Halley's comet

Science investigations designed to image the surface of a comet close up and determine its exact chemical and mineralogical composition have been provisionally selected by NASA and the French space agency, to be carried out early next century on a comet lander named Champollion.

Slated for launch aboard the international Rosetta mission, Champollion and a similar comet lander named RoLand, to be provided by a German-led consortium, will be the first spacecraft ever to land on one of these ancient clumps of icy rubble. Champollion is managed by the Jet Propulsion Laboratory.

Planetary scientists believe that comets were the primary building blocks for the outer planets of the solar system. Cometary bombardment also may have provided a significant fraction of the atmosphere, oceans and organic materials of Earth when it was a young planet.

The overall scientific objective of the Rosetta mission is to produce a global picture of a comet called Wirtanen, including its shape and composition, the nature of the volatiles that it spews out, and the comet surface phenomena that contribute to this process.

The mission is named after the Rosetta Stone, an ancient Egyptian tablet discovered near the town of Rosetta in 1799 that provided a major key to the translation of Egyptian hieroglyphic writing. Jean-Francois Champollion of France, for whom one of the Rosetta mission comet landers is named, played a large part in deciphering it.

"The new knowledge about comets that Rosetta and Champollion promise to return will help us decipher important clues about the earliest stages of the formation of our solar system, just as the Rosetta Stone did with ancient Egyptian hieroglyphics," said Dr. Wesley T. Huntress, associate administrator for space science at NASA Headquarters in Washington, D.C. "The most intriguing potential result from Champollion's investigations is the possible presence of complex organic molecules, which would tell us whether these precursors of life might have been brought to Earth by comets."

Rosetta will be the first spacecraft to orbit a comet. It represents the next major step in cometary science, following several recent reconnaissance flybys of comets by other international spacecraft. Rosetta is the third cornerstone mission in the European Space Agency's long-term space science program called Horizon 2000.

After its scheduled launch aboard an Ariane 5 vehicle in January 2003, the Rosetta spacecraft will perform gravity-assist flybys of Mars and Earth, and then rendezvous with comet Wirtanen in August 2011. It will deploy the Champollion and RoLand surface landers about one year later. Two asteroid flyby encounters also are planned for about halfway through the mission.

The selected Champollion experiments incorporate a number of new technologies, including high-density, three-dimensional electronics, an advanced infrared spectrometer, active pixel imaging sensors with on-chip electronics, an advanced gamma-ray sensor, and a miniaturized, low-power gas chromatograph/mass spectrometer.

A suite of a dozen cameras will provide Earth-bound scientists with their first close-up look at the surface of a comet. One set of cameras, to be provided by Dr. Jean- Pierre Bibring of the Institut d'Astrophysique Spatiale in Orsay, France, will create stereo images of the landscape surrounding the lander. A second camera set supplied by Dr. Roger Yelle of Boston University, Boston, MA, will photograph the surface close to the lander. An even closer look will be generated by a microscope, also supplied by Yelle, which should reveal individual grains in the comet nucleus.

Organic molecules, which may provide clues to the origin of life on Earth, will be identified by a gas chromatograph/mass spectrometer to be contributed by a group led by Dr. Paul Mahaffy of NASA's Goddard Space Flight Center, Greenbelt, MD. Determining the chemical composition of the comet itself is the task of an international consortium headed by Dr. Claude d'Uston of the Centre d'Etude Spatiale des Rayonnements in Toulouse, France. They will use a gamma-ray spectrometer to measure the radiation generated from inside the comet by the cosmic rays that bombard it continuously.

The strength, density and temperature of the comet surface will be measured by probes placed on spikes driven into the surface. These spikes, to be provided by Dr. Thomas Ahrens of the California Institute of Technology, will also serve to anchor the spacecraft firmly to the comet and prevent it from drifting off into space.

The scientific investigators for the provisionally selected instruments are based at 10 U.S. universities, three NASA field centers, three other U.S. laboratories, 10 French institutes and nine institutes in other countries.

A radio sounding tomographic experiment that would produce a CAT scan-like, three-dimensional image of the comet nucleus is under consideration as an additional Champollion investigation, if financial and technical resources can be made available. It would be provided by Dr. Wlodek Kofman of the Centre d'Etude des Phenomenes Aleatoires et Geophysiques, St. Martin d'Heres, France.

Full confirmation of the Champollion instrument payload is anticipated in about one year, after a formal review and endorsement by the ESA Space Program Committee in February 1996, and verification by NASA and the French space agency, Centre National d'Etudes Spatiales (CNES), that the selected investigators are able to accommodate changes required to increase instrument collaboration and decrease their costs.

JPL manages the Champollion project for the Solar System Exploration Division of NASA's Office of Space Science, Washington, D.C., and the CNES Scientific Program Division, Paris. CNES will contribute several key elements of the mission, including its telecommunication subsystem, batteries, spacecraft separation mechanism, and its ground-based control system.

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