Mission Information
MISSION_NAME NEAR EARTH ASTEROID RENDEZVOUS
MISSION_ALIAS NEAR
MISSION_START_DATE 1996-02-17T12:00:00.000Z
MISSION_STOP_DATE 2001-02-28T12:00:00.000Z
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.
MISSION_OBJECTIVES_SUMMARY
Science Objectives
    ==================
 
        The overall objectives of the NEAR mission are to rendezvous
        with a near-Earth asteroid, achieve orbit around such an
        asteroid, and conduct the first systematic scientific
        exploration of a near-Earth asteroid. NEAR studied the nature
        and evolution of S-type asteroids, improved our understanding
        of processes and conditions relevant to the formation of
        planets in the early solar system, and clarified the
        relationship between asteroids and meteorites.
 
        Specific science questions addressed by NEAR are as follows.
        What are the morphological and textural characteristics of the
        asteroid surface, and how do they compare with those on larger
        bodies? What is the elemental and mineralogical composition of
        the asteroid? Is there evidence of compositional or structural
        heterogeneity? Is the asteroid a solid fragment of a larger
        parent body or a rubble pile? Is the asteroid's precursor
        body(ies) primitive or differentiated? Is there evidence of
        past or present cometary activity? Is the asteroid related to
        a meteorite type or types? Does an intrinsic magnetic field
        exist? What is it like?  Does the asteroid have any
        satellites, and how might they compare with Eros?
REFERENCE_DESCRIPTION