Much of what we know about the physical state and composition of materials that make up the surface of the Moon comes from the field of remote sensing, whereby a wide variety of radiation emitted from planetary surfaces is carefully measured and analyzed. Typically, these emissions reflect the interaction of the Moon with incident radiation from external sources, such as galactic cosmic rays and solar illumination. When this radiation interacts with the minerals and elements of lunar materials, characteristic absorptions and emissions of energy result, which can then be detected and measured with instruments from Earth, in lunar orbit, on the surface or in the subsurface of the Moon. These measured emissions vary in wavelength and magnitude in ways characteristic of their compositional properties.
Remote sensing allows us to map both elemental composition (i.e., which elements are present and in what quantity) and mineralogical make-up (i.e., how the elements coordinate with each other in crystal structures) of the lunar surface. While optical sensing can be focused with lenses, particle sensors have a more difficult issue. Cosmic rays hit the Moon from all directions, inducing nuclear reactions that produce gamma-rays and other emissions. Detectors for this radiation have a “four-pi” (4p) field of view, meaning that they constantly look in all directions. This effectively limits the resolution of such techniques; for gamma-ray and neutron mapping, the best resolution obtainable is about equal to the orbital altitude of the detector. Because of the large mountains on the Moon, we cannot orbit much closer than about 20-30 km above the surface. Thus, elemental maps from gamma-ray or neutron sensing cannot show features much smaller than about 30 km across.
This poses a problem for prospectors looking for mineable deposits of lunar polar ice. We need information about ice distribution on scales of tens to hundreds of meters. The small (meter)-scale structure of the polar deposits can be mapped using surface rovers, but how can we get the data needed to determine where to land the surface explorers? Typically, the most promising areas are identified remotely (e.g., from orbital mapping), followed by surface exploration (e.g., by a rover). How do we bridge the gap in knowledge of the distribution of water at those intermediate scales?
One way to sharpen the remote view is to use a collimator, i.e., a shield around the detector that excludes most radiation from non-lunar directions, producing a narrow field of view that can generate a high-resolution map. This approach was attempted on the Lunar Reconnaissance Orbiter mission’s LEND experiment. That instrument used a collimator to restrict the signal to an area around the sub-spacecraft point, with the objective of mapping lunar neutron flux, indicative of hydrogen concentration. However, two problems arose with the LEND instrument. First, the detector measured some higher energy neutrons that are reflective of factors other than hydrogen, making data interpretation difficult. Second, the collimator did not work as advertised, with significant leakage around the margins.
Collimated detectors have one other significant problem—even if they work as advertised, they rely on the very low, natural flux of incident energy, which means that precision suffers. It would take over a year of mapping to get good results from collimated neutron data, and even then, the uncertainties would equal the typical concentrations. The smaller the areas that we attempt to map, the lower the precision of the measurement, making such data inadequate for prospecting purposes.
But there is another approach to getting intermediate-scale, high-precision data. We can use active sensing, a technique in which we illuminate the target (i.e., the lunar surface) with the appropriate energy and then measure the surface response with a detector in orbit. In the case of elemental mapping, we create a particle beam, focus it into a small spot on the surface, and use the induced radiation to measure composition. We do this on laboratory scales, where electrons or ions are formed into a beam and pointed at geological samples. This technique (called microprobe) allows us to analyze the chemical composition of minerals at microscopic scales, and is the basic tool of modern sample science. By creating a particle beam in orbit, then pointing it at the Moon and measuring the induced radiation from the illuminated surface, we are in effect, creating a giant microprobe in space. The technique of generating a neutral particle beam in space was demonstrated during a suborbital mission called BEAR (Beam Experiment Aboard a Rocket), flown by the Los Alamos National Laboratory in 1989.
Interestingly, this scheme was proposed for a lunar orbital mapping mission more than 20 years ago. The Double Eagle Space Experiment was to fly a particle accelerator in lunar orbit and use this beam to illuminate spots on the surface, which would then be analyzed by measuring the induced gamma rays and X-rays emitted by the target. To generate a powerful beam, the spacecraft needed a power source of high output. The plan was to purchase and fly a Russian-made nuclear reactor to power the experiment. The TOPAZ-II reactor used plutonium fuel to produce about 5 kW of power, which was fed to a particle accelerator, generating a neutral particle beam. This beam could illuminate a spot a couple of hundred meters across on the lunar surface from a 50-100 km orbit. The illuminated spot would emit the induced reactions that would be received on either the beaming spacecraft or from a separate, detector spacecraft in a following orbit. The Double Eagle mission was initially pursued as part of the Strategic Defense Initiative to develop ballistic missile defense. Particle beams could be used to destroy incoming nuclear warheads still in space. But Double Eagle was shelved along with much of the rest of the SDI program in the early 1990s.
The significant power levels produced by the TOPAZ-II reactor made the Double Eagle mission possible. Although the TOPAZ reactors are no longer available, high power levels can be generated by current state-of-the-art solar arrays. Many solar panels used in communications satellites (intended for deployment in GEO) can generate up to 12 kW of electrical power. So a Double Eagle mission using such solar arrays is feasible. By employing active particle beam sensing, we can map the composition of the permanently dark areas near the poles with both high precision and high resolution. Maps produced by a modified Double Eagle could help us select the best possible candidate sites for surface exploration. Proceeding from the general to specific makes logical sense, and is the most efficient way to assure we have found the best possible prospects for ice harvesting and processing. Such information is vital for the development of extensible space transportation architectures, as we pursue a path toward learning how to harvest and use the Moon’s resources.