IEEE - Aerospace and Electronic Systems - March 2020 - 48

Lunar Flashlight: Illuminating the Lunar South Pole

Figure 2.
Mission profile and timeline of the $8 mo Lunar Flashlight primary mission showing major events during the cruise and the 2 mo science
phase. All delta-V values are notional until after SLS releases the final launch trajectories to the secondary payloads.

mixtures), the observed depth of each absorption band will
be proportional to the relative contribution of ice and dry
regolith. Lunar Flashlight will measure surface reflectance
in the two near-IR wavelengths diagnostic of water ice
presence, along with two continuum wavelengths, to calculate the band depth and thereby constrain the water ice
abundance.
Because ambient sunlight does not reflect off the surface of PSRs to be collected by a spectrometer, Lunar
Flashlight carries an active laser illumination system and
a multiband optical receiver to measure surface reflectance in PSRs. The Lunar Flashlight illumination system
uses stacked laser diode bars to emit energy pulses at four
discrete wavelengths in rapid sequence, while a receiver
system detects the reflected light. We optimized the laser
wavelengths to distinguish the water ice absorption bands
from dry lunar regolith using two pairs of molecular
absorption bands and continuum measurements (see
Figure 1). The selected central wavelengths (and requirements) are 1.064 (-0.060/ þ 0.230) mm and 1.850 (-0.030/
þ 0.020) mm for continuum measurements and 1.495
(-0.015/ þ 0.015) mm and 1.990 (-0.020/ þ 0.025) mm for
absorption bands. The Lunar Flashlight 1.064 mm laser is
the same wavelength used by the Lunar Orbiter Laser
Altimeter instrument on LRO, potentially enabling a tie
point of absolute surface reflectance, although at a different spatial scale. Reflectance and water ice band depths
will be calculated along the track of the spacecraft in order
to identify locations where H2O ice is present at the scale
of $10 km along-track and about 35 m cross-track.
48

The laser diodes were procured from DILAS, Inc.; the
continuum bands are off-the-shelf procurements and custom laser epitaxies were grown for the water band wavelengths. The diode lasers, supplied with 45 A current from
batteries, emit 14-72 W (depending on wavelength).
About 99.6% of the emitted energy is encircled within a
full-angle of 17 mrad. The receiver is an aluminum offaxis 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%. The signal to noise realizable by the entire instrument will depend heavily on the noise within the electronics used to read the detector. The detector electronics are
currently in their final design and test phase; the achievable signal-to-noise ratio of the entire instrument system
will be determined through end-to-end characterization.
See the work presented in [17] and [18] for a more thorough description of the design and characterization of the
Lunar Flashlight multiband reflectometer.
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

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