The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument

The OSIRIS-REx Thermal Emission Spectrometer (OTES) is designed to acquire remote infrared spectral measurements of the asteroid Bennu in order to determine the mineralogy and thermophysical properties of its surface, help select the OSIRIS-REx sampling site, and investigate the Yarkovsky effect. OTES is a Fourier Transform spectrometer covering the spectral range 5.71-100 mm (1750-100 cm^-1) with a spectral sample interval of 8.66 cm^-1 and a 6.5-mrad field of view. The OTES telescope is a 15.2-cm diameter Cassegrain that feeds a flat-plate Michelson interferometer. A single uncooled deuterated l-alanine doped triglycine sulfate (DLATGS) pyroelectric detector is used to sample the interferogram every two seconds. A laser diode is used in a metrology interferometer to provide precise moving mirror control and IR sampling at 772 Hz. A diamond beamsplitter was developed for this instrument to provide spectral measurements to 100 µm, and is the first successful space application of this material. It is a 38-mm diameter, 1-mm thick chemical vapor deposited diamond with an antireflection microstructure to minimize surface reflection. An internal calibration cone blackbody target provides radiometric calibration. The radiometric precision in a single spectrum is ≤2.2 × 10^-8 W cm^-2 sr^-1 /cm^-1 between 300 and 1350 cm^-1. The absolute integrated radiance error is <1% for scene temperatures ranging from 150 to 380 K. The overall OTES envelope size is 37.5 × 28.9 × 52.2 cm, and the mass is 6.27 kg. The power consumption is 10.8 W average. OTES was developed and integrated, tested, and radiometrically calibrated at Arizona State University, with Moog Broad Reach developing the electronics.

OTES data were collected of the Earth and Moon during the OSIRIS-REx Earth gravity assist flyby. These data provided an excellent confirmation of the OTES performance and have also been used to validate the instrument pointing and data processing methodology. If OTES are successfully acquired during the approach phase, we will present our initial analysis of these spectra.