The Johns Hopkins Gazette: February 14, 2000
February 14, 2000
VOL. 29, NO. 22


NEAR to Encounter Asteroid

By Helen Worth
Johns Hopkins Gazette Online Edition

The NEAR spacecraft is straightening its orbit and putting its best solar panels forward as it approaches asteroid 433 Eros for a Valentine's Day rendezvous more than 160 million miles from Earth. Its intended is a near-Earth asteroid named for the Greek god of love.

The Near Earth Asteroid Rendezvous mission, a NASA Discovery Program being conducted by the Applied Physics Laboratory, is the first mission to orbit an asteroid. For a year, the spacecraft will use its instruments to scrutinize the potato-shaped space rock to learn about its chemical and physical features and evolutionary history. The asteroid is known to be 21 by 8 by 8 miles--about twice the size of Manhattan Island.

The spacecraft's multispectral imager has been taking daily images for the past few weeks to confirm that the spacecraft is on track, to look for any moons orbiting the asteroid and to measure brightness variations to help map its rotation.

On Feb. 13, at about 11:33 p.m. EST, the spacecraft was scheduled to fly directly between the sun and the asteroid, enabling NEAR's near-infrared spectrometer to take critical observations of Eros' northern hemisphere under near-perfect lighting conditions, which will allow it to characterize the asteroid's mineral composition. In October, a similar sweep will be made over its southern hemisphere.

On Feb. 14, at 10:33 a.m. EST, when NEAR is approximately 207 miles from the center of Eros, it will fire its hydrazine engines to slow it enough to be captured by the asteroid's weak gravitational pull. Confirmation of orbit is expected to come at about 11:30 a.m. EST, to waiting team members in the Mission Operations Center on the APL campus.

During the first few weeks after achieving orbit the spacecraft will slowly descend toward the asteroid. Because Eros is irregularly shaped and rotating (it rotates once every 5.27 hours), this early stage of the mission can be very tricky, says Robert Farquhar, NEAR mission director. "No one has ever orbited a small body in space," Farquhar says. "The orbital stability is rather tenuous, and as we travel around Eros our navigation maneuvers must be perfect to keep us from crashing into it."

Using an onboard multispectral imager, laser rangefinder and radio science experiment, mission scientists and engineers will acquire enough information on Eros' shape, mass and gravity field to allow the spacecraft to come closer. "Soon after we go into orbit we should know the asteroid's mass and therefore its density to within 5 percent," says Andrew Cheng, mission scientist.

The onboard magnetometer will determine the strength of the asteroid's magnetic field--if there is one. "This will give the scientific community the first definitive measurement of an asteroid's magnetism, which contains clues to its thermal and geologic history," Cheng says. "The results of these measurements and others that we will take over the next year will help us to determine the origin of the asteroid and give us an unprecedented understanding of asteroids in general."

For the first two months NEAR will slowly descend to within 31 miles from Eros. During this low-orbit phase the x-ray/gamma-ray spectrometer will measure elemental abundances--important information to help determine the relationship between meteorites and asteroids.

In late August, the spacecraft will begin to climb from 31 to 311 miles above the center of Eros. During this ascent the multispectral imager will continue to take images of the asteroid's surface that will be compiled into a complete map of the asteroid. In December the spacecraft will descend, possibly to less than a mile, from the surface of the asteroid. From this vantage point the near-infrared spectrometer can collect extremely high-resolution data of the asteroid's surface, making it possible to characterize the composition of rocks as small as a grapefruit. Final events of the mission, which will end in February 2001, will be determined sometime this summer.

NEAR was launched Feb. 17, 1996, from Cape Canaveral Air Station, Fla. Its original rendezvous date of Jan. 10, 1999, was postponed when a firing of the spacecraft's bipropellant engine--designed to put the spacecraft on target for the rendezvous--exceeded preset acceleration limits and caused the spacecraft to retreat into safe-mode. But valuable information about the asteroid was collected by a hastily programmed flyby of Eros on Dec. 23, 1998. "The fact that the mission is still on track is a tribute to the robustness of the spacecraft and a mission design that included planning for adversity," Farquhar says.

For the latest mission news visit the NEAR website: