| MISSION_DESCRIPTION |
Mission Overview
================
The Near Earth Asteroid Rendezvous (NEAR) mission inaugurated
NASA's Discovery Program. It was the first mission to orbit an
asteroid and made the first comprehensive scientific
measurements of an asteroid's surface composition, geology,
physical properties, and internal structure. NEAR was launched
successfully on 17 February 1996 aboard a Delta II-7925. It made
the first reconnaissance of a C-type asteroid during its flyby
of the main-belt asteroid 253 Mathilde in June 1997. It became
the first spacecraft to enter orbit around an asteroid, doing so
at the large near-Earth asteroid 433 Eros in February 2000. The
spacecraft, renamed NEAR Shoemaker, landed on Eros at 37.2 South
by 278.4 West, ending its mission on February 12, 2001 with
another spacecraft first. NEAR obtained new information on the
nature and evolution of asteroids, improved our understanding of
planetary formation processes in the early solar system, and
clarified the relationships between asteroids and meteorites.
The NEAR Mission Operations Center and Science Data Center were
both located at APL. The latter maintained the entire NEAR data
set on-line and made data from all instruments accessible over
the Internet to every member of the NEAR science team. For a
detailed description of the mission see [CHENGETAL1998].
Introduction
============
Of the more than 7000 asteroids that have been named, most are
found in the main asteroid belt between the orbits of Mars and
Jupiter, but those that come within 1.3 AU of the Sun are known
as near-Earth asteroids. The orbits of these dynamically young
bodies have evolved on 100-million-year timescales because of
collisions and gravitational interactions with planets. The
present-day orbits of such asteroids do not necessarily indicate
where they formed. Some are already in Earth-crossing orbits,
and those that are not are highly likely to evolve into one.
More than 250 near-Earth asteroids are known, and they appear to
typify a broad sample of the main-belt population. Before NEAR,
knowledge of the nature of asteroids came from three sources:
Earth-based remote sensing, data from the Galileo spacecraft
flybys of the two main-belt asteroids 951 Gaspra and 243 Ida,
and laboratory analyses of meteorites. Most meteorites are
believed to be collisional fragments of asteroids, but they may
represent a biased and incomplete sampling of the materials
actually found in near-Earth asteroids. Firm links between
meteorite types and asteroid types have been difficult to
establish [GAFFEYETAL1993A]. The uncommon eucrite (a basaltic
achondrite) meteorites have been linked by visible and
near-infrared reflectance measurements to the relatively rare
V-type asteroids [MCCORDETAL1970], [BINZEL&XU1993].
However, a major controversy has been whether and how the most
common meteorite types (the ordinary chondrites) may be linked
to the most common asteroid types (the S-type or stony
asteroids) in the inner part of the asteroid belt
[BELLETAL1989], [GAFFEYETAL1993B]. Galileo and NEAR targets 951
Gaspra, 243 Ida, and the 433 Eros are all S-type asteroids.)
The S-type asteroids are a diverse class of objects known to
contain the silicate minerals olivine and pyroxene plus an
admixture of iron/nickel metal. Some appear to be fragments of
bodies that underwent substantial melting and differentiation.
Others may consist of primitive materials like ordinary
chondrites that never underwent melting and that may preserve
characteristics of the solid material from which the inner
planets accreted. The Galileo flybys provided the very first
high-resolution images of S asteroids, revealing complex
surfaces covered by craters, fractures, grooves, and subtle
color variations [BELTONETAL1992], [OSTROETAL1990]. Galileo
also discovered a satellite at Ida, which is a member of the
Koronis family (Eros is not an asteroid family member). The
near-infrared spectrum of Gaspra indicates a high olivine
abundance such that it is inferred to be a fragment of a
differentiated body. Conversely, Ida and Eros display infrared
spectra that may be consistent with a silicate mineralogy like
that in ordinary chondrites [CHAPMAN1996],
[MURCHIE&PIETERS1996]. The Galileo instrument complement did not
include any capability to measure elemental composition, and
debate continues about whether ordinary chondrites are related
to S-type asteroids.
