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Science File

Tailing Comets for Clues to Life

NASA plans this month to launch Contour, its latest probe to explore these celestial balls of dust and ice, which hold secrets older than Earth.


Comets are among the last of the solar system's mysteries, holding secrets from the time before the birth of the planets. Scientists have long been fascinated by these strange brews of ice and dust that have been lighting up Earth's sky for billions of years.

Scientists believe that early in the formation of the solar system, the young Earth was bombarded with a rain of comets. They have wondered if these small and mysterious visitors delivered organic molecules to Earth that became the seeds of life.

"Are we, indeed, the progeny of comets?" asked Joseph Veverka, chairman of Cornell University's astronomy department.

NASA is hoping its latest comet probe, Contour, will help answer that tantalizing question. The spacecraft is scheduled to launch this month from Cape Canaveral, Fla. Its launch window extends through July 25.

Contour (Comet Nucleus Tour) will embark on a four-year (and perhaps longer), comet-chasing journey to study one of the "least understood bodies of our solar system," said Veverka, who is the mission's principal investigator.

The spacecraft is expected to collect a treasure trove of gas, dust and images as it makes the closest approach yet to the small (often just a few miles wide) nugget of ice and dust at a comet's core. The probe aims to come within 60 miles of its target, analyzing chemical compounds in the comet's tail and snapping the best nucleus pictures to date in the few minutes before its approach.

Contour is NASA's second mission dedicated to cometary exploration. It is part of a series of probes that will visit at least five comets over the next decade, heralding a "golden age" of cometary science, said Colleen Hartman, director of NASA's Solar System Exploration Division.

Contour is scheduled to make contact with comet Encke in 2003, followed by an encounter with Schwassmann-Wachmann 3 in 2006.

One of its key missions is to explore the diversity of comets, said Donald K. Yeomans, a scientist at NASA's Jet Propulsion Laboratory in Pasadena and one of the leaders of the Contour science team.

Encke is "a tough, blackened old comet" that has been in a close orbit around the sun, he said. Schwassmann-Wachmann 3 is a "young, fragile object" that journeyed to the inner solar system much more recently. It broke into pieces in 1995, giving scientists unprecedented access to the comet nucleus.

Contour's instruments will collect data on the comets' composition as it flies through the haze of speeding dust surrounding its targets, sending information to Earth for detailed analysis.

The information will help determine if the air we breathe, the water in our oceans and the ingredients for life came from comets.

A silver plate on the front of the spacecraft will collect specks of dust, which will be analyzed by a spectrometer to determine their chemical composition.

Scientists will look at the ratio of deuterium, a heavy form of hydrogen, to regular hydrogen in comet ice. If the ratio is similar to that of Earth's water, it is likely that comets were the source of our oceans, Yeomans said.

By measuring gas ratios, the scientists will be able to determine if Earth's atmosphere came from comets. Analysis of the comet's organic molecules will help determine if they are similar to complex compounds found on Earth.

Contour will take detailed pictures of each comet's nucleus. These images will be 25 times better than those taken by the European Space Agency's Giotto, which flew by comet Halley in 1986 and was the first spacecraft to get close enough to see a comet's core.

Analysis of Giotto's data revealed that Halley was 80% water and that the surface of the comet was covered in a layer of organic molecules.

NASA's first comet-chasing expedition was Deep Space 1, which was launched in 1998. Its primary mission was to test instruments for solar system exploration, but it flew by comet Borrelly in 2001, collecting data that confirmed that comets are rich in complex organic molecules.

Researchers have also used telescopes to analyze the compounds emitted from comets and found differences in the composition of comet water that are linked to their area of origin.

Measurements from telescopes and spacecraft show that water from comets Halley and Hale-Bopp, which are thought to have originated far from the sun, have a higher deuterium-to-hydrogen ratio than the water in Earth's oceans.

Comet Linear (named after the Lincoln Near Earth Asteroid Research observatory) is thought to have formed in the warmer regions near Jupiter. It is the first comet whose water seemed to have the same isotopic composition as Earth's. Linear's nucleus mysteriously vanished in 2000.

Contour's future after its two comet encounters is wide open. NASA engineers have designed the spacecraft to be durable and flexible enough to chase a bright new comet if one should appear in the Earth's neighborhood, as Hale-Bopp did in 1997.

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