Department of Climate and Space Sciences and Engineering in the College of Engineering at the University of Michigan

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Dr. Combi Watches a Comet Fading Away

Posted: July 29, 2011

Dr. Combi Watches a Comet Fading Away

On Nov. 4, 2010, Comet Hartley 2, a small comet not even a mile in diameter, became the focus of two NASA spacecraft, giving comet researchers such as AOSS Research Professor Michael Combi a close-up view of a comet fading away.

NASA's EPOXI spacecraft came within 450 miles of Comet Hartley 2, which was also observed from 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 Dr. Combi. Dr. Combi has an international reputation in cometary science and is known around U-M by his car license plate: “DRCOMET”.

His findings about Comet Hartley 2 appeared in the 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.

SWAN has collected data on nearly one hundred comets, so when Combi and his colleagues learned EPOXI was destined to get a closer view of Hartley 2, they pored over old data from that comet's most recent approach in 1997. They compared this to SWAN's 2010 observations from September to December 2010.

Surprisingly, the comet's water production in 1997 was three times the amount of water put out in 2010. "We've analyzed multiple comets with short periods like Hartley 2 on repeated trips around the sun," says Combi. "But none of them has shown such a drastic change from one close pass by the sun to the next."

"Analysis of all this data on Hartley 2 is just beginning," says Combi, "So it will be awhile before we figure out all that's happening. But we have here an example of an unusual comet. We don't know if this one had odd behaviors or some different kind of composition – but maybe we'll start seeing things like this, perhaps even in hindsight, in other comets."

Source article: http://www.nasa.gov/mission_pages/soho/hartley2-fades.html

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 dayOn 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.) Credit: NASA/Steele Hill
Inset upper right: Comet Hartley 2 as viewed by EPOXI from 435 miles away on November 4, 2010. Credit: NASA/JPL-Caltech/UMD

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