The NEAR mission spent about a year in orbit around Eros,
entering 14 February 2000 and landing at the asteroid surface 12
February 2001 from when spacecraft operations were continued
until 28 February 2001. It acquired the first comprehensive,
spatially resolved measurements of the geomorphology,
reflectance spectral properties, and shape of an asteroid, and
X-ray and gamma-ray spectral measurements of elemental
abundances from orbit and the surface. The ambient magnetic
field in the vicinity of the asteroid was also measured. NEAR
orbited Eros at low altitude, as close as about 1 body radius
above the surface, for several months so as to allow NEAR's
instruments to to acquire their highest spatial resolution
measurements. The NEAR data, especially when combined with those
from the Galileo flybys, greatly advanced our understanding of
S-type asteroids and their possible relationships to meteorites
and other small bodies of the solar system. NEAR also conducted
a thorough search for satellites.
Spacecraft Design
=================
NEAR was a solar-powered, three-axis-stabilized spacecraft
[SANTOETAL1995] with a launch mass, including propellant, of 805
kg and a dry mass of 468 kg. The spacecraft was simple and
highly redundant. It used X-band telemetry to the NASA deep
space network; data rates at Eros were selectable in the range
of 2.9 to 8.8 kbps using a 34-m high-efficiency antenna. With a
70-m antenna, the data rates from Eros ranged from 17.6 to 26.5
kbps. The command and telemetry systems were fully redundant.
Two solid-state recorders were accommodated with a combined
memory capacity of 1.6 Gbit.
Spacecraft attitude was determined using a star camera, a fully
redundant inertial measurement unit, and redundant digital Sun
sensors. The propulsion sub-system was dual mode (hydrazine was
used as fuel for both the monopropellant and bipropellant
systems) and included one 450-N bipropellant thruster for large
maneuvers, four 21-N thrusters, and seven 3.5-N thrusters for
fine velocity control and momentum dumping. Attitude was
controlled by a redundant set of four reaction wheels or by the
thruster complement to within 1.7 mrad. NEAR's line-of-sight
pointing stability was within 20 microrad 1 s, and
postprocessing attitude knowledge was within 130 microrad.
Forward and aft aluminum honeycomb decks were connected with
eight aluminum honeycomb side panels. Mounted on the outside of
the forward deck were a fixed, 1.5-m-dia. X-band high-gain
antenna (HGA), four fixed solar panels, and the X-ray solar
monitor system. When the solar panels were fully illuminated,
the Sun was in the center of the solar monitor field of view
(FOV). No booms were accommodated on the spacecraft. The
electronics were mounted on the inside of the forward and aft
decks.
NEAR contained six scientific instruments, which are detailed in
the next section.
1. Multispectral Imager (MSI)
2. Near-Infrared Spectrograph (NIS)
3. X-Ray Spectrometer (XRS)
4. Gamma-Ray Spectrometer (GRS)
5. NEAR Laser Rangefinder (NLR)
6. Magnetometer (MAG)
The MAG was mounted on top of the HGA feed, where it was exposed
to the minimum level of spacecraft-generated magnetic fields.
The remaining instruments (MSI, NIS, XRS, GRS, and NLR) were all
mounted on the outside of the aft deck. They were on fixed
mounts and were co-aligned to view a common boresight direction.
The NIS had a scan mirror that allowed it to look 30 degrees
forward and 110 degrees aft from the common boresight. Key
properties of the mission design permitted the use of this fixed
spacecraft geometry. Throughout most of the orbital rendezvous
with Eros, the angle between the Sun and the Earth, as seen from
the spacecraft, remained less than about 30 degrees. In
addition, the mission aphelion was reached during cruise. Hence,
if the solar panels were sufficiently large to sustain NEAR at
aphelion, there was sufficient power margin at Eros for the
spacecraft to pull its solar panels over 30 deg off full
illumination to point the HGA at Earth. Moreover, the rendezvous
orbit plane was maintained so that the orbit normal pointed
approximately at the Sun. In this case, as NEAR orbited Eros,
it was usually able to roll around the HGA axis so as to keep
the instruments pointed at the asteroid while maintaining adequate
solar panel illumination. The instruments were usually pointed
away from the asteroid when the HGA was used to downlink to
Earth. This mode of operation motivated the requirement for
on-board data storage. With on-board image compression, NEAR
could store more than 1000 images and downlink them within 10
hrs at its maximum data rate of 26.5 kbps.
