Goddard's Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument will sample ambient gas atoms and molecules in the comet's atmosphere (coma) as CONTOUR zips past the nucleus. Sampling the coma gives information about a comet's composition because the coma is comprised of dust and gas evaporated from the nucleus.

The coma appears as the bright "head" of a comet, and it develops if a comet's orbit takes it close to the Sun, where solar heat and radiation vaporize the comet's ices. (The nucleus is technically part of the head also, but it is too small to see, and it is obscured by the coma.)

"It's truly exciting to have the opportunity to fly this close to the nucleus of a comet for the first time," said Dr. Paul Mahaffy, a Goddard Co-Investigator with the CONTOUR mission for NGIMS.

"With NGIMS we'll sample gases from Comet Encke and at least one more comet and hope to learn much about the diversity of comets and how these bodies may have helped shape our own environment in the distant past through delivery of water and organic molecules. It has been a real privilege working with a uniquely talented team at Goddard to develop this measurement capability."

NGIMS will identify the types and relative amounts of atoms and molecules in the coma by creating ions (electrically charged atoms and molecules) from neutral gas and then separating these by their mass to charge ratio using electromagnetic fields. Since this ratio is often unique to a particular atom or molecule, this separation, or mass spectrometry, can be used to identify the atoms and molecules present in the coma.

Comets are thought to be relics left over from the formation of the solar system. They are comprised of dust and frozen gasses that were present in the primordial cloud that became the Sun and planets. Comets normally inhabit the outer regions of the solar system, far from the Sun, where frigid temperatures keep their ices completely frozen. There, a comet is just a nucleus, very small and faint, without the much larger and brighter coma or tail structure.

Occasionally, however, gravity from another solar system object, or perhaps a passing star, disrupts a comet's orbit and sends it hurtling into the inner solar system, where the coma forms. Sunlight and a stream of electrified gas that blows constantly from the Sun, called the solar wind, push dust and gas away from the coma in the direction opposite from the Sun, giving a comet its familiar tail-like shape.

"Comets are the solar system's smallest bodies, but among its biggest mysteries," says Dr. Joseph Veverka, CONTOUR's Principal Investigator from Cornell University, Ithaca, N.Y. "We believe they hold the most primitive materials in the solar system and that they played a role in shaping some of the planets, but we really have more ideas about comets than facts. CONTOUR will change that by coming closer to a comet nucleus than any spacecraft ever has before and gathering detailed, comparative data on these dynamic objects."

CONTOUR's flexible four-year mission plan includes encounters with comets Encke (Nov. 12, 2003) and Schwassmann-Wachmann 3 (June 19, 2006), though it can add a study of a "new" comet from the outer solar system should one be discovered in time for CONTOUR to catch it. CONTOUR will examine each comet's "heart," or nucleus, which scientists believe is a chunk of ice and rock, often just a few kilometers across and hidden from Earth-based telescopes beneath a dusty atmosphere and long tail.

The 8-sided solar-powered craft will fly as close as 100 kilometers (62 miles) to each nucleus, at top speeds that could cover the 56 kilometers between Washington and Baltimore in two seconds. A 5-layer dust shield of heavy Nextel and Kevlar fabric protects the compact probe from comet dust and debris. CONTOUR's four scientific instruments will take pictures and measure the chemical makeup of the nuclei while analyzing the surrounding gases and dust.

The $159 million CONTOUR is the sixth mission in NASA's Discovery Program of lower cost, scientifically focused exploration projects. The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., manages the mission, built the spacecraft and its two cameras, and will operate CONTOUR during flight.

NASA's Goddard Space Flight Center, Greenbelt, Md., provided CONTOUR's neutral gas/ion mass spectrometer and von Hoerner & Sulger, GmbH, Schwetzingen, Germany, built the dust analyzer. NASA's Jet Propulsion Laboratory, Pasadena, Calif., will provide navigation and Deep Space Network (DSN) support. Cornell's Veverka leads a science team of 18 co-investigators from universities, industry, and government agencies in the U.S. and Europe.

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