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.
Specifications:
- Mass: Spectrometer, 10.15 kg; Electronics, 5.0 kg
- Power: Spectrometer, 4.3 W; Electronics, 10.8 W
- Instantaneous field of view: adjustable to
0.38° X 0.76° or 0.76° X 0.76°; 0.65 X 1.3 km or
1.3 X 1.3 km from a 100 km distance
- Total field of view: scannable over 140° in
one axis; second dimension obtained by spacecraft motion
- Integration time: 1 second; onboard summation of
up to 16 spectra
- Detectors: 804- wavelength range:1506 nm in
21.6-nm increments
- 32-element InGaAs array: 1348-2732 nm in 43.1-nm
increments
- Data Character: 64 channels, 12 bits per
channel, summable to 16 bits
Description
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.
- After slowly approaching and flying by Eros in February 1999, the
spacecraft will be inserted into orbits of progressively lower radii,
culminating in a 35-km low orbit
- NIS will return spectroscopic measurements during approach, flyby,
and orbit at progressively higher spatial resolutions.
- This first, high-resolution mineralogic mapping investigation of an asteroid will finally clarify the composition and geologic variety of an asteroid and shed light on what processes have produced the asteroids and meteorites.
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)
Links to the other NEAR instruments:
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Near-Infrared Spectrograph