Comet LINEAR 2001/A2: The Little Comet That Could

(A revised and expanded version of an article originally published in the September 2001 issue of Eyepiece)


Comet LINEAR 2001/A2 on July 26, 2001. The image shows isophotes, zones of equal
brightness around the coma. Photo copyright 2001 Gianluca Masi, used by permission.
For more of Gianluca's comet photos, see
www.bellatrixobservatory.org


When Comet LINEAR (2001/A2) was discovered in January as a 19th-magnitude object, nobody expected it to become more than an unassuming blob, barely visible from the New York City area in any telescope, when at its best. Yet the comet exceeded all expectations, peaking around June 13 at better than magnitude 3.5—over 100 times its predicted brightness—thanks to a very active nucleus that crumbled and split several times, releasing a large amount of material to fuel its brightness. By May it had become an easy naked-eye sight for people in southern latitudes, and it remained bright through mid-July as it swung north into our skies. This surprising object has put on the best cometary show since Hale-Bopp’s passage in 1997. As David Levy once said, "Comets are like cats; they both have tails, and they do exactly what they want."

For the first couple of months after discovery, LINEAR showed no unusual characteristics, brightening to magnitude 13 by late March. But around March 25, an Australian observer, Michael Mattiazzo, noted that it had suddenly brightened to magnitude 10; by March 30 it had jumped to magnitude 8, a hundredfold increase in brightness in a matter of days. Cometary outbursts are often short-lived, but LINEAR showed no signs of flagging. By late April, the comet was a naked-eye object, and remained so for observers with dark skies for over two months. On April 30, what many astronomers had suspected was shown to be true: LINEAR’s nucleus had split in two, feeding the cloud of gas and dust known as the coma that surrounds the nucleus, and spurring its binge in brightness. In mid-May, one of the two fragments itself split in two, and now the comet had three nuclei moving in formation. LINEAR reached perihelion on May 24, passing about 73 million miles from the sun, and came closest to Earth around June 30, just 23 million miles away.

I first saw LINEAR on July 2, from Ann Arbor, Michigan, a city of nearly 100,000 even with the University of Michigan’s campus in its summer slow season. A couple of hours before dawn, I walked out to a nearby park. I knew the comet was located below the Great Square of Pegasus, so I scanned with my binoculars and soon found it: a bright fuzzball with no discernable tail. It looked rather similar in brightness and appearance to the Andromeda Galaxy, well to the comet’s upper left. Though it was about magnitude 4.5, I was unable to see it with the naked eye; summer haze and light pollution impeded the view. I saw five days later, again from Ann Arbor; it had rapidly moved north through Pegasus away from the predawn glare. On July 14 and 15, when I observed LINEAR from Westport, Connecticut, it had already become visible in the midnight sky as it pulled away from both Earth and Sun and headed into the depths of space. It was still quite bright, magnitude 5 or better. It turns out it was undergoing another mini-outburst, but it soon faded; later in the week, I tried to observe it on several nights from Queens, NYC, with haze and nearby lighting to add to the normal severe light pollution, without any luck. By month’s end it had already faded to 7th magnitude. As of September 19, it was down to magnitude 13.

Comet LINEAR is in a highly elongated orbit, with a period of approximately 30,000 years, originating in the outer Kuiper Belt. It is making one of its first visits to the inner solar system, and is unusually active for such a "new" comet. This has given astronomers a rare opportunity to study relatively pristine matter left over from the formation of the solar system that has existed in a frozen form for billions of years.

This comet was the 54th comet discovered by Project LINEAR (Lincoln Near-Earth Asteroid Research), an automated sky survey designed to locate objects with orbits that intersect with Earth’s and could conceivably collide with our world. It is an MIT project funded by the U.S. Air Force that uses a 1-meter Cassegrain telescope at the White Sands Missile Range in Socorro, New Mexico to collect CCD images of the night sky. The images are then transmitted to MIT’s Lincoln Laboratory in Massachusetts, where they are checked and sent on to the Minor Planet Center, where any discoveries are recorded. In addition to discovering numerous near-Earth asteroids, Project LINEAR has picked up large numbers of comets (over 60 at last count), including many that would have otherwise been discovered visually by amateurs. Through 1999, about a half-dozen comets were found each year by amateurs; in each of the past two years, only one amateur visual discovery was made. Another Comet LINEAR, this one designated 2000 WM1, may approach naked-eye visibility when at its best this winter (though it is currently lagging well behind early predictions; as October opens it’s about magnitude 11.5).


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