Guest Blogger: Vicky Hamilton, OTES Deputy Instrument Scientist
To help characterize the asteroid Bennu and aid in the selection of a scientifically compelling sampling site, OSIRIS-REx carries a pair of complementary spectrometers, one of which is called the OSIRIS-REx Thermal Emission Spectrometer, or OTES. OTES is being developed and built at the School of Earth and Space Exploration at Arizona State University. During several phases of the mission, OTES measures the energy emitted by Bennu over wavelengths of approximately 5 – 50 microns, also called the thermal infrared. At these wavelengths, virtually all minerals have unique spectral signatures that are like fingerprints, which will help the science team to understand what minerals are present on the surface of Bennu and search for minerals of particular interest, such as those that contain water. Additionally, the emitted heat energy (temperature) at these wavelengths can tell the science team about physical properties of the surface, such as the mean particle size.
OTES is an uncooled, Fourier transform infrared point spectrometer. The design of OTES is heritage from the Mars Global Surveyor TES and the Mars Exploration Rovers Mini-TES instruments. The heart of the instrument is a Michelson interferometer that collects one interferogram every two seconds. OTES’s spectral resolution is 10 cm-1 and its field of view is 8 mrad, achieved with a 15.2-cm f/3.91 Ritchey-Chretien telescope. A key component of OTES is its beamsplitter, which is the part of the interferometer that splits the incoming light beam into two pathways before they are recombined and measured at the detector. Unlike the TES and Mini-TES beamsplitters, which were made of CsI (cesium iodide) and KBr (potassium bromide) the OTES beamsplitter is made of chemical vapor deposited (CVD) diamond. A diamond beamsplitter is physically stronger than the CsI and KBr and it is not hygroscopic, which means that it does not absorb water from the atmosphere (which will cause CsI and KBr beamsplitters to become cloudy, making them less effective).
OTES looks at just one spot on the asteroid’s surface at a time, and it does not need to focus in the same way the human eye or a camera does. OTES’s telescope collects all of the infrared energy emitted by whatever is in its field of view. The area that is observed (called spatial resolution) varies depending on the distance of the spacecraft from the target (in this case, Bennu). It’s a bit like looking through the tube from a roll of paper towels – the farther away you are from what you’re looking at, the more things you see; when you get closer to whatever you’re looking at, you see a smaller portion of it. When the spacecraft is at a moderate distance from Bennu, such as 5 kilometers (during the survey part of the mission), OTES sees a spot on the surface that is about 40 m in diameter. In the reconnaissance phase of the mission, OTES has a spatial resolution that is closer to 4 m. OTES has no ability to point itself – it looks straight out from the spacecraft – so to see other places on the surface, OTES relies on the spacecraft to move the OTES field of view across the surface of Bennu.
As OTES measures the mineral signatures and temperatures of many spots, we put the information from each spot on a map to understand the whole of Bennu. In this way, we can look at where on the surface different minerals are found, how particle sizes change across the face of the asteroid, and obtain critically important context information for the samples that OSIRIS-REx will return to Earth.
Engineering models (EMs) of key sub-assemblies help the OTES engineers assess the performance of parts of the instrument before they build the flight instrument. EMs are very similar to the flight hardware and function in the same way; they can be subjected to repeated testing to verify performance, and they can be taken apart and rebuilt if necessary so that the flight instrument doesn’t need to be subjected to any unnecessary hazards other than the required testing before launch. The EM of the OTES moving mirror assembly (MMA) recently went through vibration testing to ensure that it can withstand the rigors of spaceflight. The OTES MMA passed this test with flying colors – there was no measureable change in the performance of the MMA after being shaken.
Now that both OTES and the OSIRIS-REx mission have passed their critical design reviews, OTES flight hardware is being procured, and assembly of the flight instrument is beginning. The flight OTES instrument is expected to be completed by mid-2015, just a little over a year from now.