NASA's Deep
Impact spacecraft has observed a massive, short-lived outburst of ice or other
particles from comet Tempel 1 just days before the
craft will release a probe to slam
into the comet.
The outburst,
which occurred June 22 and was announced today, was larger than one photographed
recently by the Hubble Space Telescope.
The latest
eruption temporarily expanded the size and reflectivity of the cloud of dust
and gas, called a coma, that surrounds the comet
nucleus.
"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.
An animation
of the June 22 eruption is available here.
Early results
The Deep
Impact mission is designed to carve a crater in the comet, releasing primordial
material for study in an effort to learn specifically what comets are made of.
The
spacecraft's instruments found 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," A'Hearn said. "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."
Two eruptions
in as many weeks suggests the activity is common.
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," said Deep Impact co-investigator Jessica Sunshine, with Science
Applications International Corporation.
Looking good
The latest
event was observed with the spacecraft's spectrometer, which splits light into
its constituent parts for chemical analysis.
"The
spectrometer is working very well and we already are able to see changes in the
makeup of the fresh material extruded from the comet," Sunshine said.
"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."
Eruptions like
this 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."
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