Near-Infrared Spectrograph

The Near-Infrared Spectrograph (NIS), one of the five instruments on the NEAR spacecraft, is designed to map the mineralogic composition of the near-Earth asteroid 433 Eros using the spectrum of reflected sunlight. Spectra measured during approach, flyby, and orbit of Eros will cover surface regions as small as 300 meters. Each spectrum has 64 spectral channels covering the near-infrared wavelength region 800-2700 nm, which allows mapping of the mineral composition of Eros.

NIS data will provide the main evidence for the rock types present at the asteroid's surface. Together with measurements of elemental composition from the X-ray/Gamma-ray Spectrometer and color imagery from the Multispectral Imager, NIS data will provide a link between asteroids and meteorites and clarify the processes by which asteroids formed and evolved.



The NIS is a scanning spectrometer that measures near-infrared light in the wavelength range 800-2700 nm. A gold scan mirror that rotates over an angle of 140° controls the direction of viewing. Light reflected from the scan mirror enters through either of two shutters that provide a 0.38° X 0.76° or a 0.76° X 0.76° field of view, to accommodate different illumination conditions. These provide spot sizes of 0.65 X 1.3 km or 1.3 X 1.3 km from a 100-km distance.

The light is dispersed off a diffraction grating onto two detectors. A 32-element germanium (Ge) detector measures the wavelengths 804-1506 nm in 21.6-nm increments; a 32-element indium-gallium arsenide (InGaAs) detector measures the wavelengths 1348-2732 nm in 43.1-nm increments.

The gain of the Ge detector can be set at 1X or 10X, to accommodate various illumination conditions. A movable shutter can block the aperture to provide calibration measurements of the background dark level. A solar-illuminated gold calibration target is viewable for radiometric calibration.

Individual spectra consist of 12-bit measurements, and up to 16 individual spectra for one spot can be summed onboard. Mirror scanning combined with spacecraft motion will be used to build up spectral images.

The computer (digital processing unit, or DPU) is shared with the NEAR magnetometer, and provides the software to control instrument function.

Near-Infrared Spectroscopy at Eros

Asteroid composition is measured using the spectrum of reflected sunlight. Different minerals have characteristic reflectance spectra which serve as "fingerprints" for their identification. "S" asteroids, including 433 Eros, are composed mostly of iron-containing silicates (the minerals olivine and pyroxene) and metal (an iron-nickel alloy). However, the mineralogies of different S-asteroids span a wide range. Meteorite types having these mineralogies have divergent histories: some have remained largely pristine since the beginning of the solar system, while others consist of extremely ancient volcanic rocks, and still others have melted and cooled inside large parent asteroids. The correspondence between different types of meteorites and the different kinds of S asteroids remains unknown. Resolving this issue will provide an understanding of the processes that occurred early in the solar system and led to the formation of asteroids and meteorites.

433 Eros is in the middle of the range of mineralogic variation among S asteroids. In this sense it is a "representative sample" of these bodies. However, Earth-based spectroscopic studies show that different parts of Eros have different abundances of minerals; that is, Eros contains different types of rocks. The composition and especially the nature of the boundaries between different rock types are key pieces of evidence about how these asteroids formed.

NEAR's battery of instruments is designed to measure the composition and compositional variations of Eros's surface using several complementary strategies. NIS is the key to this approach. NIS data will be used to map the distribution and abundance of minerals, at scales as small as 300 meters.

The Multispectral Imager has additional spectroscopic capabilities, at 70 times the spatial resolution of NIS. That instrument will allow compositional measurements from NIS to be extrapolated down to the scale of meters, revealing details of the physical interrelationship of discrete materials identified by NIS. The X-ray/Gamma-ray Spectrometer measures the distribution and abundance of elements. Measurements from XGRS, together with NIS's measurement of the distribution and abundance of minerals, will allow definitive identification of the rock types composing Eros.

This information together will be used to synthesize the history of geologic processes that formed Eros, and to clarify how the materials on Eros are related to meteorites.

Experiment Profile

The NEAR spacecraft will follow a 2-year DVEGA trajectory beginning with a launch in February 1996.

NIS Team

Science Team:
Joseph Veverka (Cornell Univ.), Team Leader
James F. Bell, III (Cornell Univ.)
Clark R. Chapman (Planetary Science Institute)
Michael C. Malin (Malin Space Science Systems)
Lucy-Ann A. McFadden (Univ. of Maryland)
Mark S. Robinson (U. S. Geological Survey)
Peter C. Thomas (Cornell Univ.)
Lead Engineer:
Jeffery W. Warren (JHU/APL)
Instrument Scientist:
Scott L. Murchie (JHU/APL)
NEAR Payload Manager:
Robert E. Gold (JHU/APL)

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