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NASA NTRS1999

Near Earth Asteroid Rendezvous (NEAR) Revised Eros Orbit Phase Trajectory Design

Trajectory design of the orbit phase of the NEAR mission involves a new process that departs significantly from those procedures used in previous missions. In most cases, a precise spacecraft ephemeris is designed well in advance of arrival at the target body. For NEAR, the uncertainty in the dynamic environment around Eros does not allow the luxury of a precise spacecraft trajectory to be defined in advance. The principal cause of this uncertainty is the limited knowledge oi' the gravity field a,-id rotational state of Eros. As a result, the concept for the NEAR trajectory design is to define a number of rules for satisfying spacecraft, mission, and science constraints, and then apply these rules to various assumptions for the model of Eros. Nominal, high, and low Eros mass models are used for testing the trajectory design strategy and to bracket the ranges of parameter variations that are expected upon arrival at the asteroid. The final design is completed after arrival at Eros and determination of the actual gravity field and rotational state. As a result of the unplanned termination of the deep space rendezvous maneuver on December 20, 1998, the NEAR spacecraft passed within 3830 km of Eros on December 23, 1998. This flyby provided a brief glimpse of Eros, and allowed for a more accurate model of the rotational parameters and gravity field uncertainty. Furthermore, after the termination of the deep space rendezvous burn, contact with the spacecraft was lost and the NEAR spacecraft lost attitude control. During the subsequent gyrations of the spacecraft, hydrazine thruster firings were used to regain attitude control. This unplanned thruster activity used Much of the fuel margin allocated for the orbit phase. Consequently, minimizing fuel consumption is now even more important.

Helfrich, J↗
NASA NTRS1999

Bulk Compositional Trends in Meteorites: A Guide for Analysis and Interpretation of NEAR XGRS Data from Asteroid 433 EROS

The Near Earth Asteroid Rendezvous (NEAR) spacecraft is to orbit the S-class asteroid 433 Eros for about one year beginning on February 14, 2000. The X-ray/gamma-ray O, Mg, Al, Si, Fe, and K; possibly H, Ca, S, Ti, and Th) of Eros with a spatial resolution ranging from a few km for X-rays to approx. 25% of the asteroid's surface for gamma-rays. The major scientific goals for the NEAR XGRS are to relate the composition of Eros to known classes of meteorites, to assess compositional heterogeneity and to identify geological processes that have occurred on the asteroid. Comparing remote-sensing data from asteroids to laboratory data from meteorites requires that the latter be well determined and understood. How well particular classes of meteorites can be identified as analogues of Eros depends not only on the error of the XGRS measurement, but also on the spread in abundances observed among different members of a given meteorite class. To prepare for the return of XGRS data from Eros, we have compiled a large database of bulk elemental compositions of meteorites, using data from a wide variety of published and unpublished sources. Custom software was developed to easily extract statistical information and make plots of data from different meteorite classes. Here, we use the meteorite compositional database to investigate which abundances and abundance ratios, of those measureable by the NEAR XGRS, are most diagnostic for distinguishing meteorite classes and identifying geological processes that have occurred on the samples' parent asteroids.

Nittler, Larry R.↗
NASA NTRS2004

Modeling Mosaic Degradation of X-Ray Measurements of 433 Eros by NEAR-Shoemaker

One of the primary accomplishments of the NEAR-Shoemaker mission to 433 Eros was the first measurement of the elemental composition of an asteroid. X- and -ray detectors measured characteristic emission lines for a variety of elements. The X-ray results are usually presented as elemental ratios, which are thought to eliminate geometric and mosaic degradation effects. One of the mission goals was to determine the best meteoritic analog for Eros. The X-ray data obtained from the surface of Eros appear consistent with an ordinary chondrite composition that has been depleted in sulfur.

Burbine, T. H.↗
NASA NTRS1995

(abstract) Satellite Dynamics About Eros

The Near Earth Asteroid Rendezvous (NEAR) mission spacecraft will arrive at Asteroid 433 (Eros) in February 1999. Following an initial period of characterization, the science phase of the mission will commence, and unprecedented information concerning an asteroid's shape, mass, density, composition, and rotational dynamics will be sent to Earth for analysis. It will also mark the start of orbital operations about the most complex and irregular gravity field ever encountered in the history of space exploration. The severity of Eros' distortion from the usual spheroid bodies encountered in planetary exploration leads to fundamental differences in the orbital dynamics about it. Operations about Eros will also serve as an exciting test of orbital determination and prediction in an orbital environment which can be chaotic in some instances. General orbital dynamics about Eros are discussed. Some simple formulae are given for the secular motion of the node and argument of periapsis. Conditions for a satellite orbit to escape from Eros are discussed, and some simple results derived. Finally, the chaotic nature of certain orbits about Eros are discussed, and the consequences for orbit prediction are investigated.

Near↗
NASA NTRS1999

Estimating the Mass of Asteroid 433 Eros During the Near Spacecraft Flyby

The terminal navigation of the Near-Earth Asteroid Rendezvous (NEAR) spacecraft during its flyby of asteroid 433 Eros on December 23, 1998 involved coordinated efforts to determine the heliocentric orbits of the spacecraft and Eros and then to determine the relative trajectory of the spacecraft with respect ot Eros.

