Spacecraft will wink at Earth during flyby Probe will swing past tomorrow on its way to visit asteroid Eros

January 22, 1998|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

It's back.

NASA's Near Earth Asteroid Rendezvous spacecraft, built in Maryland and launched toward the Asteroid Belt almost two years ago, is zooming back toward a close encounter with Earth early tomorrow morning.

The $108 million, 1,800-pound NEAR is returning for an energy boost and a course change toward its ultimate target, the asteroid Eros.

It will also take snapshots and movies of Earth and calibrate its instruments as it soars within 333 miles of its home planet.

Along the way, NEAR will be maneuvered to reflect flashes of sunlight onto more than a dozen U.S. cities, giving millions of Americans a chance to see an interplanetary spacecraft as it zips around the solar system.

That's "very unusual," said Robert Farquhar, a mission planner at the Johns Hopkins University's Applied Physics Laboratory, where NEAR was built.

"But the primary purpose [of the Earth flyby] is to get us to Eros," he said. "We would not get there without it."

When NEAR was launched in February 1996, its Delta II rocket packed too little power to get the craft to Eros in a single bound. That's because the 25-mile-long rock circles the sun in an orbit tilted 11 degrees from Earth's.

NEAR did fly past another asteroid, called Mathilde, last June. It sent back 300 close-up (750 miles) photographs of the battered rock as it orbited the sun.

But days after that encounter, APL engineers rocketed NEAR into a new trajectory. That allowed NEAR to gain speed as it raced back into the pull of Earth's gravity. It's just enough added energy so that when NEAR swings by tomorrow, it will be flung into a new trajectory leading to Eros by January 1999.

Two similar Earth flybys in 1990 and 1992 gave the Galileo spacecraft a push toward Jupiter.

About 1: 25 a.m. tomorrow, NEAR will be 16,200 miles above the Pacific northeast of Hawaii, streaking toward Earth at 20,000 mph, said APL's David Dunham.

That's when controllers will wag the spacecraft's four solar panels to beam flashes of reflected sunlight across major U.S. cities, including New York, Baltimore and Washington, Atlanta, Miami and points west.

Under clear skies, the sunlight reflected off NEAR will look like a rapidly brightening star, about 30 degrees above the west-northwest horizon. That's a hand's breadth below Capella, the brightest star in the northwest sky.

"At first you would see nothing," Dunham said. "Then, when the first 'mirror' starts reflecting, you would see a star suddenly appear, as bright as Polaris [the North Star] or a little brighter.

"Then it would get brighter, to something near as bright as the brightest star. Then it would fade back," he said.

"There is some chance we could see four glints or two glints," depending on how well the four solar panels aligned after launch in 1996, Dunham said. Scientists also want a more precise fix on the spacecraft's trajectory.

The flashes will last 30 to 40 seconds at any given location. The whole exercise will end by 1: 48 a.m., with NEAR 9,000 miles above Alaska's Aleutian Islands.

From there, Dunham said, NEAR will soar over northern Siberia and Kazakstan, making its closest approach to Earth 333 miles over southwestern Iran.

As NEAR skims south over the day side of Earth, its camera, spectrograph and other scientific instruments will take thousands of photographs, while measuring the chemical composition of the air, land and water, said Scott Murchie, a member of APL's NEAR science team.

The photos and readings are designed to help engineers calibrate their instruments -- to be sure they're all seeing what we already know is there. "Earth is our cheat sheet," Murchie said. "We know the answers, and we can tell if we understand the way the instruments are operating and if we've done our job correctly."

Later tomorrow and Saturday, as NEAR races back into interplanetary space, it will point its camera at Antarctica and assemble a movie of the receding planet.

For more information, and an animation of the sun glint experiment, try the APL Web page: sd-www.jhuapl.edu /NEAR/

Pub Date: 1/22/98

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