[meteorite-list] Deep Impact Observes Huge Comet Outburst

[Ron Baalke]

http://www.newsdesk.umd.edu/scitech/release.cfm?ArticleID=1090

University of Maryland
For Immediate Release
June 28, 2005

Contacts: Lee Tune, 301 405 4679 or ltune at umd.edu 

UM-Led Deep Impact Observes Huge Comet Outburst

COLLEGE PARK, Md. - NASA's Deep Impact spacecraft have observed
a massive, short-lived outburst of ice or other particles from comet
Tempel 1 that temporarily expanded the size and reflectivity of the
cloud of dust and gas (coma) that surrounds the comet nucleus. (See a
movie <http://www.newsdesk.umd.edu/movies/dioutburst.mpt> of the outburst)

The outburst, which was detected as a dramatic brightening of the comet
on June 22, is the second of two such events observed in the past two
weeks. A smaller outburst was seen on June 14 by ground-based observers,
the Hubble Space Telescope and Deep Impact.

"This most recent outburst was six times larger than the one observed on
June 14, but the ejected material dissipated almost entirely within
about a half day," said University of Maryland astronomer Michael
A'Hearn, who leads the Deep Impact mission. A'Hearn noted that data 
from the spectrometer aboard the spacecraft showed that during the 
June 22 outburst, the amount of water vapor in the coma doubled, while 
the amount of other gases, including carbon dioxide, increased even more.

"Outbursts such as this may be a very common phenomenon on many comets,
but they are rarely observed in sufficient detail to understand them
because it is normally so difficult to obtain enough time on telescopes
to discover such phenomena," said A'Hearn. "We likely would have missed
this exciting event, except that we are now getting almost continuous
coverage of the comet with the spacecraft's imaging and spectroscopy
instruments."

Deep Impact co-investigator Jessica Sunshine, with Science
Applications International Corporation (SAIC), agreed that observing
such activity twice in one week suggests outbursts are fairly common.
"We must now consider them as a significant part of the processing that
occurs on comets as they heat up when approaching the sun," she said.

The spectrometer is working very well and we already are able to see
changes in the make up of the fresh material extruded from the comet,"
said Sunshine, who leads the analysis of data from the spectrometer. "We
are still a long way from the comet, so this bodes very well for our
ability to observe and characterize changes in the comet's materials,
before, during, and after our impact."

Science team member Tony Farnham, also of the University of Maryland,
noted that although the duration (less than 18 hours) of both outburst
events was comparable, the June 22 event was much more spectacular,
exhibiting intricate features in the coma where material was blown off
the surface.

A'Hearn and other mission scientists say that most outbursts are
believed to be associated with the heating of comet material by the Sun.
Comet Tempel 1 is near perihelion, or the point in its orbit at which it
is closest to the Sun.

"For the June 22 event, it is the rapid dispersal of this outburst that
raises the most questions," said A'Hearn. "It looks as though the puff
was nearly instantaneous and that simple radial expansion is not enough
to make the brightness go down as fast as it did. Thus the particles
must also either be vaporizing, and thus disappearing, or getting much
darker after release, and "disappearing" in that way."

"This adds to the level of excitement as we come down to the final days
before encounter," said Rick Grammier, Deep Impact
project manager at NASA's Jet Propulsion Lab in Pasadena, Calif. "But
this comet outburst will require no modification to mission plan and in
no way affects spacecraft safety."

DI Instruments

Deep Impact -- which consists of a sub-compact-car-sized flyby
spacecraft and an impactor spacecraft about the size of a washing
machine -- carries a spectrometer and three imaging instruments. The
three imaging instruments, two on the flyby spacecraft and one on the
impactor, are essentially digital cameras connected to telescopes. The
spectrometer is on the flyby spacecraft and uses the same telescope as
the flyby's high-resolution imager.

A spectrometer takes light that is emitted, absorbed, or scattered by
materials (such as the dust and gas of the comet) and breaks it into its
component wavelengths, or spectrum. It does this in much the same way
that a prism breaks visible light into its component spectrum: red,
orange, yellow, green, blue, indigo, violet). However, visible light
represents only a narrow range of the spectrum of light. The Deep Impact
spectrometer measures light in the infrared range rather than visible
light. By analyzing data from a spectrometer scientists can determine
the composition of the materials being studied. The Deep Impact
scientists will be able to analyze data from their spectrometer to
determine the composition of the material that is ejected from the
crater during impact and to observe how the composition of that material
changes over time.

The final prelude to that impact will begin early on July 3, some 24
hours before the 1:52 a.m. (EDT) July 4th impact, when the flyby
spacecraft will launch the impactor into the path of the onrushing comet.

Like a copper penny pitched up into the air just in front of a speeding
tractor-trailer truck, the 820-pound impactor will be run down by the
comet, colliding with the nucleus at an impact, or closing, speed of
some 23,000 miles per hour. A'Hearn and his fellow mission scientists
expect the impact to create a crater several hundred feet in size;
ejecting ice, dust and gas from the crater and revealing pristine
material beneath. The impact will have no significant affect on the
orbit of Tempel 1, which poses no threat to Earth.

Nearby, Deep Impact's 'flyby' spacecraft will use its medium and high
resolution imagers and infrared spectrometer to collect and send back to
Earth pictures and spectra of the event. In addition, the Hubble and
Spitzer space telescopes, the Chandra X-ray Observatory, and large and
small telescopes on Earth also will observe the impact and its aftermath.

The University of Maryland, College Park, conducts
the overall mission for Deep Impact, which is a Discovery class NASA
program. NASA's Jet Propulsion Laboratory
(JPL) handles project management and mission operations. The spacecraft
was built for NASA by Ball Aerospace & Technologies Corporation,
Boulder, Colo.