SOHO Mission Discovers Rare Comet

GREENBELT, Md., Sept. 26 /PRNewswire-USNewswire/ -- The Solar and
Heliospheric Observatory (SOHO) has discovered a rare periodic comet.
During a mission where 1,350 comets have been discovered, this is the first
time one has been officially designated "periodic."

Many of the comets SOHO has observed are believed to be periodic --
they follow their orbits around the sun more than twice and have orbital
periods of less than 200 years. Thousands of comets have been seen by
astronomers, but only around 190 are classified as periodic. Many more are
believed to be. The most famous periodic comet is Halley's comet, which
comes near to the Earth every 76 years. Its most recent close pass to the
sun was in 1986.

SOHO's new find has a much smaller orbit than Halley's comet. It takes
the comet approximately four years to travel once around the sun. It was
first seen in September 1999 and then again in September 2003. In 2005,
German PhD student Sebastian Hoenig realized that the two comets were so
similar in orbit that they might actually be the same object. To test his
theory, he calculated a combined orbit for the comet and consequently
predicted that it would return on Sept. 11, 2007. Hoenig's prediction
proved to be extremely accurate -- the comet reappeared in SOHO's Large
Angle and Spectrometric Coronagraph camera right on schedule and has now
been given the official designation of P/2007 R5 (SOHO). The credit for
original discovery and recovery of the object goes to Terry Lovejoy
(Australia, 1999), Kazimieras Cernis (Lithuania, 2003) and Bo Zhou (China,
2007).

A puzzling aspect to P/2007 R5 (SOHO) is that it does not look exactly
like a comet. It has no visible tail or coma of dust and gas, as is
traditionally associated with the phenomena. Initially, this led some
scientists to wonder if the object was actually an asteroid, a chunk of
space-rock, rather than a chunk of space-ice. However, P/2007 R5 (SOHO) did
exhibit some characteristics consistent with a comet. As scientists watched
the object pass close to the sun, drawing to within 4.9 million miles, or
around 5% of the distance between the Earth and the sun, they saw it
brighten by a factor of around a million, which is common behavior for a
comet.

"It is quite possibly an extinct comet nucleus of some kind," says Karl
Battams of the Naval Research Laboratory, Washington, who runs SOHO's comet
discovery program. Extinct comets have expelled most of their volatile ices
and retain little to form a tail or coma. They are theorized to be common
objects among the celestial bodies orbiting close to the sun.

This comet faded as quickly as it brightened, and soon became too faint
for SOHO's instruments to see. Estimates show that P/2007 R5 (SOHO) is
probably only 100 to 200 yards in diameter. Given how small and faint this
object is, and how close it still is to the sun, it is an extremely
difficult target for observers on Earth to pick out in the sky.

Now we know for certain that P/2007 R5 (SOHO) is there, astronomers
will be watching closely for it during its next return in September 2011.

SOHO is a cooperative project between the European Space Agency and
NASA.