THE NEAR-INFRARED SPECTROMETER

The Near-Infrared Spectrometer (NIS) instrument will map the spectral properties of asteroid 433 Eros in the range 800-2600 nm to determine its mineral content. Optically, NIS uses a scan mirror and an imaging mirror to project an image at a field stop. A grating spectrometer dis perses the energy, and a dichroic beamsplitter separates two spectral orders, reflecting the short wavelengths, 816-1500 nm, to a 32-element Ge detector, and transmitting the long wavelengths, 1500-2600 nm, to a 32-element InGaAs detector. The spectral resolutions are 22 and 44 n m for the second and first spectral orders, respectively. A scan mirro r rotates the field of view over a 140 degrees range that includes a s olar-illuminated calibration target. Spectra are measured once per sec ond using a selection of data acquisition macros. The properties of th e instrument were extensively measured during on-ground calibration, a nd in-flight calibration was begun shortly after launch. The NIS will observe Eros during approach to the asteroid and will produce composit ion maps of the entire illuminated surface at spatial resolutions as h igh as approximate to 300 m.