NASA Spacecraft Sees Cosmic Snow Storm During Comet Encounter

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 vapor."

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:


Source: NASA

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