NEAR Spacecraft Set For January 3 Main Engine Burn

December 31, 1998

A confident NEAR mission team is preparing for a noon, Jan. 3, 1999, engine burn that will put the Near Earth Asteroid Rendezvous (NEAR) spacecraft in line for a rendezvous with its target, asteroid 433 Eros, in just over a year.

The upcoming burn will last 24 minutes and will increase the spacecraft's speed by 2,100 mph (939 meters per second), putting it close to the same speed as Eros. The burn will be divided into an initial 3-minute, small hydrazine settling burn that will change the velocity by only 11 mph (5 meters per second) and a 21-minute, bipropellant main engine burn that will provide the rest of the velocity change.

The burn will lessen the distance between NEAR and Eros as they orbit the sun. For the next year NEAR will travel behind Eros in a slightly closer orbit to the sun. By mid-February 2000, NEAR will catch up to Eros. The spacecraft will then enter orbit around Eros and begin its year-long study of the asteroid.

The spacecraft's first attempted rendezvous burn was aborted Dec. 20, 1998, just seconds after the settling burn was completed. An investigation by mission personnel revealed that the brief engine burn exceeded certain safety limits associated with the onboard system that autonomously controls the spacecraft. This resulted in the engine abort. Reprogramming of these values is now being completed and the spacecraft will be ready for the Jan. 3 burn.

"We're very confident that we've found the problems associated with the Dec. 20, abort," says Thomas B. Coughlin, NEAR project manager at The Johns Hopkins University Applied Physics Laboratory (APL), which is managing the NASA mission. "The abort lost us time but the flyby gave us valuable information about Eros' shape and mass that we wouldn't have had --information that will help us during our orbital phase a little more than a year from now."

Because the spacecraft did not complete the Dec. 20 burn, the opportunity to orbit Eros next month as originally planned was lost. The setback presented mission planners with a huge challenge but also, an unexpected opportunity. Within days of the abort the NEAR team developed a complicated command sequence for a Dec. 23 flyby of Eros to obtain multicolor images, near-infrared spectra, and magnetic field measurements. The commands were uploaded to the spacecraft in record time and executed as planned, producing images of the asteroid and other valuable data.

Despite the delay, NEAR is expected to complete all its science objectives.
To follow the NEAR mission as it unfolds, visit Web site: Updates of mission activities and science returns are available on the Web site and the NEAR Hot Line: 240-228-5413.

Due to the rescheduling of mission events, the previously announced Jan. 10 and Jan. 14 press briefings have been canceled.

Johns Hopkins University

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