A shoebox-sized NASA-funded cubesat should create the most detailed, high-resolution map of lunar water ice ever attempted. Now scheduled for launch in 2020* as a secondary payload on NASA’s Space Launch System (SLS) Exploration Mission (EM-1), the Lunar Polar Hydrogen Mapper Mission (LunaH-Map) will spend two months mapping the abundance of hydrogen, and by rote water (H2O) ice, in the lunar South Pole’s deep craters.
The 30-pound cubesat, proposed, designed and built by a team at Arizona State University (ASU), will be deployed by SLS once within the lunar vicinity. From there, the tiny spacecraft will use ion propulsion to conduct its own lunar flyby, then wait to be gravitationally captured into lunar orbit.
The spacecraft will then spiral into a very elliptical polar orbit which will take it as low as 10- to 25-km above the lunar South Pole and some 4,000-km above the lunar North Pole. And it will do so at a tiny fraction of the cost of a conventional NASA planetary science mission.
Once deployed, its solar arrays unfold, the spacecraft powers on, finds the Sun, and starts charging the batteries, ASU planetary geologist Craig Hardgrove, the mission’s principal investigator, told me. We begin communication via NASA’S Deep Space Network shortly after deployment, he says.
The mission’s primary goal is to measure the spatial distribution and extent of hydrogen enrichments at the lunar South Pole’s permanently shadowed craters. If there is an enrichment within a region of less than 20 sq.-km then LunaH-Map will be able to resolve it, says Hardgrove.
The cubesat spacecraft is equipped with a star tracker, an x-band radio, a command and data handling system, power control system, ion propulsion system and its primary science instrument — a miniaturized neutron spectrometer.
The spectrometer measures high-energy galactic cosmic ray interactions with hydrogen embedded in the lunar surface, says Hardgrove.
How does the spectrometer detect such hydrogen interactions with neutrons from cosmic rays?
With each hydrogen collision, these neutrons lose about half their energy. Therefore, the energy distribution of neutrons emitted from the lunar surface is a good proxy for the amount of hydrogen and water ice within the top meter of the lunar surface, says Hardgrove. Detection of more high-energy neutrons indicates less water ice; while fewer high-energy neutrons indicate more water ice. But the goal is to create a detailed map of the hydrogen abundance within and around these permanently shadowed regions.
“Planetary scientists have used this property to map water ice on Mars, Ceres, Mercury and the Moon with great success,” said Hardgrove.
During its science mission, LunaH-Map will make repeated flyovers of several well-known South Pole lunar craters ; including, Cabeus, Shackleton, Amundsen, and Sverdrup. The spacecraft will produce maps of bulk lunar regolith water-ice at spatial scales that allow us to “see” into the permanently shadowed regions, says Hardgrove.
Once LunaH-Map has completed its nominal mission length of 60 days and transmitted its critical data back to Earth, it will de-orbit and crash into a permanently shadowed crater.
What’s the biggest puzzle that this mission might solve?
We don’t understand the distribution of water-ice within permanently shadowed regions of the lunar poles , says Hardgrove. There is a theory that the irregular distribution from one crater to the next is due to the wobble of the Moon’s poles over geologic time, he says.
As the ancient Moon’s poles changed position, regions that are currently not in permanent shadow may have been in shadow in the past. So, traces of water ice and water-rich compounds may still linger, despite those regions no longer being in permanent shadow. But only the craters that were previously in permanent shadow at prior pole positions would have significant enrichment, says Hardgrove.
This is a hypothesis that LunaH-Map could test, says Hardgrove.
If LunaH-Map’s observed distributions of polar ices and water-rich compounds match models of the Moon’s ancient polar wander, then it’s likely that their current distribution can be explained by the Moon’s ancient wobbles, says Hardgrove. Models predict that the enrichments that remain at the Moon’s former ancient poles may lie on spatial scales of less than 10-20 sq.-km. This is a scale which LunaH-Map should be able to see but previous lunar missions with neutron spectrometers could not.
Observations may not match what’s expected from the wandering poles, however. If so, alternative ideas about the origin of lunar water ice and other water-rich compounds include delivery via asteroids and comets, or eruptions spewing water and other minerals from deep within the Moon’s interior.
“But water ice is not stable on the Moon in regions that get any sunlight,” said Hardgrove.
Delivery by passing and/or impacting asteroids and comets may not account for the amount of existing water ice in the permanently shadowed regions, says Hardgrove. However, it is possible that subsurface lunar processes could account for some of these hydrogen-rich deposits, but that’s more speculative, he says.
The hope, nevertheless, is that the mission will not only do cutting edge lunar science but also further the cause of future human exploration and settlement at the lunar South Pole.
These maps will be on the spatial scale of a landing ellipse for a future human mission, says Hardgrove. Thus, he says, they could be used to determine that the proposed landing site is closest to regions with the most possible water ice.
*Correction: An earlier version of this article mistakenly reported a September launch date for the cubesat. The mission is now scheduled for next year.