A Comet Comes for Christmas

By Natalie Angier

Three satellites will present a show of lights

Early on Christmas morning, just before the faintest glimmerings of dawn over the Pacific, a group of scientists from the U.S., West Germany and Britain will begin their holiday celebrations by monitoring a unique experiment: the creation of the first man-made comet. A satellite orbiting some 70,000 miles above earth will release four canisters containing about 90 lbs. of barium and copper powder, worth $240,000. The powder will swell into a gaseous cloud 100 miles across that will glow pale yellow-green and then a dusky purple; as it expands, the cloud will grow a comet's classic tail.

The precise shape and behavior of the comet will give scientists insights into an array of physics problems, particularly some of the interactions between the sun and the earth. The release is part of a study of the magnetosphere, the powerful magnetic bubble that surrounds the earth; of the solar wind, the stream of supersonic particles that blows from the sun out to the planets; and of the bow-shock region, which lies between them. Aware of the comet's seasonal significance, NASA, one of the mission's main participants, has only too happily dubbed the performance the "Christmas comet." Sadly, the spectacle will be visible only to those living roughly west of St. Louis to Hawaii, from Canada down to northern Mexico, and then only for about ten minutes, beginning at 4:18 a.m. Pacific Standard Time.

Collectively called the active magnetospheric particle tracer explorers (AMPTE), three satellites, each designed by one of the participating nations, were launched on a single Delta rocket from Cape Canaveral in August. Since then the probes have been working together, although in different orbits, on a series of seven experiments. Space scientists must normally restrict their research to the passive observation of the heavens. AMPTE was designed to turn space into an active laboratory. "Rather than wait for chance events to happen, we decided to go out there and simulate natural conditions," says Mario Acuna, an astrophysicist with NASA'S Goddard Space Flight Center in Greenbelt, Md.

In the first experiment, begun last fall, the satellites provided some indication that the magnetosphere presents a more solid barrier to the solar wind than had previously been believed. Soaring beyond the magnetosphere, the Christmas comet will enable scientists to study the effects of the solar wind on an object without a magnetic field. The West German satellite will release the barium, while the British craft records the progress of the comet, measuring the tail and noting how long it takes for the solar wind to disperse it. The U.S. satellite will track how much barium is able to penetrate the magnetosphere.

The three spacecraft will round off their cosmic dance in the spring of 1985, with the release of barium and lithium into the so-called magnetotail, the very edge of the earth's magnetic field. The findings could shed some light on what might happen if the earth loses its magnetic field in the future. There is evidence that the magnetic poles switch every several thousand years and in the process can lose their strength for years at a time. Without a magnetic field to protect the atmosphere, more potentially dangerous high-energy particles could reach the earth. There is a need, says Acuna, "to better understand what would happen if the magnetosphere disappeared."

--By Natalie Angler. Reported by Jerry Hannifin/Washington

With reporting by Jerry Hannifin