June 27, 2002. Copyright 2002. Graphic News. All rights reserved.

Close encounter with a Òdirty snowballÓ

LONDON, June 27, Graphic News: Comets are among the least understood objects in the heavens, yet these frigid remnants of the birth of our solar system, some 4.6 billion years ago, may be among the most important.

Comets contain amino acids, which are the building-blocks of proteins. If comets ever did contribute to the EarthÕs primordial oceans, they may have supplied these complex organic molecules -- the chemical precursors of life. The big question is exactly how complex the molecules on comets actually are. 

On July 1, all being well, NASA will launch CONTOUR (Comet Nucleus Tour) which should provide the first detailed look at the differences between these enigmatic Òdirty snowballs.Ó The instruments on CONTOUR, and other missions, will analyse dust for carbon-based molecules and answer questions about how comets act and evolve. 

CONTOUR is scheduled to blast-off from Cape Canaveral on a Boeing Delta II rocket during a launch window lasting just 12 seconds on Monday morning, at 2:56am (EDT). (6:56 GMT). If there are any hitches there are other launch opportunities on most mornings throughout July. 

CONTOUR is the first mission to use the Indirect Launch Mode, a clever plan designed to repeatedly bring the spacecraft back to the vicinity of Earth for velocity-boosting flybys, using the planetÕs gravity to supply the energy that otherwise would have to be provided by rocket fuel.

After launch the craft will stay in a highly elliptical Earth orbit, from as low as 200 kilometers (124 miles) out to nearly 115,000 kilometers (71,300 miles), until August 15, when it should fire its main solid-fuel rocket motor and enter a comet-chasing orbit around the sun.

Set to fly within a stoneÕs throw of two comets, CONTOURÕs four-year mission plan includes close encounters with comets 2P/Encke on November 12, 2003, and Schwassmann-Wachmann 3 on June 19, 2006. CONTOUR will examine each cometÕs Òheart,Ó 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 speeds of 26 kilometers (16 miles) per second. A five-layer dust shield of heavy Nextel and Kevlar fabric protects the compact probe from comet dust and debris.

CONTOURÕs four scientific instruments will take pictures and measure the chemical make-up of the nuclei while analyzing the surrounding gases and dust. Approaching Encke head-on CONTOURÕs side-looking camera (CRISP) will be able to snap digital pictures at least 10 times sharper than any ever taken, revealing car-sized surface features as small as four metres (about 13 feet) across. 

The camera is expected to reveal the precise size of the nucleus, its shape, its rotation, brightness and colour. Other instruments will characterize the gas and dust blowing away from the comet.

After the Encke flyby, CONTOUR will go into hibernation and head back toward Earth for three more gravity-assisted flybys to set up the encounter with Schwassmann-Wachmann 3.

ÒSW-3 is a young, fragile object; it split off at least three pieces in late 1995 for no obvious reason,Ó Donald Yeomans, a CONTOUR co-investigator at NASAÕs Jet Propulsion Laboratory in Pasadena, California, explained. ÒItÕs probably a rubble pile-type structure thatÕs held together by little more than its own self gravity. And if indeed some of these pieces have left the interior of the main nucleus exposed, then weÕll get a chance to look at the structure of this comet.Ó

ÒComets are the building blocks of the early solar system. If it were not for comets, perhaps we wouldnÕt even be here, and for more reasons than one,Ó Yeomans added. The impact of a comet or asteroid 65 million years ago likely resulted in the extinction of the dinosaurs and scores of other species, enabling mammals -- and humans -- to rise to the pre-eminent position atop the worldÕs food chain.

The CONTOUR mission is scheduled to end in September 2006, but the trajectory is designed to permit flight controllers to send the spacecraft on to a third comet if a worthwhile target is identified.
/ENDS

Sources: Sources: NASA, Johns Hopkins University Applied Physics Laboratory, Cornell University