Astronomers analyzing debris from a comet that broke apart last summer spied pieces as small as smoke-sized particles and as large as football-field-sized fragments. But it's the material they didn't see that has aroused their curiosity.
Tracking the doomed comet, named C/1999 S4 (LINEAR), NASA's Hubble Space Telescope's Wide Field and Planetary Camera 2 found tiny particles that made up the 62,000-mile-long (100,000-kilometer-long) dust tail and 16 large fragments, some as wide as 330 feet (100 meters). Hubble detected the small particles in the dust tail because, together, they occupy a large surface area, which makes them stand out in reflected sunlight. However, the estimated mass of the observed debris doesn't match up to the comet's bulk before it cracked up.
"The mass of the original, intact nucleus is estimated to be about 660 billion pounds (300 billion kilograms), according to some ground-based observers who were measuring its gas output," says Hal Weaver, an astronomer at the Johns Hopkins University in Baltimore, Md., who studied the comet with the Hubble telescope, the European Southern Observatory's Very Large Telescope (VLT) in Chile, and other ground-based telescopes.
"However, the total mass in the largest fragments measured by the Hubble telescope and the VLT is only about 6.6 billion pounds (3 billion kilograms), and the dust tail has an even smaller mass of about 0.7 billion pounds (0.3 billion kilograms). In other words, the total mass measured following the breakup is about 100 times less than the estimated total mass prior to the breakup." Weaver's results will be published in a special May 18 issue of Science devoted to the transitory comet.
So where is the rest of the comet's fractured nucleus? Perhaps, suggest Weaver and other investigators, most of the comet's bulk after the breakup was contained in pieces between about 0.1 inches (2.5 millimeters) and 160 feet (50 meters) across. These pebble-sized to house-sized fragments cannot be seen by visible-light telescopes because they do not have enough surface area to make them stand out in reflected sunlight. Comets are leftover debris from the creation of the solar system 4.6 million years ago. They're made up of a combination of solid rock and frozen gases held together by gravity.
If the midsized cometary fragments exist, then the fundamental building blocks that comprised LINEAR's nucleus may be somewhat smaller than what current "rubble pile" theories of the solar system's formation suggest. These theories generally favor football-field-sized fragments, like the ones observed by the VLT and the Hubble telescope. The analysis of LINEAR's fragments indicates that the "rubble" comprising cometary nuclei may be somewhat smaller than previously thought.
Another puzzling question is why the comet broke apart between June and July of last year as it made its closest approach to the Sun.
"We still don't know what triggered the comet's demise," Weaver says. "But we do know that carbon monoxide (CO) ice probably did not contribute to the breakup."
Hubble's Space Telescope Imaging Spectrograph detected low levels of this volatile material, about 50 times less than was observed in comets Hale-Bopp and Hyakutake. Carbon monoxide ice sublimates [changes directly from a solid to a vapor] vigorously, even at the cold temperatures in a comet's interior. This activity could lead to a buildup of pressure within the core that might cause the nucleus to fragment.
"The scarcity of carbon monoxide in LINEAR's nucleus is problematic for any theory that attempts to invoke it as the trigger for the comet's demise," Weaver says.
An armada of observatories, including the Hubble telescope, watched the dazzling end to the transitory comet. Hubble was the first observatory to witness LINEAR breaking apart, spying in early July a small piece of the nucleus flowing down the doomed comet's tail. LINEAR completely disintegrated in late July as it made its closest approach to the Sun, at a cozy 71 million miles. Again, the Hubble telescope tracked the comet, finding at least 16 fragments that resembled "mini-comets" with tails. Now LINEAR is little more than a trail of debris orbiting the Sun. The comet is believed to have wandered into the inner solar system from its home in the Oort Cloud, a reservoir of space debris on the outskirts of the solar system.
"We were witnessing a rare view of a comet falling to pieces," Weaver says. "These observations are important because, by watching comet LINEAR unravel, we are essentially seeing its formation in reverse. The nucleus was put together 4.6 billion years ago when the Earth and other planets were forming, so by watching the breakup we are looking backwards in time and learning about conditions during the birth of the solar system."
Weaver notes, however, that astronomers may have witnessed an "oddball" comet break apart.
"I've never seen anything like this," he says. "I know of no other example of a comet falling to pieces like this. Comet Shoemaker-Levy 9 fell apart, but tidal forces from Jupiter caused that disintegration. LINEAR didn't come close to any other large object. Comet Tabur (C/1996 Q1) also seemed to vanish without a trace, but it already was the fragment of another comet nucleus [C/1988 A1 (Liller)]. Some investigators concluded that Tabur did not even break up but rather, became 'invisible' only because the icy area on its surface was no longer in sunlight, and its activity shut down as a result."
Comet LINEAR was named for the observatory that first spotted it, the Lincoln Near Earth Asteroid Research (LINEAR) program.
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