NASA—Deep Impact Delivers Deep Disappointment (Satellite)

After almost nine years in space that included an unprecedented July 4th impact and subsequent flyby of a comet, an additional comet flyby, and the return of approximately 500,000 images of celestial objects, NASA's Deep Impact mission has ended.

The project team at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, has reluctantly pronounced the mission at an end after being unable to communicate with the spacecraft for over a month. The last communication with the probe was August 8th. Deep Impact was history's most traveled comet research mission, going about 4.7 billion miles (7.58 billion kilometers).

"Deep Impact has been a fantastic, long-lasting spacecraft that has produced far more data than we had planned," said Mike A'Hearn, the Deep Impact principal investigator at the University of Maryland in College Park. "It has revolutionized our understanding of comets and their activity."

Deep Impact successfully completed its original bold mission of six months in 2005 to investigate both the surface and interior composition of a comet, and a subsequent extended mission of another comet flyby and observations of planets around other stars that lasted from July 2007 to December 2010. Since then, the spacecraft has been continually used as a space-borne planetary observatory to capture images and other scientific data on several targets of opportunity with its telescopes and instrumentation.

Launched in January 2005, the spacecraft first traveled about 268 million miles (431 million kilometers) to the vicinity of comet Tempel 1. On July 3, 2005, the spacecraft deployed an impactor into the path of comet to essentially be run over by its nucleus on July 4. This caused material from below the comet’s surface to be blasted out into space where it could be examined by the telescopes and instrumentation of the flyby spacecraft. Sixteen days after that comet encounter, the Deep Impact team placed the spacecraft on a trajectory to fly back past Earth in late December 2007 to put it on course to encounter another comet, Hartley 2 in November 2010.

"Six months after launch, this spacecraft had already completed its planned mission to study comet Tempel 1," said Tim Larson, project manager of Deep Impact at JPL. "But the science team kept finding interesting things to do, and through the ingenuity of our mission team and navigators and support of NASA’s Discovery Program, this spacecraft kept it up for more than eight years, producing amazing results all along the way."

The spacecraft's extended mission culminated in the successful flyby of comet Hartley 2 on Nov. 4, 2010. Along the way, it also observed six different stars to confirm the motion of planets orbiting them, and took images and data of the Earth, the Moon and Mars. These data helped to confirm the existence of water on the Moon, and attempted to confirm the methane signature in the atmosphere of Mars. One sequence of images is a breathtaking view of the Moon transiting across the face of Earth.

The spacecraft's extended mission culminated in the successful flyby of comet Hartley 2 on Nov. 4, 2010. In January 2012, Deep Impact performed imaging and accessed the composition of distant comet C/2009 P1 (Garradd). It took images of comet ISON this year and collected early images of comet ISON in June.

After losing contact with the spacecraft last month, mission controllers spent several weeks trying to uplink commands to reactivate its onboard systems. Although the exact cause of the loss is not known, analysis has uncovered a potential problem with computer time tagging that could have led to loss of control for Deep Impact's orientation. That would then affect the positioning of its radio antennas, making communication difficult, as well as its solar arrays, which would in turn prevent the spacecraft from getting power and allow cold temperatures to ruin onboard equipment, essentially freezing its battery and propulsion systems.

“Despite this unexpected final curtain call, Deep Impact already achieved much more than ever was envisioned," said Lindley Johnson, the Discovery Program Executive at NASA Headquarters, and the Program Executive for the mission since a year before it launched. "Deep Impact has completely overturned what we thought we knew about comets and also provided a treasure trove of additional planetary science that will be the source data of research for years to come.”

The mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. JPL manages the Deep Impact mission for NASA's Science Mission Directorate in Washington. Ball Aerospace & Technologies Corp. of Boulder, Colorado, built the spacecraft.

In other news

>NASA’s Deep Impact Produced Deep Results<

The Deep Impact spacecraft, history’s most traveled deep-space comet hunter, provided many significant results for the science community. Here are the top five, according to the mission’s principal investigator, Michael A’Hearn of the University of Maryland, College Park.

Studies of imagery showed that that the luminous flash created within a fraction of a second after Deep Impact’s impactor was atomized by comet Tempel 1 was much fainter than expected. Comparison with experimental impacts at the Vertical Gun Range at NASA Ames Research Center in Moffett Field, Calif., showed that such a faint flash was consistent only with a surface layer (depth a few times the diameter of the impactor) that was more than 75 percent empty space. This surprisingly high porosity was in contrast with theories that predicted comets were armored with a stronger, solid crust that impeded outgassing.

Observations of comet Tempel 1 by Deep Impact’s spectrometer instrument showed that water was arising primarily at longitudes near noon and peaking near the equator, whereas most of the carbon dioxide was arising from far southern latitudes, not too far from comet Tempel 1’s south pole. This could be due to seasonal effects (southern hemisphere just going into winter darkness) or due to differences in the chemical composition in different parts of the nucleus. During the mission extension, the EPOXI observations of comet Hartley 2 showed that the comet’s smooth waist was emitting pure water, while the small end was emitting excess carbon dioxide, regardless of time of day. This was a clear sign that chemical diversity was the important factor in a comet’s chemical makeup.

For many years we have known that a handful of comets (fewer than 10 percent) produced more water vapor than should be possible by sublimation of nucleus of water ice, in which the sizes of the nuclei are known. The flyby of comet Hartley 2 showed a large number of icy grains in the coma are driven out of the nucleus by the outgassing of carbon dioxide. These icy grains are plausibly the source of much of the water coming from the comet.

Observations of Hartley 2 by the Deep Impact spacecraft showed the importance of carbon-dioxide ice relative to carbon-monoxide ice in comets, and led to reexamination of all previous observations of these two ices in comets. The relative abundances in short-period and long-period comets imply that the short-period comets formed under warmer conditions than did the long-period comets. Thus, the short-period comets must have formed closer to the sun than their longer-period brethren. This is contrary to popular belief in the astronomical community (for many decades) that the short-period comets formed in the Kuiper belt beyond Neptune, while the long-period comets formed in the vicinity of the giant planets. The new model fits well with measurements by other astronomers of heavy water in Hartley 2, and with the newest dynamical studies of planetary migration.

The excavation of a crater on Tempel 1 was the trigger that allowed the proposal for the Stardust NExT mission to succeed. In addition to searching for the crater formed by Deep Impact, a key goal of that Stardust-NExT mission was to measure changes in the surface of the comet over an orbital period. This second set of measurements of Tempel 1 surface features showed that much of the evolution was in discrete, large areas, i.e., there was not a small, uniform erosion of the all parts of the surface, but rather large changes in a few places. Thus, comets evolve in a manner anaologous to erosion - most erosion takes place in discrete events (floods that make large, local changes) rather than as a slow, continuous process.

http://www.jpl.nasa.gov/news/news.php?release=2013-286

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