Reflectance Spectrometer: Receiver
Lunar Flashlight's science payload is an active infrared reflectance spectrometer utilizing an on-board laser, capable of emitting at 4 distinct wavelengths. The receiver is an aluminum off-axis paraboloidal mirror with a focal length of 70 mm, which collects the reflected light from the lunar surface onto a single-pixel InGaAs detector with a 2-mm diameter, providing a 20-mrad field of view (FOV). The detector temperature is cold biased and stabilized by a heater. At a spacecraft altitude of 12.6 km above the lunar surface, the receiver subsystem signal-to-noise ratio on the measured reflectance band ratio is 1000–2000, corresponding to a water ice discrimination from dry regolith of 0.2–0.3 wt%.
Reflectance Spectrometer: Laser
The Lunar Flashlight illumination system uses stacked laser diode bars to emit energy pulses at four discrete wavelengths in rapid sequence. The laser wavelengths (and requirements) are optimized to distinguish the water ice absorption bands from dry lunar regolith using two pairs of molecular absorption bands and continuum measurements. In nominal operation, the LF lasers will fire sequentially for 1–6 ms each, followed by a pause of 1–6 ms with all lasers off. At an altitude of 20 km, the lasers will have a footprint on the surface of approximately 35 m in diameter. The optical receiver collects and measures the light reflected from this FOV on the lunar surface.
Data Acquisition and Mapping
The measurement with all lasers off quantifies the background, which is the sum of detector dark current, thermal emission from the receiver itself incident on the science detector, and solar and Earthshine illumination reflected from the lunar surface and detected by the instrument from both inside and outside its FOV. This background measurement will be subtracted from each laser light measurement. The strength of a water-ice absorption feature would be determined by taking the ratio of each band measurement to the adjacent continuum measurement.
Reflectance and water ice band depths will be calculated along the track of the spacecraft in order to identify locations where H2 O ice is present at the scale of ~10 km along-track and about 35 m cross-track. Data will be processed by adding successive measurements along-track for each spectral band to achieve the required discrimination; the number of coadded spectra will define the mapping resolution (anticipated to be ~2–10 km along-track resolution). The total duration of the laser-firing per pass will be approximately 2–3 min during the closest approach. By repeating these measurements over multiple points, Lunar Flashlight will create a map of surficial water frost concentration that can be correlated with previous mission data and used to guide future missions. All calibrated data and derived data products will be publicly archived in NASA’s Planetary Data System.
