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Hubble, Spanish Observatory Catch Comet Linear Breaking Up
By Maia Weinstock
Staff Writer
posted: 08:32 am ET
29 July 2000
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Hubble, Spanish Observatory Catch Comet LINEAR Breaking Up 
These three Hubble Space Telescope images chronicle a violent outburst in the life of Comet Linear. A chunk of material that was blown from the comet's nucleus can be see in the final picture. At that point, the chunk was about 290 miles (470 kilometers) away from the nucleus.
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As far as naked-eye stargazers are concerned, this year's passing of Comet Linear was a bit of a dud. Though hyped as the next big thing since Comet Hale-Bopp's brilliant performance in the spring of 1997, Linear has commanded stargazers to rely on at least a pair of binoculars if they wanted to catch a glimpse. But several major discoveries made by more high-tech eyes have now turned Linear into one of the most intriguing comets ever to zoom by Planet Earth. With NASA's Hubble Space Telescope staring straight at the icy traveler earlier this month, astronomers were treated to a cometary dazzler when a chunk of Linear's nucleus (icy core) literally blew off. Then, observations made this week with the 40-inch (1-meter) Jacobus Kapteyn Telescope in the Spanish Canary Islands showed another major explosion of the comet's nucleus. Thanks to these rare sky shows, astronomers say they may soon learn about some of the more critical details of cometary structure and dynamics. "This was a complete surprise to us," said Harold Weaver, one of the researchers at John's Hopkins University who viewed the comet's first outburst. "I couldn't believe it when I brought these images up on the screen -- the first day it looked like a normal comet, and then the next day was just completely different." At the time of the Hubble observations, Linear (named for the acronym that stands for Lincoln Laboratory Near-Earth Asteroid Research) was between 0.7 and 0.8 astronomical units, or about 74 million miles (120 million kilometers), from Earth. Scientists say the side-splitting event started on July 5 as the comet began spitting out a large amount of dust, increasing the its brightness by over 50 percent in just four hours. "It was probably the biggest outburst in terms of the magnitude of change that I had ever seen in a comet," said Weaver. | Chandra Solves LINEAR Mystery | | On July 14, NASA's Chandra X-ray Observatory also made a spectacular Linear observation -- it detected X-rays from the comet's oxygen and nitrogen ions. "This observation solves one mystery," said Cary Lisse of the Space TelescopeScience Institute in Baltimore. "It proves how comets produce X-rays." The observation was made with Chandra's Advanced CCD Imaging Spectrometer instrument. | Next, a significant chunk of the nucleus blasted off and was blown back by the solar "wind" that heats up comets as they streak closer to our closest star. Finally, Linear's brightness normalized, and the split-off chunk was noticed further on down in the comet's "tail," though it had already started to disintegrate. Scientists don't know how big the chunk was that flew off, but evidence so far points to it being what's called "macroscopic" in size -- in other words, inches to feet (centimeters to meters) in diameter. Although Hubble's sharp eye gave astronomers a relatively detailed view of the cometary fragmentation, Weaver explained that it's virtually impossible to tell the size of the chunk because "we don't even know how big the nucleus is." ~ On July 25, a similar event was captured by astronomers working at the Observatorio del Roque de Los Muchachos in La Palma, Spain. But this time, rather than spitting off a chunk, the nucleus appeared to have completely blown apart. "Something very odd was happening to the comet," said Mark Kidget, of the Instituto de Astrofisica de Canarias. "The central condensation was seen to be strongly elongated." In other words, the comet had almost completely broken up. 
| An image of the nucleus of Comet Linear, taken by the Jacobus Kapteyn Telescope on July 26. The elongated shape of the comet's bright head indicates that its nucleus was disrupted. |
Astronomers believe that cometary fragmentation is a normal occurrence, though they haven't been able to catch many in action. In fact, Weaver said that the only time scientists have seen something similar to these most recent sightings was with Comet Hyakutake in March of 1996. Though astronomers haven't had sufficient time to perform detailed analysis of the Hubble and Jacobus Kapteyn Telescope images, the events should give them a greater understanding for how the contents of cometary nuclei are put together. "The forces that caused it to break up are incredibly miniscule," said Weaver. "It just shows you how fragile cometary nuclei are."
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