The Near Earth Asteroid Mission to 433 Eros

Lucy A. McFadden, Christopher T. Russell, Andrew F. Cheng

  Originally appeared in: EOS, Trans AGU, 77 , 73 and 79, 1996


  The mortal beauty, Psyche, of Greek mythology, was driven by curiosity to gaze upon Eros, son of Aphrodite, defying his counsel not to look upon him by the light of day. In February, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft will be on its way to reveal the geophysical and geochemical wonders of the asteroid, Eros (Figure 1), bearing the name of the god who fell in love with and secretly married Psyche against his mother's wishes.

Fig. 1
Fig. 2

          The spacecraft (Figure 2) , built and managed by the Johns Hopkins Applied Physics Lab, Laurel, MD was launched aboard a Delta II-7925-8 rocket from Cape Canaveral just after Valentine's Day. In order to further our understanding of the nature of asteroids and their role in the formation of the Solar System, the three-axis stabilized spacecraft, which is passively cooled and powered with fixed solar panels, will orbit the asteroid for a year after its three-year trajectory through the inner Solar System (Figure 3). Arriving in February, 1999, the spacecraft will make measurements with five scientific instruments and transmit data to Earth through a 1.5-m, fixed, high-gain antenna at rates up to 27 kbits/s. Upon analysis of the data, this planet-crossing asteroid will be brought into the realm of geophysical and geological study, extending our knowledge of the small bodies near Earth not only to understand their role in the formation of our Solar System, but to understand the physics and chemistry of the impactors that have significantly affected the surface and atmospheres of all planets.


          433 Eros, discovered in 1898 by G. Witt, was the first asteroid discovered to cross within the orbit of Mars and to approach that of Earth. Its approximate diameters are 40 x 14 x 14 km making it the second largest planet-crosser and larger than the Martian moons Phobos and Deimos. Our selection of Eros as a mission target was driven partially by curiosity based on its proximity to Earth and its size. It was also selected because we anticipate geological diversity in terms of both surface features and composition. With the set of instruments on board there is a possibility of establishing the relationship between ordinary chondrite meteorites, those most abundant on Earth, and S-type asteroids found in the inner portion of the Main Asteroid Belt. These asteroids have moderate albedo and spectral reflectance absorption bands indicative of mafic silicate mineralogy. With ground-based measurements from asteroids alone, whether these asteroids are chemically and mineralogically similar to the ordinary chondrites, and thus are their parent bodies, is a subject of debate among scientists.

          NEAR will expand both scientific and technical horizons. The asteroid will be by far the smallest body in the solar system to have its mass measured and its gravity field mapped from orbit. It will be the first small body to have its elemental composition probed with x-ray and gamma-ray spectrometers; will have its shape measured with a laser rangefinder; and the first small body to be magnetically surveyed. NEAR's Multispectral Imager (MSI) will return images with the highest spatial resolution ever. The mission will provide the most comprehensive analysis to date of the surface and interior of any solar system body beyond the Moon. After analysis of the data, we will have knowledge of Solar System material extending into a new size regime. This information will undoubtedly force us to modify some details in our models of Solar System formation.

          The mission achieves a number of technical and financial firsts as well. Perhaps its most daring technological achievement will be its navigation in orbit about a highly irregularly-shaped body. A financial first is coming under the cost cap of $150 million for the development of the spacecraft and its first 30 days of operation, on time and under budget. We are clearly engaged in an experiment with a strong economical component to it as well. Unlike Psyche and Eros in their mythological world, we are not living in a jeweled palace.

          Just after launch, the engineers perform their spacecraft check-out and open the cover on the imager (other covers are deployed after the last major trajectory correction maneuver in July, 1997). The MSI team is eager to point their camera at the Moon for a calibration measurement before it gets too far away. Back on Earth, while the spacecraft is en route to Eros, scientists and mission planners will be putting their observation sequences into their final form. Science team members will be examining their preflight instrument calibrations and designing rapid and efficient data reduction and analysis procedures for the year-long data collection effort. In this trim budget environment, small teams will be working longer to prepare for the data stream from the spacecraft.

          If NEAR is launched early in its launch window, it will be able to fly by the 60-km diameter, low-albedo, main belt asteroid named 253 Mathilde in June, 1997. This target of opportunity is anticipated with excitement as the planetary community may get its first look at the surface of an asteroid which has the photometric properties occurring most frequently among objects in the Main Asteroid Belt. Mathilde is a C-type asteroid, a type designated by a low albedo of 0.03-0.06, (carbon black has an albedo of 0.01-0.005) and neutral colors in the visible and near- infrared. Spectra with these characteristics have few absorption bands from which to extract mineralogical surface information. Recent ground- based brightness measurements indicate that Mathilde has the third longest rotation period of any known asteroid, 417 hours. It is difficult to understand how such a long rotation period comes about. Some process is braking this asteroid's rotation rate. We eagerly await images of Mathilde that might bring additional insights to this observation.

          What scientific return do we expect from the primary mission at Eros? The objectives include an inventory of basic physical properties: shape, volume, rotational state and rate, mass and a search for satellites. The MSI will be used for optical navigation as the spacecraft approaches Eros and for shape determination. Our enthusiasm for a satellite search, to be conducted with MSI during the approach phase, is piqued by the unexpected discovery of Dactyl at the asteroid Ida as the Galileo spacecraft flew by it in August, 1993.

          After the spacecraft goes into orbit about Eros, the spacecraft's position will be monitored and Eros' total mass and its distribution will be derived from the radio science experiment using the spacecraft's main antenna. A model of the asteroid's interior structure will be derived from varying acceleration of the spacecraft as it moves around the asteroid. The NEAR Laser Rangefinder (NLR) and MSI will build shape models from which the volume and then bulk density of Eros will be derived. These data will address one of the fundamental debates in the planetology community, whether the asteroid is a solid fragment or is a loosely bound conglomerate of fractured debris. Knowledge of its mass distribution will shed light on this question at least for this particular asteroid.

          While in orbit the rest of the spacecraft's instrument complement consisting of: a magnetometer, a Near-Infrared Spectrometer (NIS) and X-ray and Gamma-ray Spectrometer (XGRS) will proceed with their mapping tasks. The orientation of the magnetic field as a function of location at the surface will reveal whether or not the asteroid has an intrinsic or a remanent magnetic field. The nature of this magnetic field will in turn constrain the state of thermal evolution of the parent body (or bodies) of Eros when the material in the present asteroid cooled. The NLR and MSI will continue mapping topography and morphology at spatial resolutions surpassing any on previous spacecraft missions. These data will provide a basis for discovering previously unknown processes active on surfaces of small bodies. The NIS will measure reflected sunlight between the spectral region of 0.8-2.7 microns, a region sensitive to electronic transitions in major rock- forming minerals. As the spacecraft is lowered into closer and closer orbits, the X-ray spectrometer will map the resonance fluorescence spectra from elemental Mg, Al, and Si, S, Ca, Ti, and Fe. Two solar monitors will continuously measure the x-ray output from the sun to enable quantitative elemental abundance measurements. Elemental abundances are measured independently by the gamma-ray spectrometer that will count emissions from elements that are stimulated by cosmic rays and energetic solar particles. Naturally occurring gamma radiation from K, U, and Th will also be measured.

          After gazing upon the surface of Eros for a year, bringing mortal scientists happiness and fulfillment from a deeper understanding of the early Solar System, what will be the fate of the spacecraft and asteroid? Mythology predicts they will live happily ever after, remain tightly bound forever. Celestial mechanics provides an alternative ending in which the wrath of Aphrodite reigns and the spacecraft is ejected from the gravitational sphere of Eros from a short-lived, chaotic orbit.

          The members of the NEAR science team are:

Multispectral Imager/Near-Infrared Spectrograph

Joseph Veverka, Cornell University (Team Leader), Ithaca, NY.

James F. Bell III, Cornell University, Ithaca, NY.

Clark R. Chapman, Southwest Research Institute, Boulder, CO.

Michael C. Malin, Malin Space Science Systems, Inc., San Diego, CA.

Lucy-Ann A. McFadden, University of Maryland, College Park, MD.

Mark S. Robinson, U.S. Geological Survey, Flagstaff, AZ.

Peter C. Thomas, Cornell University, Ithaca, NY.

X-ray/Gamma-Ray Spectrometer

Jacob I. Trombka, NASA Goddard Space Flight Center (Team Leader),

Greenbelt, MD.

William V. Boynton, University of Arizona, Tucson, AZ.

Johannes Bruckner, Max Planck Institut fur Chemie, Mainz, Germany

Steven W. Squyres, Cornell University, Ithaca, NY.


Mario H. Acuna, Goddard Space Flight Center (Team Leader), Greenbelt,MD

Christopher T. Russell, University of California, Los Angeles

NEAR Laser Rangefinder (NLR)

Maria T. Zuber, Massachusetts Institute of Technology, Cambridge, MA and

Goddard Space Flight Center, Greenbelt, MD (Team Leader)

Radio Science

Donald K. Yeomans, NASA Jet Propulsion Laboratory (Team Leader),

Pasadena, CA.

Jean-Pierre Barriot, Centre National D'Etudes Spatiales, Toulouse, France

Alexander S. Konopoliv, Jet Propulsion Laboratory, Pasadena, CA.

Project Scientist

Andrew F. Cheng, Applied Physics Lab, Laurel, MD.

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