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.

These images were acquired between June 22 and June 24. A brightening by a factor of about 5 and a rapid decay to baseline brightness were observed on June 22. As the comet moves through space, background stars pass in and out of the field of view. Credit: NASA/JPL-Caltech/UMD

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 early on July 4. It will collide 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 effect 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.