Guest Blogger: Amy Simon – OVIRS Deputy Instrument Scientist
OSIRIS-REx will map the entire surface of asteroid Bennu to pick a site that is most accessible and likely to yield a sample, but is also scientifically interesting. Choosing the best site requires understanding both the local geology and the local composition; a prime goal of the two OSIRIS-REx spectrometers.
OVIRS, the OSIRIS-REx Visible and near-IR Spectrometer, is sensitive to wavelengths from the blue through the visible and into the near infrared (0.4 to 4.3 microns), beyond where the human eye can see. The spectrometer divides reflected sunlight and thermal radiation from the asteroid into separate wavelengths, much like a prism splits sunlight into a visible rainbow. The key to OVIRS is the use of special filters called linear variable (or wedged) filters. These filters only allow certain wavelengths of light to pass to the detector, and the wavelength transmitted varies along the length of each filter. The filters are bonded together like a butcher block and sit in front of a sensitive detector covering 512×512 pixels.
OVIRS is a point spectrometer, meaning it collects all wavelengths from a single spot on the surface at the same time. The electronics determine how long to expose the pixels and which pixels to read out, avoiding the seams of the butcher block. Because OVIRS observes at infrared wavelengths, the detector is cooled using a passive radiator that points to cold, dark, deep space; this lowers the electrical noise and gives a better science signal.
Different elements and compounds on the surface each have a unique spectral signature, much like a fingerprint, allowing us to diagnose the composition of Bennu’s surface as OSIRIS-REx searches for the perfect site for sampling.
OVIRS uses the motion of the spacecraft to build up maps of the asteroid. Each slew of the spacecraft will provide another strip that can be reassembled like a beach ball to make a global map. The main map is acquired from a distance of 5 kilometers, yielding 20-meter (65 ft) spatial resolution. Candidate sample sites will also be mapped with a resolution of 2 meters (6.5 ft). Maps of composition will be combined with maps from the other instruments to choose the best sample site.
OVIRS has now passed its Critical Design Review and the team is shifting from building and testing engineering units to final flight parts. After full assembly and testing at Goddard Space Flight Center, OVIRS will be delivered to Lockheed Martin in 2015 for integration into the spacecraft.