PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = " 2002-06-28 Received from A. Raugh; 2002-07-11 A.Raugh: Added citation and copyright acknowledgment; " OBJECT = MISSION MISSION_NAME = "NEAR EARTH ASTEROID RENDEZVOUS" OBJECT = MISSION_INFORMATION MISSION_START_DATE = 1996-02-17 MISSION_STOP_DATE = 2001-02-28 MISSION_ALIAS_NAME = "NEAR" MISSION_DESC = " The following text is reprinted by permission from the Johns Hopkins Technical Digest, Volume 19, Number 2, (April--June 1998), The Johns Hopkins University Applied Physics Laboratory. The text was updated by members of the NEAR team to reflect the final status of the mission after its completion. 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?" 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