LRAD – The Lunar Radiometer on Commercial Lunar Mission IM-2

Kurzfassung

Permanent shadowed regions (PSR) in the polar regions of the Moon are craters with extremely low temperatures below 100 K, low enough to bear water ice that is stable on geological timescales. Spacecraft observations, e.g., UV albedo, Neutron suppression and geomorphology indicate the presence of such ice deposits in PSRs. Water ice deposits are a highly valuable resource for human space exploration serving as water source for astronauts and fuel source for deep space exploration missions. To foster the development of a Cis-Lunar economy, NASA started the Commercial Lunar Payload Service (CLPS) program. As part of this program the US company Intuitive Machines launched the mission IM-2 (Athena) to the Lunar southern polar region. IM-2 carried a Hopper, a detachable spacecraft that would perform a series of short flights, attempting to land within a PSR. The Hopper carried cameras, a neutron detector and LRAD, a thermal infrared radiometer that could measure the temperature of the PSR. Athena did not succeed to land upright such that the Hopper did not get a chance to launch. Nevertheless, the LRAD instruments capabilities and small engineering budgets make it suitable for future commercial lunar missions. The Lunar Radiometer uses thermopile sensors to measure the net radiative flux in the thermal infrared wavelength range. Its sensor head carries six thermopile sensors, equipped with individual IR-filters to fulfill specific scientific measurement goals:

1. Determination of the surface brightness temperature in the illuminated and shadowed terrain.

2. Derivation of the thermal inertia

3. Derivation of the mm to cm-scale surface roughness.

The instrument design is based on the miniRAD radiometer of the Idefix rover, which will launch soon with JAXA MMX sample return mission to Phobos. The choice of filters is flexible, and in the case of LRAD was optimized for very low surface temperatures. LRAD underwent radiometric calibration in a small vacuum chamber equipped with a He-cooled cold head. The sensor head was placed inside a temperature-controlled aluminum box representing the thermal environment while viewing a blackbody mounted to the cold head. By varying the blackbody temperature from 70-330 K and the box temperature between 200 K and 280 K the five calibration coefficients could be fitted for each thermopile channel. The estimated uncertainty of the brightness temperature measurement (using longpass filters), including systematic disturbances, is 10 K for a target temperature of 80 K and 5 K for a target temperature of 100 K.