The spacecraft was designed using a distributed architecture,
partitioned so that subsystems generally did not share common
hardware or software. One major benefit of this approach was
that careful design of interfaces allowed development, test, and
integration of sub-systems in parallel. In addition, this
architecture had a natural advantage of built-in contingencies
and design margins. Truly parallel subsystem development
required independence at the subsystem interface, through
careful partitioning of functional requirements and ample design
margins at subsystem inter-faces. On NEAR, subsystems were
interfaced through a MIL-STD-1553 data bus, chosen because it
was compatible with many off-the-shelf industry components. The
data bus had additional attractive features: fewer
interconnecting cables; built-in redundancy and cross-strapping;
simplification of interface definition; a fault-tolerant,
transformer-coupled interface; a common data architecture for
sharing information among subsystems; and a flexible
software-defined interface instead of a rigid hardware-defined
interface.
When it was launched, NEAR was the lowest-cost U.S. planetary
mission ever. The spacecraft's 27-month development schedule was
unusually rapid. The distributed architecture and the selection
of the 1553 data bus were key to developing NEAR on time and
under budget. Previous planetary missions have not used a
distributed architecture because they have been optimized for
performance, i.e., to return maximum science within available
technology. The distributed architecture approach comes with a
mass penalty, and therefore a performance penalty: some hardware
that can be combined at the system level is duplicated at the
subsystem level. The distributed architecture approach for NEAR
features interface margin and testability, optimizing the
spacecraft for low cost and rapid schedule. Nevertheless, the
performance penalty is minuscule, and the mass penalty for using
the distributed architecture approach is only about 10 kg.
Instrument Tasks
================
Details on the many science objectives of the NEAR instruments
can be found elsewhere [VEVERKAETAL1997A], [TROMBKAETAL1997],
[ACUNAETAL1997], [ZUBERETAL1997], [YEOMANSETAL1997] and
[CHENGETAL1997]. A brief summary of instrument characteristics
is given in this section. (Full descriptions of each science
investigation and instrument appeared in a special issue of
Space Science Reviews, vol. 82, 1997.) Detailed instrument
descriptions and results of ground and in-flight calibrations
appear in the companion articles of this issue of the Technical
Digest.
Multispectral Imager
--------------------
The main goals of the MSI were to determine the shape of Eros
and to map the mineralogy and morphology of features on its
surface at high spatial resolution. MSI was a 537 x 244 pixel
charge-coupled device camera with five-element
radiation-hardened refractive optics. It covered the spectral
range from 0.4 to 1.1 microns, and it had an eight-position
filter wheel. Seven narrow-band filters were chosen to
discriminate the major iron-bearing silicates present (olivine
and pyroxene); the eight, broad-band filter was for fast
exposures and high sensitivity, including optical navigation.
occur on Eros. The camera had an FOV of 2.93 x 2.26 degrees
and a pixel resolution of 96 x 162 microrad. It had a maximum
framing rate of 1 per second with images digitized to 12 bits
and a dedicated digital processing unit with an image buffer
in addition to both lossless and lossy on-board image
compression.
Near-Infrared Spectrograph
--------------------------
NIS measured the spectrum of sunlight reflected from Eros in
the near-infrared range from 0.8 to 2.5 microns to determine
the distribution and abundance of surface minerals like
olivine and pyroxene. This grating spectrometer dispersed the
light from the slit FOV (0.38 x 0.76 degrees in its narrow
position and 0.76 x 0.76 degrees in the wide position) across
a pair of passively cooled one-dimensional array detectors. A
32-channel germanium array covering the lower wavelengths,
with channel centers at 0.82 to 1.49 microns with a 0.022
micron spacing between channels. A 32-channel indium/gallium-
arsenide array covering longer wavelengths, with channel
centers at 1.37 to 2.71 microns with a 0.043 micron spacing
between channels. Due to configuration of the optics and the
sensitivity of this array, useful measurements were acquired
by it over the wavelength range 1.5 to 2.5 microns. The slit
could be closed for dark current measurements, which were
routinely interleaved with measurements of the asteroid. NIS
had a scan mirror that enabled it to step across the range
from 30 degrees forward of the common boresight to 110 degrees
aft, in 0.4 degree steps. Spectral images were built up by a
combination of scan mirror and spacecraft motions. In
addition, the NIS had a gold calibration target that viewed at
the forward limit of the mirror's scan ranges. It scattered
sunlight into the instrument and provided a quantitative,
in-flight calibration of instrument stability.
