NEAR-INFRARED SPECTROMETER FOR THE NEAR-EARTH ASTEROID RENDEZVOUS MISSION

The Near-Infrared Spectrometer (NIS) instrument on the Near-Earth Aste roid Rendezvous (NEAR) spacecraft is designed to map spectral properti es of the mission target, the S-type asteroid 433 Eros, at near-infrar ed wavelengths diagnostic of the composition of minerals forming S ast eroids. NIS is a grating spectrometer, in which light is directed by a dichroic beam-splitter onto a 32-element Ge detector (center waveleng ths, 816-1486 nm) and a 32-element InGaAs detector (center wavelengths , 1371-2708 nm). Each detector reports a 32-channel spectrum at 12-bit quantization. The field-of-view is selectable using slits with dimens ions calibrated at 0.37 degrees x 0.76 degrees (narrow slit) and 0.74 degrees x 0.76 degrees (wide slit). A shutter can be closed for dark c urrent measurements. For the Ge detector, there is an option to comman d a 10x boost in gain. A scan mirror rotates the field-of-view over a 140 degrees range, and a diffuse gold radiance calibration target is v iewable at the sunward edge of the field of regard. Spectra are measur ed once per second, and up to 16 can be summed onboard. Hyperspectral image cubes are built up by a combination of down-track spacecraft mot ion and cross-track scanning of the mirror. Instrument software allows execution of data acquisition macros, which include selection of the slit width, number of spectra to sum, gain, mirror scanning, and an op tion to interleave dark spectra with the shutter closed among asteroid observations. The instrument was extensively characterized by on-grou nd calibration, and a comprehensive program of in-flight calibration w as begun shortly after launch. NIS observations of Eros will largely b e coordinated with multicolor imaging from the Multispectral Imager (M SI). NIS will begin observing Eros during approach to the asteroid, an d the instrument will map Eros at successively higher spatial resoluti ons as NEAR's orbit around Eros is lowered incrementally to 25 km alti tude. Ultimate products of the investigation will include composition maps of the entire illuminated surface of Eros at spatial resolutions as high as similar to 300 m.