The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument

Abstract

The OSIRIS-REx Thermal Emission Spectrometer (OTES) will provide remote measurements of mineralogy and thermophysical properties of Bennu to map 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 µm (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 telescope that feeds a flat-plate Michelson moving mirror mounted on a linear voice-coil motor assembly. A single uncooled deuterated L-alanine doped triglycine sulfate (DLATGS) pyroelectric detector is used to sample the interferogram every two seconds. Redundant ~0.855 µm laser diodes are used in a metrology interferometer to provide precise moving mirror control and IR sampling at 772 Hz. The beamsplitter 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. The OTES was developed by Arizona State University with Moog Broad Reach developing the electronics. OTES was integrated, tested, and radiometrically calibrated on the Arizona State University campus in Tempe, AZ.

Evidence for widespread hydrated minerals on asteroid (101955) Bennu

“…The calibration of OTES data generally consists of an automated processing pipeline that transforms OTES raw interferograms into voltage spectra and then into absolute radiance units 3 . More specifically, the measured voltage spectrum is the difference between the radiance of the scene, foreoptics, and the detector times the instrument response function (IRF); the radiance of the detector and the IRF are unknowns, but can be determined by periodically observing space and an internal calibration target, at which point it becomes possible to solve for the scene radiance and account for temperature fluctuations of the instrument (detector) that result from the instrument heater cycling during the observations.…”

“…The OVIRS instrument 2 is a hyperspectral, point spectrometer that measures the reflected and emitted energy of Bennu across the spectral region from 0.4 to 4.3 μm (25,000 to 2,300 cm −1 ) with a circular, 4-mrad field of view (FOV). The OTES instrument 3 , the first thermal infrared spectrometer to visit an asteroid, is a hyperspectral, point spectrometer that measures the emitted radiance of Bennu across the spectral region from ~1750 to 100 cm −1 (~5.71 to 100 μm) with a circular, 8-mrad FOV. The primary role of visible-to-infrared spectroscopy on the OSIRIS-REx mission is to characterize the mineralogy and chemistry of Bennu and aid in sample site selection 4 .…”

Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis

“…The OSIRIS-REx Camera Suite (OCAMS [2]) PolyCam panchromatic camera confirmed the top-like shape of the asteroid [3] and imaged the surface at scales down to 0.33 m/pixel, while the MapCam instrument imaged at scales down to 1.1 m/pixel in four colors and a panchromatic filter. The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS [4]) and the OSIRIS-REx Thermal Emission Spectrometer (OTES [5]) measured disk-integrated spectra covering a full asteroid rotation before Bennu filled their respective fields of view.…”

Constraining the thermal properties of planetary surfaces using machine learning: Application to airless bodies

“…These mission requirements will constrain the choice of the latitude of the sample selection area, the local time, and the arrival date at the asteroid (Delbo et al 2015). The OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al 2018) will collect the infrared fluxes emitted by the surface of the asteroid covering the spectral range 5.71 − 100 µm (1750 − 100 cm −1 ) with a spectral sample interval of 8.66 cm −1 . The thermal properties of the surface will inform the team about the sampleability of the different regions of the asteroid.…”