X-Ray Spectrometer
------------------
The XRS was an X-ray resonance fluorescence spectrometer that
detected the characteristic X-ray line emissions excited by
solar X-rays from major elements in the asteroid's surface. It
covered X-rays in the energy range from 1 to 10 keV using
three gas proportional counters. The balanced, differential
filter technique was used to separate the closely spaced Mg,
Al, and Si lines lying below 2 keV. The gas proportional
counters directly resolved higher energy line emissions from
Ca and Fe.
A mechanical collimator gave the XRS a 5 degree FOV, with
which it mapped the chemical composition of the asteroid at
spatial resolutions as fine as 2 km in the low orbits. It also
included a separate solar monitor system to measure
continuously the incident spectrum of solar X-rays, using both
a gas proportional counter and a high-spectral-resolution
silicon X-ray detector. The XRS performed in-flight
calibration using a calibration rod with Fe-55 sources that
could be rotated into or out of the detector FOV.
Gamma-Ray Spectrometer
----------------------
The GRS detected characteristic gamma rays in the 0.3- to
10-MeV range emitted from specific elements in the asteroid
surface. Some of these emissions were excited by cosmic rays
and some arose from natural radioactivity in the asteroid. The
GRS used a body-mounted, passively cooled NaI scintillator
detector with a bismuth germanate anticoincidence shield that
defined a 45 degree FOV. Abundances of several important
elements such as K, Si, and Fe were measured.
NEAR Laser Rangefinder
----------------------
The NLR was a laser altimeter that measured the distance from
the spacecraft to the asteroid surface by sending out a short
burst of laser light and then recording the time required for
the signal to return from the asteroid. It used a
chromium-doped neodymium/yttrium-aluminum-garnet (Cr-Nd-YAG)
solid-state laser and a compact reflecting telescope. It sent
a small portion of each emitted laser pulse through an optical
fiber of known length and into the receiver, providing a
continuous in-flight calibration of the timing circuit. The
ranging data were used to construct a global shape model and a
global topographic map of Eros with horizontal resolution of
about 300 m. The NLR also measured detailed topographic
profiles of surface features on Eros with a best spatial
resolution of under 5 m. These topographic profiles enhanced
and complemented the study of surface morphology from imaging.
Magnetometer
------------
The fluxgate magnetometer used ring core sensors made of
highly magnetically permeable material. MAG searched for any
intrinsic magnetic fields of Eros. The recent Galileo flybys
of the S-type asteroids Gaspra and Ida yielded evidence that
both of these bodies are magnetic, although this evidence is
ambiguous [KIVELSONETAL1993]. Discovery of an intrinsic
magnetic field at Eros would have been the first definitive
detection of magnetism at an asteroid and would have yielded
important insights about its thermal and geological history.
Radio Science
-------------
In addition to the six major instruments, a coherent X-band
transponder was used to conduct a radio science investigation
by measuring the Doppler shift from the spacecraft's radial
velocity component relative to the Earth. Accurate
measurements of the Doppler shift and the range to Earth as
the spacecraft orbited Eros allowed mapping of the asteroid's
gravity field. In conjunction with MSI/NIS and NLR data,
gravity determinations were combined with global shape and
rotation data to constrain the internal density structure of
Eros and search for heterogeneity.
Mission Profile
===============
The NEAR spacecraft was successfully launched in February
1996, taking advantage of the unique alignment of Earth and
Eros that occurs only once every 7 years [FARQUHARETAL1995].
