Photo: NASA/ESA

Amateur astronomers worldwide placed their bets, crossed their fingers and waited in anticipation for the ESA-NASA Solar & Heliospheric Observatory (SOHO) spacecraft to spot its 500th comet. Their patience was rewarded on August 12 as SOHO, a mission actually designed to research the Sun, revealed its own type of royal flush with comet C/2002 P3 (SOHO).

It is the 500th comet to be discovered by SOHO since it launched on December 2, 1995, making the spacecraft by far the most prolific comet-hunting observatory in history.

Between May 2 and May 31, using the SOHO website, 1,256 amateur astronomers tracked SOHO's observations of comets and made predictions about the date and time that the 500th comet would be at perihelion (the closest approach to the Sun). Comet C/2002 P3 (SOHO) had a perihelion time of August 12 at 12:04:48 p.m. EST.

Diane McElhiney won the sweepstakes with a prediction only one hour and 43 minutes away from the correct time.

The website also allows comet aficionados around the world to look at real-time SOHO data and images. Once they see what appears to be a new comet, they seek confirmation from others on-line. If confirmation is provided, they have accomplished a feat that is the goal of many amateur astronomers.

More than 75 percent of the comet discoveries have come from amateurs from Australia, Canada, China, France, Germany, Italy, Lithuania, Russia, the United Kingdom and the United States.

"Analyzing SOHO data is a great challenge and you have to combine many skills," said Rainer Kracht, the discoverer of C/2002 P3 (SOHO). "But every amateur astronomer dreams of finding a comet."

Kracht, a mathematics, physics, computer science and astronomy teacher at the Kooperative Gesamtschule Elmshorn in Elmshorn, Germany, has discovered 63 comets since August 2001 with the help of SOHO data and images. An entire group of near Sun comets that he discovered is even named after him - the Kracht group.

C/2002 P3 (SOHO) is not only an unanticipated milestone for the mission, but whereas most near Sun comets are of the Kreutz family, C/2002 P3 (SOHO) is a member of a family of comets called the Meyer group. The Meyer group is one of three new families that have been discovered with SOHO data. A comet family is a group of comets that originated from the same place and follow the same path. C/2002 P3 (SOHO) has a perihelion distance of 5.37 million km from the Sun.

"We don't yet know much about this family, or the other new families," said Dr. Doug Biesecker, the head of SOHO's comet discovery program stationed at NASA's Goddard Space Flight Center and a solar physicist with L3 Com Analytics Corporation. "With such few known members of this family, we need every additional comet we can get."

The use of SOHO as a tool to discover comets is a byproduct of the mission's original purpose - to continuously monitor solar weather and assist scientists in analyzing how the Sun functions. The Large Angle and Spectrometric Coronagraph (LASCO), one of the instruments on SOHO, blocks out the Sun to create an artificial eclipse and then views the space around the Sun, looking for outbursts of solar activity. This eclipse allows the Sun's faint outer atmosphere (corona) to be seen and studied.

Like the corona, comets that would usually be lost in the glare of the Sun unexpectedly come into LASCO's field of view. Almost all of them end up vaporized in the solar atmosphere. In spite of the fact that the SOHO Kreutz family comets tend to be only 10 meters in diameter, they are fragments of a comet that was originally much larger, maybe 100 km in diameter. This is so large, in fact, that it might have been visible during daylight more than 2000 years ago.

"Though LASCO was not originally built for this purpose, no other instrument has discovered more of these comets," said Dr. Bernhard Fleck, ESA Project Scientist for SOHO stationed at Goddard. "It was a wonderful surprise, of the kind we get in science sometimes."