SOHO Watches a Comet Fading Away

On September 30, 2010, water production on Comet Hartley 2 -- as represented by the hydrogen cloud surrounding the comet seen by SOHO – jumped by a factor of two and half in a single day. (Comet and cloud size are not to scale.)

On Nov. 4, 2010, NASA’s EPOXI spacecraft came within 450 miles of Comet Hartley 2, a small comet not even a mile in diameter, which takes about six and a half years to orbit the sun. Designated officially as 103P/Hartley 2, the comet thus became the fifth for which scientists have collected close-up images.

But the comet was also observed from another spacecraft: the Solar and Heliospheric Observer (SOHO), better known for its observations of the sun. Together, the two returned data about what appears to be an irregular comet, belching chunks of ice and losing water at a surprisingly fast pace.

“By combining EPOXI’s direct imaging with several months of SOHO data, we had a rare chance to see a comet in the process of shedding off large amounts of water,” says Michael Combi, a space scientist at the University of Michigan in Ann Arbor, Mich., who wrote about his findings in a June 10, 2011 issue of the Astrophysical Journal Letters. “Comets always lose water as they heat up during the approach to the sun, but this was much more than usual. Something pretty dramatic happened in those weeks.”

Understanding the composition and behavior of comets intrigues scientists because they are some of the first objects that formed around our sun some 4.5 billion years ago and they’ve evolved little since. These chunks of ice, rock, and frozen gas hold clues to what existed in those early days of the solar system’s formation, says Combi. So he uses an instrument onboard SOHO called SWAN – for Solar Wind ANistropy – to observe how water streams off of comets…. Continue reading SOHO Watches a Comet Fading Away