A Delta-II 7925 rocket placed NEAR into a 2-year DV
(trajectory correction maneuver)/Earth gravity-assist
trajectory (DVEGA). This trajectory represents a new
application of the DVEGA technique: Instead of using an Earth
swingby maneuver to increase the aphelion of the spacecraft
trajectory, the maneuver actually decreased the aphelion
distance while increasing the inclination from 0 to about 10
deg. The circuitous 3-year flight path to Eros was the result
of a Discovery Program requirement to use an inexpensive, but
less capable, launch vehicle. With a larger launch vehicle
such as an Atlas or Titan, a 1-year direct trajectory could
have been used, but the total mission cost would have
increased by at least $50 million.
The Mathilde encounter occurred 1 week before the deep space
maneuver on 3 July 1997. The Earth swingby occurred on 23
January 1998. Rendezvous operations at Eros were scheduled to
begin on 20 December 1998, but a main rocket engine abort
occurred. A flyby of Eros was accomplished on 23 December
1998, and the rendezvous was rescheduled for 14 February 2000,
when orbit insertion occurred. On 12 February 2001, NEAR
accomplished a soft landing on Eros.
Mathilde Flyby
--------------
Asteroid 253 Mathilde was discovered on 12 November 1885 by
Johann Palisa in Vienna, Austria. The name was suggested by V.
A. Lebeuf (1859-1929), a staff member of the Paris
Observatory, who first computed an orbit for the new asteroid.
The name is thought to honor the wife of astronomer Moritz
Loewy (1833-1907), then the vice director of the Paris
Observatory. Although Mathilde's existence has been known
since 1885, it was only following the announcement of NEAR's
possible flyby that extensive physical observations were
carried out using telescopes on Earth. These showed that
Mathilde was an unusual object, especially because of its
rotation, which is at least an order of magnitude slower than
typical main-belt asteroids.
Using a series of observations of this asteroid made in the
first half of 1995, Stefano Mottola and his colleagues
[MOTTOLAETAL1995] determined that Mathilde's rotation period
is an extremely long 17.4 days. Only two asteroids, 288
Glauke and 1220 Clocus, have longer periods (48 and 31 days,
respectively), and there is no obvious mechanism that can
account for these extremely long asteroid 'days.'
The only previous spacecraft encounters with asteroids, as
noted earlier, had been the Galileo flybys of 951 Gaspra in
October 1991 and 243 Ida in August 1993. Both of these
objects, as well as Eros, are S-type asteroids. However, the
most common type of asteroid in the outer asteroid belt, the
dark and primitive C-type objects, had not yet been
investigated.
Spectral observations of Mathilde showed that its spectrum was
consistent with those of C-type asteroids and that it was
similar to those of the large carbonaceous asteroids 1 Ceres
and 2 Pallas (the two largest asteroids). (Mathilde is about
twice the size of Ida and four times the size of Gaspra.)
Before the NEAR spacecraft executed its flyby of Mathilde on
27 June 1997, these additional facts were known about the
asteroid: estimated diameter, 61 km; H magnitude (a measure of
absolute visual brightness), 10.30; perihelion, 1.94 AU;
aphelion, 3.35 AU; and orbital inclination, 6.71 degrees.
Prior to the NEAR spacecraft encounter with Mathilde, on
27 June 1997 Mission Operations sent a command to the NEAR
spacecraft that had the effect of advancing the Mission
Elapsed Time (MET) clock by 10 seconds. This command was
issued in order to correct for a timing error in the Mathilde
fly-by observing sequence due to ephemeris uncertainties which
existed at the time the sequence was generated and loaded to
the spacecraft. After analysis of the final optical navigation
data, the navigation team determined an additional shift
decrementing the MET clock by 1 second was necessary. Mission
Operations sent the additional command to the spacecraft; thus
collectively these commands had the effect of incrementing the
MET clock on board the NEAR spacecraft by 9 seconds. The NEAR
spacecraft fly-by of Mathilde was then successfully executed.
Following the Mathilde fly-by Mission Operations commanded the
spacecraft to restore the MET clock.
