In a rare celestial spectacle, two comets were observed plunging into the Sun's atmosphere in close succession on June 1 and 2, 1998. Photo: NASA/ESA

Both amateur and professional astronomers are using the joint European/American Solar and Heliospheric Observatory to find so-called "sungrazing" comets that pass very close to our Sun.

After its launch in 1995, the SOHO spacecraft was placed at one of the LaGrange points--the point where the influence of gravity from Earth and the Sun is equal. It is currently located around 1.5 million kilometers from Earth and gets an uninterrupted view of the Sun.

SOHO's intended mission is to monitor the solar environment for solar flares and solar storms. This is needed to provide advanced space weather warnings to protect Earth and satellites from the dangerous effects of violent space weather.

During the first three years of its mission, SOHO's videos and images of the Sun were available only to team members at ESA and NASA centers. But in late 1998, SOHO's realtime videos were posted on the internet to be viewed live by anyone in the world.

Now, with the help of both project scientists and amateur astronomers, SOHO has found 200 of the sungrazing comets. The 200th comet was discovered earlier this month when amateur astronomer Michael Oates of England searched archival data. The image including the comet was taken on April 11, 1997. Doug Biesecker, a solar physicist at Goddard Space Flight Center (GSFC), says that he sent an e-mail to Oates in February that advised that he try using the "C2" coronagraph, which does not have as wide a field-of-view as the "C3" coronagraph. Since using both instruments, scientists and astronomers have doubled the number from 100 to 200 in just over months. It took over four years to find the first 100.

Said Biesecker of the "round-the-world helpers: "Amateurs have even taken the lead on realtime discoveries," added Biesecker. "If a comet zooms through the coronagraph'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!"

SOHO's 111th sungrazing comet, discovered in April 2000. Photo: NASA

SOHO uses a coronagraph instrument to find faint objects around the Sun. A coronagraph is a disk-like device that blocks out the intense light of the Sun so that only the surrounding corona is visible. This enables the spacecraft instruments able to see planets, stars, and comets in its field of view.

"No one expected to find all these comets when we launched SOHO nearly 5 years ago," says Biesecker. "They came as a huge surprise, there's no doubt about it."

Sungrazing comets are believed to be parts of larger and older comets that have broken up while traveling close to the Sun in the past. As they near the Sun, they brighten briefly then disappear behind the Sun. Most of the 200 sungrazing comets found to date were destroyed just a few hours after being found as they passed close to the Sun.