[meteorite-list] History's Greatest Comet Hunter Approaches Major Milestone (SOHO)

[Ron Baalke]

Dolores Beasley
Headquarters, Washington                       July 6, 2005
(Phone: 202/358-1753)

Bill Steigerwald
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-5017)

RELEASE: 05-173

HISTORY'S GREATEST COMET HUNTER APPROACHES MAJOR MILESTONE

The Solar and Heliospheric Observatory (SOHO) spacecraft is 
expected to discover its 1,000TH comet this summer.

The SOHO spacecraft is a joint effort between NASA and the 
European Space Agency. It has accounted for approximately 
one-half of all comet discoveries with computed orbits in 
the history of astronomy.

"Before SOHO was launched, only 16 sun grazing comets had 
been discovered by space observatories. Based on that 
experience, who could have predicted SOHO would discover 
more than 60 times that number, and in only nine years," 
said Dr. Chris St. Cyr. He is senior project scientist for 
NASA's Living With a Star program at the agency's Goddard 
Space Flight Center, Greenbelt, Md. "This is truly a 
remarkable achievement!"

About 85 percent of the comets SOHO discovered belongs to 
the Kreutz group of sun grazing comets, so named because 
their orbits take them very close to Earth's star. The 
Kreutz sun grazers pass within 500,000 miles of the 
star's visible surface. Mercury, the planet closest to 
the sun, is about 36 million miles from the solar surface.

SOHO has also been used to discover three other 
well-populated comet groups: the Meyer, with at least 55 
members; Marsden, with at least 21 members; and the Kracht, 
with 24 members. These groups are named after the 
astronomers who suggested the comets are related, because 
they have similar orbits.

Many comet discoveries were made by amateurs using SOHO 
images on the Internet. SOHO comet hunters come from all 
over the world. The United States, United Kingdom, China, 
Japan, Taiwan, Russia, Ukraine, France, Germany, and 
Lithuania are among the many countries whose citizens 
have used SOHO to chase comets.

Almost all of SOHO's comets are discovered using images 
from its Large Angle and Spectrometric Coronagraph (LASCO) 
instrument. LASCO is used to observe the faint, 
multimillion-degree outer atmosphere of the sun, called 
the corona. A disk in the instrument is used to make an 
artificial eclipse, blocking direct light from the sun, 
so the much fainter corona can be seen. Sun grazing 
comets are discovered when they enter LASCO's field of 
view as they pass close by the star.

"Building coronagraphs like LASCO is still more art 
than science, because the light we are trying to detect 
is very faint," said Dr. Joe Gurman, U.S. project 
scientist for SOHO at Goddard. "Any imperfections in the 
optics or dust in the instrument will scatter the light, 
making the images too noisy to be useful. Discovering 
almost 1,000 comets since SOHO's launch on December 2, 
1995 is a testament to the skill of the LASCO team."

SOHO successfully completed its primary mission in April 
1998. It has enough fuel to remain on station to keep 
hunting comets for decades if the LASCO continues to 
function.

For information about SOHO on the Internet, visit:

http://www.nasa.gov/vision/universe/solarsystem/1000comet.html

For information about NASA and agency programs on the 
Internet, visit:

http://www.nasa.gov/home/index.html

-end-