NEAR's encounter with Mathilde occurred at about 2 AU from the
Sun, where available power from the solar panels was reduced
to about 25% of its maximum mission level. Furthermore, a
requirement to point the solar panels about 50 deg away from
the optimal solar direction during the encounter reduced the
available power by another 36%. Because of this power
constraint, the only science instrument operated during the
encounter period was MSI [LANDSHOF&CHENG1995]. However,
spacecraft tracking data for the radio science experiment were
obtained for an asteroid mass determination [CHENGETAL1994].
The imaging experiment during the flyby had three major
objectives:
1. Most importantly, to obtain at least one image of Mathilde
near closest approach to provide the
highest-spatial-resolution view of the surface
2. To obtain an image of the complete illuminated portion of
the asteroid visible during the flyby
3. To acquire images of the sky around the asteroid to search
for possible satellites
The entire imaging sequence was accomplished in about 25 min
around closest approach (1200 km) at a speed of 9.93 km/s (Sun
distance, 1.99 AU; Earth distance, 2.19 AU). A total of 534
images (24 high phase angle, 144 high-resolution, 188 global
color imaging, 178 satellite search) were obtained during this
interval. The whole illuminated portion of the asteroid was
imaged in color at about a 500 m/pixel at a phase angle near
40 degrees. The best partial views were at 200 to 350 m/pixel.
Mathilde's mass was determined by accurately tracking NEAR
before and after the encounter. Apart from an interval of 1 to
2 h during the closest approach period, when imaging
experiments were conducted, continuous tracking of the
spacecraft was conducted for 3 days on either side of closest
approach. During the flyby, Mathilde exerted a slight
gravitational tug on NEAR. The corresponding gravitational
tugs on the Galileo spacecraft at Gaspra and Ida were too
small to allow mass determinations. However, because
Mathilde's mass is so much larger than either Gaspra's or
Ida's, its effects on NEAR's path were detectable in the
spacecraft's radio tracking data.
Earth Swingby
-------------
The next critical phase of NEAR's
flight profile was scheduled for 23 January 1998, when the
spacecraft would pass by the Earth at an altitude of only 532
km. This maneuver was expected to drastically alter NEAR's
heliocentric trajectory, changing the inclination from 0.52 to
10.04 deg, and reducing the aphelion distance from 2.18 to 1.77
AU and perihelion distance from 0.95 to 0.98 AU. An interesting
consequence of the Earth flyby was that the post-swingby
trajectory remained over the Earth's south polar region for a
considerable time.
During the encounter MSI and NIS observations of both Earth
and the Moon were acquired from 23 January through 26 January,
to test instrument performance during extended operations like
at Eros, and to perform inflight radiance and alignment
calibrations.
Eros Encounter
--------------
The NEAR mission target, 433 Eros,
is the second largest asteroid and is intermediate in size
between Gaspra and Ida. Eros is one of only three near-Earth
asteroids with maximum diameter above 10 km, and it is the only
large one whose heliocentric orbit is accessible enough to
permit a rendezvous mission using the Delta II launch vehicle.
The mean diameter of Eros, about 17 km, is an order of magnitude
larger than that of typical known near-Earth asteroids. Eros
was discovered in 1898. It was the subject of a worldwide
ground-based observing campaign in 1975 when it passed within
0.15 AU of Earth. Visible, infrared, and radar observations
determined the approximate size, shape, rotation rate, and pole
position of Eros (Table 1) and showed that a regolith
(fragmentary material produced by impacts) was present on its
surface. 433 Eros is presently in a Mars-crossing (but not
Earth-crossing) orbit; however, numerical simulations suggest
that it may evolve into an Earth crosser within 2 million years.
[MICHELETAL1996]
Spectroscopic analyses have found the visible and
near-infrared spectra of Eros to be consistent with a silicate
mineralogy like that found in ordinary chondrite meteorites.
These measurements were extended to higher spatial resolution
by NEAR.
Rendezvous operations at Eros were scheduled to begin on 20
December 1998, culminating in orbit insertion on 10 January.
During the first of four main rocket engine firings to match
velocity with Eros, on 20 December, an abort occurred and NEAR
flew by Eros on 23 December at a relative velocity of 1 km/s.
