updated 6/28/2005 8:03:36 PM ET 2005-06-29T00:03:36

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 at NASA's Deep Impact Web site.

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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