C/1999 S4 (LINEAR)


Breaking Up Is Hard To Do

Comets used to be named after people. Now, it seems they're named after telescopes and asteroid survey programs. As a result, there are more comets with acronyms for names than comets named after people. The biggest culprit is SOHO. There are more than 200 comets named SOHO, an acronym for Solar and Heliospheric Observatory. SOHO is a space-based solar observatory that keeps a constant eye on the Sun. On occasion, SOHO will witness the passage of a special class of comet called a sungrazer. The first sungrazers were observed more than 2,000 years ago. They are believed to be remnant fragments of a comet that broke up after a close encounter with the Sun. Most sungrazers are so small that they don't become visible until they get very close to the Sun. Only the SOHO spacecraft is capable of detecting these kamikaze comets.

There are more than 50 comets named, LINEAR. LINEAR is an acronym for the Lincoln Near-Earth Asteroid Research project. LINEAR is funded by the U.S. Air Force and operated by scientists from MIT Lincoln Laboratory. LINEAR was setup to find asteroids and comets in Earth-crossing orbits. These NEOs--another acronym--pose a real threat to us. We know our home planet has been hit in the past by comets and asteroids. It is only a matter of time before we are hit, again. By cataloging all the near-Earth asteroids and comets, we give ourselves a fighting chance to prevent a future disastrous impact.

Among the more than 50 comets discovered by the LINEAR team, is C/1999 S4. The "C" tells us that the orbit is longer than 200 years. The "1999" refers to the year of discovery. The "S4" tells us that this was the fourth comet discovered during the last half of September. Within weeks of this comet being discovered, astronomers realized it would pass close enough to the Earth that it should put on a pretty good show. Early predictions indicated that comet LINEAR might be visible to the naked eye. However, several months of study revealed that this comet was probably making its first passage through the inner Solar System. The gas and dust production was not increasing as would be expected for a comet that has been around the block a few times. By December 1999, astronomers had tempered their enthusiasm somewhat, predicting LINEAR would look impressive through telescopes but would not brighten to naked eye visibility.

Fast forward to July 2000. Comet LINEAR was approaching its peak performance. The comet had indeed become a fine object for amateur telescopes. Then, something unexpected happened. The comet nucleus suddenly disappeared. Only the gas and dust left in the comet's wake could be seen. Even large professional telescope were unable to detect the comet nucleus. What had happened? The early theory was that comet LINEAR underwent a sudden and total disruption due to the intense heat and gravitational force of the Sun. Images taken by the Hubble Space Telescope (HST) supported this theory.

Comet LINEAR was not observed by many folks outside the professional and amateur astronomy community. I suppose public enthusiasm for comets has tempered a bit since the spectacular displays of Hyakutake and Hale-Bopp. Click on the below link to see my sketch and observation notes for comet 1999 S4 (LINEAR).

July 19, 2000

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