twin comets
Image: NASA

TWIN COMETS, in the lower right corner, captured by SOHO's corongraphs.

The Solar and Heliospheric Observatory (SOHO), launched in 1995, was designed to spy on the sun. More specifically, researchers wanted to study the sun's internal structure, the solar atmosphere and the origin of the solar wind. Little did they know that SOHO would also capture the ephemeral trails of sungrazing comets. Indeed, astronomers recently announced that SOHO spotted its 200th such streaker, dubbed SOHO-200, and thereby set a new record for comet discoveries. What's more, amateur observers have identified many of these comets.

As a result of their close encounter with Sol, most sungrazers succumb to the star's scorching heat, which vaporizes their frozen cores. Intriguingly, most of these brazen comets appear to be remnants of a single supercomet that fragmented near the sun more than 2,000 years ago. In fact, according to Brian Marsden of Harvard University's Minor Planet Center, that giant comet might have been the very same one described in 372 B.C. by the Greek astronomer Ephorus, who observed a bright comet that split in two. Repeated divisions might have eventually yielded the so-called Kreutz family of sungrazers, which share an elliptical orbit that brings them so dangerously close to the sun.

The abundance of such comets comes as a surprise to many astronomers, because prior to SOHO sungrazers were only known from the few large enough to withstand the sun's heat and emerge intact, visible from the ground. The other, smaller comets were far fainter. Now, however, thanks to SOHO's coronagraphs--devices that block the glare of the sun in order to reveal the corona and nearby stars and planets--many more sungrazers can be spotted.

Because the images are updated every half hour on the SOHO Web site, anyone armed with Internet access might be the next comet discoverer. Archival images are also available. In fact, it was by studying a picture from April 1997 that amateur astronomer Michael Oates identified SOHO-200. Such contributions have not escaped the attention of professional observers. "Amateurs have even taken the lead on real-time discoveries," says solar physicist Doug Biesecker of the Goddard Space Flight Center. "If a comet zooms through the coronograph's field of view at 2 a.m. here at Goddard, someone in Europe is probably looking at the Web site while we're asleep!"