NEAR Poised for Historic Valentine's Day EROS Asteroid Encounter
8 Feb 2000
(Source: NASA Headquarters)
Headquarters, Washington, DC
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
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. Its intended is a near-Earth asteroid named for the Greek god of love.
The NEAR (Near Earth Asteroid Rendezvous) mission, a NASA Discovery Program being conducted by the Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 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 (33 by 13 by 13 kilometers) - about twice the size of Manhattan Island.
NEAR is less than 2,900 miles (4,700 kilometers) from Eros and is slowly closing in at about 18 mph relative to the asteroid. The spacecraft is alive with preparations for its rendezvous. Its 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 its brightness variations for clues to its rotation.
The last scheduled rendezvous burn prior to orbit insertion will take place Feb. 8 at 5 p.m. EST. On Feb. 13 at about 11:33 p.m. EST, the spacecraft will 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 distinguish 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 207 miles (333 kilometers) 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 Applied Physics Laboratory campus.
During the first few weeks after achieving orbit the spacecraft will slowly descend toward the asteroid. Because the asteroid is irregularly shaped and rotating (it rotates once every 5.27 hours), this early stage of the mission can be very tricky, says Dr. 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 a multispectral imager, laser rangefinder, and onboard 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 Dr. 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," Dr. 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 (50 kilometers) 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 (50 to 500 kilometers) 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. At that vantage point the near- infrared spectrometer can collect extremely high resolution data of the asteroid's surface, making it possible to distinguish 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, FL. 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. Early images can be found on the Internet at: