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NEAR Shoemaker

NSSDCA/COSPAR ID: 1996-008A

Description

The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker), renamed in honor of Gene Shoemaker, was designed to study the near Earth asteroid Eros from close orbit over a period of a year. The mission was the first-ever to orbit an asteroid and to touch down on the surface of an asteroid. The primary scientific objectives of NEAR were to return data on the bulk properties, composition, mineralogy, morphology, internal mass distribution and magnetic field of Eros. Secondary objectives include studies of regolith properties, interactions with the solar wind, possible current activity as indicated by dust or gas, and the asteroid spin state. This data will be used to help understand the characteristics of asteroids in general, their relationship to meteorites and comets, and the conditions in the early solar system. To accomplish these goals, the spacecraft is equipped with an X-ray/gamma ray spectrometer, a near infrared imaging spectrograph, a multi-spectral camera fitted with a CCD imaging detector, a laser rangefinder, and a magnetometer. A radio science experiment was also performed using the NEAR tracking system to estimate the gravity field of the asteroid. The total mass of the instruments is 56 kg, and they require 81 W power.

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 14 July 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 13 December 2000. The mission ended with a touchdown in the "saddle" region of Eros on 12 February 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 20 December 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 10 January 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 23 December 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 3 January 1999 involving a thruster burn to match NEAR's orbital speed to that of Eros. A hydrazine thruster burn took place on 20 January to fine-tune the trajectory. On 12 August a 2 minute thruster burn slowed the spacecraft velocity relative to Eros to 300 km/hr.

Orbit 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 3 February at 17:00 UT, slowing the spacecraft from 19.3 to 8.1 m/s relative to Eros. Another maneuver took place on 8 February increasing the relative velocity slightly to 9.9 m/s. Searches for satellites of Eros took place on 28 January, and 4 and 9 February, none were found. The scans were for 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 14 February. The orbit was slowly decreased to a 35 km circular polar orbit by 14 July. NEAR remained in this orbit for 10 days and then was backed out in stages to a 100 km circular orbit by 5 September 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 13 December 2000 the orbit was be shifted back to a circular 35 km low orbit. where NEAR will remain until the nominal end of mission on 12 February 2001. Starting on 24 January 2001 the spacecraft began a series of close passes (5 to 6 km) to the surface and on 28 January passed 2 to 3 km from the asteroid. The spacecraft made a slow controlled descent to the surface of Eros ending with a touchdown in the "saddle" region of Eros on 12 February 2001 at 20:01:52 UT (3:01:52 p.m. EST). This was the first spacecraft touchdown on an asteroid. After landing, the spacecraft continued to operate until the final contact was made on 28 February. The gamma-ray spectrometer collected data from the asteroid's surface over this time. A later attempt to contact the spacecraft on 10 December 2002 was unsuccessful.

Spacecraft and Subsystems

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 W at 2.2 AU (NEAR's maximum distance from the Sun) and 1800 W 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 gyros 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 Gbits of storage, the other has 0.67 Gbits.

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 total cost of the mission was $220.5 million, which included $43.5 million for the launch vehicle and $60.8 million for mission operations after launch.

Alternate Names

  • NEAR
  • Near Earth Asteroid Rendezvous
  • 23784

Facts in Brief

Launch Date: 1996-02-17
Launch Vehicle: Delta II 7925
Launch Site: Cape Canaveral, United States
Mass: 487.0 kg
Nominal Power: 1800.0 W

Funding Agency

  • NASA-Office of Space Science Applications (United States)

Disciplines

  • Planetary Science
  • Space Physics

Additional Information

Questions or comments about this spacecraft can be directed to: Dr. David R. Williams.

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Andrew F. ChengProject ScientistApplied Physics Laboratoryandrew.cheng@jhuapl.edu
Mr. Thomas B. CoughlinProject ManagerApplied Physics Laboratorytom.coughlin@jhuapl.edu
Dr. Elizabeth E. BeyerProgram ManagerNASA Headquartersbbeyer@hst.nasa.gov
Dr. John KerridgeProgram ScientistNASA Headquarters 
Dr. Robert W. FarquharMission ManagerApplied Physics Laboratoryrobert.farquhar@jhuapl.edu

Selected References

Cheng, A. F., Near Earth Asteroid Rendezvous: Mission overview, Space Sci. Rev., 82, No. 1-2, 3-29, 1997.

Cheng, A. F., et al., Near-Earth Asteroid Rendezvous: Mission overview, J. Geophys. Res., 102, No. E10, 23695-23708, Oct. 1997.

Dunham, D. W., et al., Implementation of the first asteroid landing, Icarus, 159, No. 2, 433-438, Oct. 2002.

Prockter, L., et al., The NEAR Shoemaker mission to asteroid 433 Eros, Acta Astronaut., 51, No. 1-9, 491-500, 2002.

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