Image of the solar corona on December 23, 1996 from the SOHO spacecraft. The frame features Comet SOHO-6 with its tail curving towards the lower left side of the image. Photo: NASA

The record is to comet-hunting what Mark McGwire's home-run streak is to baseball: In just four years of operation, the Solar and Heliospheric Observatory (SOHO) spacecraft has found 102 comets, making it by far the most successful comet-hunter in history.

Calculations have shown that the latest comets discovered with SOHO are previously unknown (undiscovered) comets, with the 102nd comet observed by Dr. Douglas Biesecker, of SM&A, Vienna, VA, and NASA's Goddard Space Flight Center, Greenbelt, MD, a member of the SOHO team personally responsible for 45 of the discoveries.

A cooperative project between the European Space Agency and NASA, SOHO has revolutionized solar science. It also revealed an amazing number of suicidal comets plunging into the solar atmosphere.

Like nearly all of SOHO's comet discoveries, the latest comet showed up in images from the Large Angle and Spectrometric Coronagraph (LASCO) instrument. This is a set of coronagraphs that view the space around the Sun out to 12.5 million miles, while blotting out the bright solar disk with masks. LASCO watches for ejections of electrically charged gas from the Sun that threaten to disturb the Earth's space environment. As a bonus of unanticipated size, it also proved ideal for capturing objects falling to the Sun. Still pictures and movies from LASCO are freely available on the Internet, and even amateur astronomers have used them to discover comets.

Ten comets discovered by SOHO, including SOHO No. 100, 101 and 102, passed the Sun at a safe distance. However, the rest of the SOHO comets vaporized in the solar atmosphere. Near misses are well known, and 100 years ago Heinrich Kreutz in Kiel, Germany, realized that several comets seen buzzing the Sun seemed to have a common origin, because they came from the same direction among the stars. These comets are now called the Kreutz sungrazers, and the 92 vanishing SOHO comets belong to that class.

Calculations confirm that a comet spotted by Kazimieris Cernis of Vilnius, Lithuania, on February 4 is a previously unknown object, making it the 100th comet discovered with the SOHO spacecraft. Photo: NASA

"SOHO is seeing fragments from the gradual breakup of a great comet, perhaps the one that the Greek astronomer Ephorus saw in 372 BC," said Dr. Brian Marsden of the Center for Astrophysics in Cambridge, MA. "Ephorus reported that the comet split in two. This fits with my calculation that two comets on similar orbits revisited the Sun around AD 1100. They split again and again, producing the sungrazer family, all still coming from the same direction."

Their ancestor must have been enormous by cometary standards. "The rate at which we've discovered comets with LASCO is beyond anything we ever expected," said Biesecker. "We've increased the number of known sungrazing comets by a factor of four. This implies that there could be as many as 20,000 fragments."

Life is perilous for a sungrazer. The mixture of ice and dust that makes up a comet's nucleus is heated like the proverbial snowball in hell, and it can survive its visit to the Sun only if it is quite large. What's more, the strong tidal effect of the Sun's gravity can tear the loosely glued nucleus apart. The disruption that created the many SOHO sungrazers was similar to the fate of Comet Shoemaker-Levy 9, which went too close to Jupiter and broke up into many pieces that eventually fell into the massive planet in 1994.

The history of splitting gives clues to the strength of comets, which will be of practical importance if ever a comet seems likely to hit the Earth. Also, the fragments seen as SOHO comets reveal the internal composition of comets, freshly exposed, in contrast to the much-altered surfaces of objects like Halley's Comet that have visited the Sun many times.

The count of SOHO's comet discoveries would be one fewer without a late bonus from SOHO's Solar Wind Anisotropies (SWAN) instrument, which looks away from the Sun to survey atomic hydrogen in the Solar System. In December 1999, the International Astronomical Union retrospectively credited SOHO with finding Comet 1997 K2 (SOHO No. 93) in SWAN full-sky images from May to July 1997. It remained outside the orbit of the Earth even at its closest approach to the Sun, and thus did not vaporize entirely.