2018.A.1.1. Lunar Flashlight CubeSat mission: a multi-band SWIR laser reflectometer to map and quantify water ice on the lunar South Pole

Author(s)

Quentin Vinckier (1)
Udo J. Wehmeier (1)
Christopher Paine (1)
Paul O. Hayne (2)
Barbara A. Cohen (3)
Glenn R. Sellar (1)

NASA Jet Propulsion Laboratory/California Institute of Technology, U.S.A.
University of Colorado, Boulder, U.S.A.
NASA Goddard Space Flight Center, U.S.A.

Session

A.1

Keywords

Lunar Flashlight, CubeSat, multi-band SWIR laser reflectometer, lunar water ice

Abstract

Mapping and quantifying lunar water ice addresses one of NASA’s Strategic Knowledge Gaps to understand the lunar resource potential for future human exploration of the moon. Lunar Flashlight (LF) is an innovative NASA CubeSat mission dedicated to mapping water ice in the permanently shadowed regions of the lunar South Pole. LF will acquire these measurements from lunar orbit using a multi-band shortwave infrared (SWIR) laser reflectometer employing an optical receiver aligned with four diode lasers emitting different wavelengths between 1 and 2 µm. The receiver measures the laser radiance reflected from the lunar surface and continuum/absorption reflectance band ratios are then analyzed to quantify water ice in the illuminated spot. The science goal is to identify locations where water ice may be present at concentrations ≥ 0.5wt% on the lunar surface with a mapping resolution of 1-2 km. During the planned 2-month mission, LF will pulse the lasers from 11 orbits, at altitudes of 12.6-52.4 km within 10° latitude of the lunar South Pole.

Here we present the instrument and measurement approach, including preliminary characterization data, and discuss about the main design challenges. The lasers output optical power between 15 and 34 W. The chosen wavelengths correspond to peak absorption for water ice (1.495 and 1.99 µm) and nearby continuum (1.064 and 1.850 µm). ˃ 99.6% of the emitted energy is encircled within a full-angle of 1.15°. The receiver is designed to maximize detection efficiency and uniformity within this field of view (FOV), and to minimize the detected stray solar light from outside the FOV, taking into account the Moon’s topography & solar illumination, and planned orbital trajectories of the LF spacecraft. The receiver is based on a 70×70-mm off-axis aluminum paraboloidal mirror with a focal length of 70 mm which collects the incoming light onto a single pixel 2 mm diameter InGaAs detector.

We also present the predicted performance. We have developed a model that predicts signal-to-noise ratio (SNR) as a function of the spacecraft position. From these SNRs, we evaluate the fraction of measurements for which the SNRs are high enough to discriminate between dry regolith and a given water ice content as a function of the confidence level.

This highly mass- and volume-constrained instrument payload will demonstrate several firsts, including being one of the first instruments onboard a CubeSat performing science measurements beyond low-Earth orbit and the first planetary mission to use multi-band active reflectometry from orbit.