NEAR Shoemaker was a NASA Discovery program spacecraft designed to
study near Earth asteroids. It completed its mission on February 28th, 2001, when it returned the last of its gamma-ray spectrometer data. NEAR, never designed to land, had touched down on 433 Eros on February 12, 2001, and was still able to gather a last few scientific readings and send them to Earth. NEAR was the first spacecraft ever to land on an asteroid.
Mission History
NEAR, or the Near Earth Asteroid Rendezvous spacecraft, recently
renamed NEAR Shoemaker in honor of Gene Shoemaker, departed from
Earth aboard a Delta II Lite launch vehicle on February 17th, 1996,
bound for a rendezvous with near Earth asteroid 433 Eros. After a
successful DETUMBLE maneuver, performed after deploying its solar
panels, and five TCMs (trajectory correction maneuvers), NEAR was on
its way to a flyby of asteroid 253 Mathilde. The flyby of Mathilde
occurred on June 27th 1997, and on July 7th, the first major deep
space maneuver using the craft's LVA (large velocity adjust) thruster
was executed. Five more TCMs saw NEAR to its Earth swingby, which
occurred on January 23rd, 1998 UTC 07:22:56.6 at an altitude of 540
km.
On December 20th 1998 at 22:00 UTC, after a successful 200 second
fuel settling burn, TCM 16 aborted after 0.2 seconds. A few seconds
later, all communication with the craft was lost. Contact was not
regained until 27 hours later, when it was discovered that NEAR had
experienced low-voltage conditions, lost 29 kg of fuel, and
encountered an attitude anomaly. The abort of TCM 16 meant that NEAR
would fly by Eros on December 23rd 1998 at 18:41:23 UTC, rather than
rendezvous with it. Mission controllers scrambled to find out what
went wrong and how the mission could be salvaged. On January 3rd
1999, TCM 17 occurred, which lasted for 24 minutes and enabled NEAR to
eventually rendezvous with Eros on February 14th 2000, thirteen months
behind schedule. An investigation team was formed to find the cause of
the December 20th burn abort, and the findings of that team are the
subject of the NEAR Burn Anomaly Report.
The OIM (orbital insertion maneuver), occurred on February 14th 2000
at 15:33:06 UTC. This event made NEAR the first craft to ever orbit an
asteroid. NEAR initially orbited in a 321 km by 366 km elliptical
path, but various maneuvers have since brought the spacecraft to within
50 km of Eros. A soft landing on Eros on February 12th, 2001, at 03:01:52 EST put NEAR in the history books as the first spacecrat ever to land on an asteriod. During its descent, NEAR was able to take 69 photographs which showed new detail never seen before. After its landing, NEAR was still able to communicate with Earth using its low gain antenna, and found itself in an excellent position to gather additional information about the composition of Eros with its gamma-ray spectrometer.
NEAR transmitted its final message to Earth on February 28th, 2001. NEAR's final resting place is just south of Himeros, a saddle shaped feature on Eros.
Mission Goals
The primary goals of the NEAR mission are to study the
properties, composition, mineralogy, morphology, internal mass
distribution, and magnetic field of the near Earth asteroid
Eros. The secondary goals include the study of regolithic
properties, solar wind interactions, dust and gas activity, and the
spin rate of the asteroid.
Hardware
The NEAR Shoemaker spacecraft has an octagonal prism shape, and is
about 1.7 meters long on each side. NEAR is three axis stabilized, it
has fixed solar panels and instruments, redundant critical
subsystems, and a passive thermal design. The overall mass of the
craft, including propellant, is 805 kg. Attitude control is
maintained through the use of reaction wheels and thrusters.
Four gallium arsenide solar panels in a windmill arrangement provide
power, 1800 watts at 1 AU. Power is stored in 9 amp, 22 cell
rechargeable batteries. A fixed 1.5 meter X-band high gain
radio antenna provides telemetry via the NASA deep space
network.
The craft's large velocity adjust thruster uses a bipropellent of
hydrazine and nitrogen trioxide, and produces an output of 450
N. Four 21 N and seven 3.5 N hydrazine thrusters provide additional
thrust and attitude control. All thrusters combined give NEAR a Delta
V of 1450 m/s. 209 kg of hydrazine and 109 kg of oxidizer (nitrogen
trioxide) are stored in two oxide and three fuel tanks.
NEAR carries 56 kg of instrumentation, which consume 81 watts of
power. These instruments are an x-ray and gamma ray spectrometer, an
infared spectragraph, a multi-spectral camera (CCD detector), a
laser rangefinder, and a magnetometer.
Guidance data is provided by five digital solar attitude detectors,
an inertial measurement unit (IMU), and a star tracker
camera. Command and data processing functions are handled by two
redundant command and telemetry processors. The spacecraft carries two
solid state data recorders, which are 16 MBit Luna-C DRAMs.
Summary
NEAR was launched under NASA's discovery program. Construction,
launch, and 30 day operating cost of NEAR is estimated at $122 Million.
NEAR Shoemaker has gathered a lot of data during its time in
space. Much of this data is available at http://near.jhuapl.edu/, along
with a lot of other information about NEAR Shoemaker.