At this time a contingency sequence was executed during which
data were collected by MSI, NIS, and MAG. The whole
illuminated portion of the asteroid was imaged in color at
about 500 m/pixel before and after closest approach at phase
angles of 80 to 110 degrees. The best partial views were at
about 400 m/pixel.
Eros Operations
===============
Beginning in January 2000, a sequence of small maneuvers
decreased the relative velocity between NEAR and Eros to only
5 m/s. On 13 Feb 2000, NEAR performed a flyby of Eros on its
sunward side at a distance as of 200 km. In addition to
gathering NIS spectra at an optimal illumination geometry,
this first pass provided improved estimates of the asteroid's
physical parameters, such as a mass determination to 1%
accuracy, identification of surface landmarks, and an improved
estimate of Eros's spin vector. Orbit insertion occurred 14
Feb. As the spacecraft orbiter altitude was subsequently
lowered, the mass, moments of inertia, gravity harmonics, spin
state, and landmark locations were determined with increasing
precision.
NEAR operated in a series of orbits that came as close as 3 km
to the asteroid's surface, culminating with a soft landing on
12 February 2001. The evolution of low-altitude orbits around
Eros was strongly influenced by its irregular gravity field.
In unstable orbits, the spacecraft could crash into Eros in a
matter of days. Safe operation of NEAR during its 11-month
prime science phase required close coordination between the
science, mission design, navigation, and mission operations
teams. [LANDSHOF&CHENG1995]
To simplify science operations, the rendezvous was divided
into distinct phases [CHENGETAL1994]. During each mission
phase, particular aspects of the science were emphasized for
science planning, so the highest priority investigation
controlled instrument pointing for the majority of the
observing time. The highest-priority science varied by mission
phase, because of the changing orbital geometry. While in
orbits at 100 km or more from the center of Eros, the highest
priority science was global mapping by MSI. In orbits at 50 km
or lower, the highest priority science was compositional
measurement by XRS/GRS. A two-week period was allocated to
altimetry by NLR at the start of the 50 km polar orbits. NEAR
spent more than 150 days in orbits at 50 km or less from the
center of Eros, plus two additional weeks on the surface
acquiring GRS data.
Data Flow
---------
All data from the NEAR mission were down-linked to the NASA
Deep Space Network and then forwarded to the Mission
Operations Center (MOC) at APL. Doppler and ranging data from
the spacecraft were analyzed primarily by the NEAR navigation
team at the Jet Propulsion Laboratory (JPL) and processed to
determine the spacecraft ephemeris as well as to perform radio
science investigations. The entire spacecraft telemetry
stream, including spacecraft and instrument housekeeping data
and all science data, was forwarded to the APL MOC together
with the radiometric Doppler and range data. Navigation data
including spacecraft Ephemeris were forwarded to MOC in the
form of SPICE kernels. (SPICE is an information system
developed by the Navigation Ancillary Information Facility at
JPL. It consists of data files and software for managing
navigation-related data including spacecraft and planetary
ephemerides, spacecraft pointing, timekeeping, gravity data,
etc.)
From the APL MOC the spacecraft telemetry stream were passed
to the Science Data Center (SDC), the project facility
responsible for low-level processing of spacecraft telemetry,
data distribution, and data archiving. As such, the SDC
supported the activities of the science team in data analysis
and mission planning. The SDC created and maintained an
archive, which was the central project repository for science
data products such as images, asteroid models, and asteroid
maps. The SDC enabled easy access to mission data sets by
members of the science team and by others, and it collected
observing requests and science priorities from the science
team. It maintained a telemetry archive, a record of
instrument and spacecraft commands as executed, and records of
science sequences as requested and as executed. It provided
ancillary data (spacecraft and planetary ephemerides,
spacecraft and planetary attitudes, shape and gravity files,
and spacecraft clock files) in the form of SPICE kernels to
the science team.
Conclusion
==========
NEAR substantially increased our knowledge
of primitive bodies in the solar system by providing a long,
up-close look at the S-type asteroid 433 Eros and the first
resolved images of the C-type asteroid 253 Mathilde. NEAR was the
first mission to a near-Earth asteroid and a C-type asteroid, and
it was the first spacecraft to flyby, orbit, and land on a small
body.
|
.png)
.png)
