Science Paper Details NASA Epoxi Flyby of Hyper Comet

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Feature: 2011-185 June 16, 2011

Science Paper Details NASA Epoxi Flyby of Hyper Comet

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-185&cid=release_2011-185

PASADENA, Calif. – Comet Hartley 2's hyperactive state, as studied by NASA's EPOXI mission, is
detailed in a new paper published in this week's issue of the journal Science.

After visiting a comet and imaging distant stars for hints of extrasolar planets, you could say the
spacecraft used for EPOXI had seen its fair share of celestial wonders. But after about 3.2 billion
miles (5.1 billion kilometers) of deep space travel, one final wonder awaited the mission's
project and science teams. On Nov. 4, 2010, the EPOXI mission spacecraft flew past a weird
little comet called Hartley 2.

"From all the imaging we took during approach, we knew the comet was a little skittish even
before flyby," said EPOXI Project Manager Tim Larson of NASA's Jet Propulsion Laboratory in
Pasadena, Calif. "It was moving around the sky like a knuckleball and gave my navigators fits,
and these new results show this little comet is downright hyperactive."

The EPOXI mission found that the strong activity in water release and carbon dioxide-powered
jets did not occur equally in the different regions of the comet. During the spacecraft's flyby of
the comet – with closest approach of 431 miles (694 kilometers) – carbon-dioxide-driven jets
were seen at the ends of the comet, with most occurring at the small end. In the middle region,
or waist of the comet, water was released as vapor with very little carbon dioxide or ice. The
latter findings indicate that material in the waist likely came off the ends of the comet and was
redeposited.

"Hartley 2 is a hyperactive little comet, spewing out more water than most other comets its
size," said Mike A'Hearn, principal investigator of EPOXI from the University of Maryland,
College Park. "When warmed by the sun, dry ice -- frozen carbon dioxide -- deep in the comet's
body turns to gas jetting off the comet and dragging water ice with it."

Although Hartley 2 is the only such hyperactive comet visited by a spacecraft, scientists know of
at least a dozen other comets that also are relatively high in activity for their size and which are
probably driven by carbon dioxide or carbon monoxide.

"These could represent a separate class of hyperactive comets," said A'Hearn. "Or they could be
a continuum in comet activity extending from Hartley 2-like comets all the way to the much less
active, 'normal' comets that we are more used to seeing."

The study provides several new twists in the unfolding story of this small cometary dynamo,
including: (1) Hartley 2 has an 'excited state of rotation' because it spins around one axis, but
also tumbles around a different axis; and (2) on its larger, rougher ends, the comet's surface is
dotted with glittering blocks that can reach approximately 165 feet (50 meters) high and 260
feet (80 meters) wide. The block-like, shiny objects, some as big as one block long and 16
stories tall, appear to be two to three times more reflective than the surface average.

EPOXI was an extended mission that utilized the already "in-flight" Deep Impact spacecraft to
explore distinct celestial targets of opportunity. The name EPOXI itself is a combination of the
names for the two extended mission components: the extrasolar planet observations, called
Extrasolar Planet Observations and Characterization (EPOCh), and the flyby of comet Hartley 2,
called the Deep Impact Extended Investigation (DIXI). The spacecraft retained the name "Deep
Impact." During its approach, encounter and departure from comet Hartley 2, the spacecraft
beamed back more than 117,000 images and spectra.

JPL managed the EPOXI and Deep Impact missions for NASA's Science Mission Directorate,
Washington. The EPOXI mission was part of the Discovery Program managed at NASA's
Marshall Space Flight Center in Huntsville, Ala. The University of Maryland, College Park, is
home to Michael A'Hearn, principal investigator for EPOXI. Drake Deming of NASA's Goddard
Space Flight Center, Greenbelt, Md., is the science lead for the EPOXI mission's extrasolar
planet observations. The spacecraft was built for NASA by Ball Aerospace & Technologies Corp.,
Boulder, Colo.

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