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NASA Spacecraft Sees Cosmic Snow Storm During Comet Encounter

Nov. 18, 2010

Dwayne Brown 
Headquarters, Washington      

Jia-Rui Cook 
Jet Propulsion Laboratory 

Lee Tune 
University of Maryland, College Park 

RELEASE: 10-304


WASHINGTON -- The EPOXI mission's recent encounter with comet Hartley 
2 provided the first images clear enough for scientists to link jets 
of dust and gas with specific surface features. NASA and other 
scientists have begun to analyze the images. 

The EPOXI spacecraft revealed a cometary snow storm created by carbon 
dioxide jets spewing out tons of golf-ball to basketball-sized fluffy 
ice particles from the peanut-shaped comet's rocky ends. At the same 
time, a different process was causing water vapor to escape from the 
comet's smooth mid-section. This information sheds new light on the 
nature of comets and even planets. 

Scientists compared the new data to data from a comet the spacecraft 
previously visited that was somewhat different from Hartley 2. In 
2005, the spacecraft successfully released an impactor into the path 
of comet Tempel 1, while observing it during a flyby. 

"This is the first time we've ever seen individual chunks of ice in 
the cloud around a comet or jets definitively powered by carbon 
dioxide gas," said Michael A'Hearn, principal investigator for the 
spacecraft at the University of Maryland. "We looked for, but didn't 
see, such ice particles around comet Tempel 1." 

The new findings show Hartley 2 acts differently than Tempel 1 or the 
three other comets with nuclei imaged by spacecraft. Carbon dioxide 
appears to be a key to understanding Hartley 2 and explains why the 
smooth and rough areas scientists saw respond differently to solar 
heating, and have different mechanisms by which water escapes from 
the comet's interior. 

"When we first saw all the specks surrounding the nucleus, our mouths 
dropped," said Pete Schultz, EPOXI mission co-investigator at Brown 
University. "Stereo images reveal there are snowballs in front and 
behind the nucleus, making it look like a scene in one of those 
crystal snow globes." 

Data show the smooth area of comet Hartley 2 looks and behaves like 
most of the surface of comet Tempel 1, with water evaporating below 
the surface and percolating out through the dust. However, the rough 
areas of Hartley 2, with carbon dioxide jets spraying out ice 
particles, are very different. 

"The carbon dioxide jets blast out water ice from specific locations 
in the rough areas resulting in a cloud of ice and snow," said 
Jessica Sunshine, EPOXI deputy principal investigator at the 
University of Maryland. "Underneath the smooth middle area, water ice 
turns into water vapor that flows through the porous material, with 
the result that close to the comet in this area we see a lot of water 

Engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., 
have been looking for signs ice particles peppered the spacecraft. So 
far they found nine times when particles, estimated to weigh slightly 
less than the mass of a snowflake, might have hit the spacecraft but 
did not damage it. 

"The EPOXI mission spacecraft sailed through the Hartley 2's ice 
flurries in fine working order and continues to take images as 
planned of this amazing comet," said Tim Larson, EPOXI project 
manager at JPL. 

Scientists will need more detailed analysis to determine how long this 
snow storm has been active, and whether the differences in activity 
between the middle and ends of the comet are the result of how it 
formed some 4.5 billion years ago or are because of more recent 
evolutionary effects. 

EPOXI is a combination of the names for the mission's two components: 
the Extrasolar Planet Observations and Characterization (EPOCh), and 
the flyby of comet Hartley 2, called the Deep Impact Extended 
Investigation (DIXI). 

JPL manages the EPOXI mission for the Science Mission Directorate at 
NASA Headquarters in Washington. The spacecraft was built for NASA by 
Ball Aerospace & Technologies Corp., in Boulder, Colo. For more 
information about EPOXI, visit: 



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