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2 Spacecraft and Subsystems 3 Reference 4 External Links |
Mission Profile
The ultimate goal of the mission was to study the near Earth asteroid 433 Eros from orbit for approximately one year. Eros is an S-class asteroid approximately 13 x 13 x 33 km in size, the second largest near-Earth asteroid. Initially the orbit was circular with a radius of 200 km. The radius of the orbit was brought down in stages to a 50 x 50 km orbit on 30 April 2000 and decreased to 35 x 35 km on July 14 2000. The orbit was raised over succeeding months to a 200 x 200 km orbit and then slowly decreased and altered to a 35 x 35 km retrograde orbit on December 13, 2000. The mission ended with a touchdown in the "saddle" region of Eros on February 12, 2001.
After launch on a Delta 7925-8 (a Delta II Lite launch vehicle with nine strap-on solid-rocket boosters and a Star 48 (PAM-D) third stage) and exit from Earth orbit, NEAR entered the first part of its cruise phase. It spent most of this phase in a minimal activity "hibernation" state, which ended a few days before the flyby of the 61 km diameter asteroid 253 Mathilde on June 27, 1997. The spacecraft flew within 1200 km of Mathilde at 12:56 UT at 9.93 km/sec, returning imaging and other instrument data. On July 3, 1997 NEAR executed the first major deep space maneuver, a two-part burn of the main 450 Newton thruster. This decreased the velocity by 279 m/sec and lowered perihelion from 0.99 AU to 0.95 AU. The Earth gravity assist swingby occurred on January 23, 1998 at 7:23 UT. The closest approach was 540 km, altering the orbital inclination from 0.5 to 10.2 degrees, and the aphelion distance from 2.17 to 1.77 AU, nearly matching those of Eros. Instrumentation was active at this time.
The first of four scheduled rendezvous burns on December 20, 1998 at 22:00 UT aborted due to a software problem. Contact was lost immediately after this and was not re-established for over 24 hours. The original mission plan called for these four burns to be followed by an orbit insertion burn on January 10, 1999, but the abort of the first burn and loss of communication made this impossible. A new plan was put into effect in which NEAR flew by Eros on December 23, 1998 at 18:41:23 UT at a speed of 0.965 km/s and a distance of 3827 km from the center of mass of Eros. Images of Eros were taken by the camera, data was collected by the near IR spectrograph, and radio tracking was performed during the flyby. A rendezvous maneuver was performed on January 3 1999 involving a thruster burn to match NEAR's orbital speed to that of Eros. A hydrazine thruster burn took place on January 20 to fine-tune the trajectory. On August 12 a 2 minute thruster burn slowed the spacecraft velocity relative to Eros to 300 km/hr.
Orbital insertion around Eros occurred on 14 February 2000 at 15:33 UT (10:33 AM EST) after NEAR completed a 13 month heliocentric orbit which closely matched the orbit of Eros. A rendezvous maneuver was completed on February 3 at 17:00 UT, slowing the spacecraft from 19.3 to 8.1 m/s relative to Eros. Another maneuver took place on February 8 increasing the relative velocity slightly to 9.9 m/s. Searches for satellites of Eros took place on January 28, and 4 and 9 February, none were found. The scans were for scientific purposes and to mitigate any chances of collision with a satellite. NEAR went into a 321 x 366 km orbit around Eros on February 14. The orbit was slowly decreased to a 35 km circular polar orbit by July 14. NEAR remained in this orbit for 10 days and then was backed out in stages to a 100 km circular orbit by September 5 2000. Maneuvers in mid-October led to a flyby of Eros within 5.3 km of the surface at 07:00 UT on 26 October.
Following the flyby NEAR moved to a 200 km circular orbit and shifted the orbit from prograde near-polar to a retrograde near-equatorial orbit. By December 13, 2000 the orbit was shifted back to a circular 35 km low orbit. where NEAR will remain until the nominal end of mission on February 12, 2001. Starting on January 24 2001 the spacecraft began a series of close passes (5 to 6 km) to the surface and on January 28 passed 2 to 3 km from the asteroid. The spacecraft then made a slow controlled descent to the surface of Eros ending with a touchdown in the "saddle" region of Eros on February 12, 2001 at approximately 20:01 UT (3:01 p.m. EST). To the surprise of the controlers, the spacecraft was undamaged and operational after the landing.
The spacecraft has the shape of an octagonal prism, approximately 1.7 m on a side, with four fixed gallium arsenide solar panels in a windmill arrangement, a fixed 1.5 m X-band high-gain radio antenna with a magnetometer mounted on the antenna feed, and an X-ray solar monitor on one end (the forward deck), with the other instruments fixed on the opposite end (the aft deck). Most electronics are mounted on the inside of the decks. The propulsion module is contained in the interior.
The craft is three-axis stabilized and uses a single bipropellant (hydrazine / nitrogen tetroxide) 450 Newton (N) main thruster, and four 21 N and seven 3.5 N hydrazine thrusters for propulsion, for a total delta-V potential of 1450 m/s. Attitude control is achieved using the hydrazine thrusters and 4 reaction wheels. The propulsion system carries 209 kilograms of hydrazine and 109 kilograms of NTO oxidizer in two oxidizer and three fuel tanks.
Power is provided by four 1.8 by 1.2 meter gallium arsenide solar panels which can produce 400 Wattss at 2.2 AU (NEAR's maximum distance from the Sun) and 1800 Watts at 1 AU. Power is stored in a 9 amp-hour, 22-cell rechargeable super nickel-cadmium battery.
Spacecraft guidance is achieved through the use of a sensor suite of five digital solar attitude detectors, an inertial measurement unit, (IMU) and a star tracker camera pointed opposite the instrument pointing direction. The IMU contains hemispherical resonator gyroscopes and accelerometers. Four reaction wheels (arranged so that any three can provide complete three-axis control) are used for normal attitude control. The thrusters are used to dump angular momentum from the reaction wheels, as well as for rapid slew and propulsive maneuvers. Attitude control is to 0.1 degree, line-of-sight pointing stability is within 50 microradians over 1 second, and post-processing attitude knowledge is to 50 microradians.
The command and data handling subsytem is composed of two redundant command and telemetry processors and solid state recorders, a power switching unit, and an interface to two redundant 1553 standard data buses for communications with other subsystems. The solid state recorders are constructed from 16 Mbit IBM Luna-C DRAMs. One recorder has 1.1 Gigabits of storage, the other has 0.67 Gigabits.
The NEAR mission was the first launch of NASA's Discovery program, a series of small-scale spacecraft designed to proceed from development to flight in under three years for a cost of less than $150 million. The construction, launch, and 30 day cost for this mission is estimated at $122 million.
Spacecraft and Subsystems
Reference
External Links