Asteroid Near-Earth Asteroid Rendezbous (NEAR) mas↗
NASA NTRS2005

Evidence for Partial Melting in Reflectance Spectra of 433 Eros

The NEAR Shoemaker spacecraft returned near-IR spectra of asteroid 433 Eros at spatial resolutions ranging from 2.5 to 100's km during its year-long orbital mission in 2000. Assuming modified Gaussian absorption bands represent the reflectance spectrum between 0.8-2.5 m we fit the average of all geometrically corrected spectra acquired by the near-IR spectrometer (NIS) with seven absorption bands. Interpretation of the absorption bands in terms of olivine and pyroxene minerals indicates that the surface of Eros contains olivine and two pyroxenes with compositions that are indicative of a partially melted assemblage. This partial melting must have occurred when the asteroid was part of a larger minor planet, prior to break up into its current elongated and irregular shape.

McFadden, L. A.↗
NASA NTRS1995

Asteroid 433 Eros: The Target Body of the NEAR Mission

Asteroid 433 Eros was the first near-Earth asteroid to be discovered and nearly 100 years of ground-based observations have allowed a rough physical characterization of the body. At the time of the NEAR spacecraft arrival in early 1999, the Eros ephemeris uncertainties will be less than 50 km based on ground-based astrometric observations alone. Characteristics are described.

astrometric↗
NASA NTRS1999

Preliminary Planning for NEAR's Low-Altitude Operations at 433 Eros

On February 14, 2000, an orbit insertion burn will place NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft (S/C) into orbit around asteroid 433 Eros. NEAR will initially orbit Eros with distances ranging from 500 to 100 km in order to characterize the shape, gravity and spin of Eros. Once the physical parameters of Eros are determined reasonably well, the plan is to establish an orbit of the NEAR S/C with increasingly lower altitudes as the one year orbital mission progresses while further characterizing the gravity and shape of Eros. Towards the end of the NEAR mission, after the shape, gravity and spin of Eros have been well characterized, the scientific interest of obtaining very close observations (< 5 km) can be realized. The navigation during this phase relies on a combination of NASA's Deep Space Network (DSN) radio metric tracking, laser ranging (LIDAR) data from the S/C to the surface of Eros, and onboard optical imaging of landmarks on Eros. This paper will provide preliminary plans for mission design and navigation during the last two months of the orbit phase, where several close passes to the surface will be incorporated to enhance the science return. The culmination of these close passes will result in the eventual landing of the S/C on the surface of Eros. Several considerations for these plans are given by Antreasian, et at. [1998]. The objective for the end of the mission will be to land the S/C autonomously using the surface relative information obtained from the onboard LIDAR instrument. The goal will be to soft land the S/C in such a way as to keep it operational. With the use of an onboard LIDAR landing algorithm as discussed by Antreasian et at. [1998], it is believed that the S/C impact velocity can be kept well under 7 m/s which is a requirement for allowing the S/C to remain operational.

Antreasian, P. G.↗
NASA NTRS2000

Orbit Operations at 433 Eros: Navigation for the NEAR Shoemaker Mission

NASA's Near Earth Asteroid Rendezvous Mission began its record-setting exploration of the asteroid 433 Eros by inserting the spacecraft into orbit about Eros on February 14, 2000. This is the first spacecraft from any country to orbit an asteroid. The mission has overcome a failed insertion burn attempt on December 20, 1998, an event that would have ended most planetary missions, to return to the same target and successfully begin its science mapping a little more than a year later. Shortly after the successful insertion into orbit, the mission was renamed NEAR Shoemaker (NEAR) in memory of the late astronomer and geologist Eugene Shoemaker. NEAR will gather science data at Eros until February 14, 2001, which is the nominal end of mission. The NEAR mission is managed by the Johns Hopkins University, Applied Physics Laboratory in Laurel, Maryland. Since the initial mission concept in 1992, the design and implementation of the NEAR navigation system have been the responsibility of the Jet Propulsion Laboratory, California Institute of Technology. This presentation will show some of the unique features of navigation and mission design related to orbiting an asteroid and to designing a robust navigation system for the NEAR spacecraft. The problem of navigating a spacecraft about an asteroid is made difficult by the relative uncertainty in the asteroid physical properties which perturb the orbit: i.e., the mass, gravity field, and spin state. To help solve this problem, the navigation system for NEAR uses traditional DSN radio metric Doppler and range tracking, along with new technologies of optical landmark tracking and laser ranging to the asteroid surface. The experiences to date for each of these data types in the navigation solutions will be presented. Plans for the remainder of the NEAR mission will be presented, which include low orbits (down to 35 km radius circular orbits), and close flybys that may pass within 1 km of the surface. In addition, at the end of mission, NASA has approved a controlled descent and hovering phase that will culminate with the spacecraft impacting the surface. The maneuver planning for this final phase will also be presented.

Williams, B. G.↗