch one isolated event," says Feldman, chair of the Department of Physics and Astronomy.
Weaver and his research team originally had trained the Hubble Telescope's Imaging Spectrograph on LINEAR to learn more about the comet's composition.
Astronomers believe comets may in many respects be relatively unchanged from the earliest days of the solar system, and could therefore offer important clues to what happened during the formation of Earth and other planets. For example, some theorists have speculated that comets may be linked to the origins of Earth's water supply.
Results from the Hubble observations showed a lack of carbon monoxide in LINEAR relative to other comets. Weaver says this suggests LINEAR originally formed much closer to the sun, where warmer temperatures would have depleted its carbon monoxide content. He hopes to further analyze the data to fix the area where LINEAR originally formed. This was probably somewhere near the planets Jupiter, Saturn, Uranus or Neptune, where a pass by the gravity fields of one of these tremendous planets could boot the comet into the Oort cloud, a vast reservoir of cometary material on the distant edges of the solar system. Astronomers believe many comets begin their periodic plunges into the solar system from this cloud.
To see color photographs, log onto http://www.pha.jhu.edu/~weaver/linear/.