USING LONG WAVE RADIO AND THE LUNAR PLASMA ENVIRONMENT FOR LUNAR EXPLO-

The Lunar Plasma Environment: Real time navigation and communication will be especially difficult in the lunar polar regions, with heavily shadowing, at best oblique sunlight and frequent lack of line of sight to the Earth or any high-altitude communications relay. Unlike the Apollo surface Extra-Vehicular Activities (EVAs), where direct line of site was maintained between astronauts and either a lander or rover in direct communication with Earth, future astronauts on polar surface EVAs will need means to communicate and perform local positioning beyond the local lines of sight to radio relays. Here we describe how these problems can be addressed through the use of low frequency radio (in this context, any usage with a radio frequency, f, with 1 Hz . f . 100 MHz). It is important to realize that at the lunar polar regions astronauts will be entering a novel and relatively poorly explored plasma and radio frequency (RF) environment, quite different from the direct sunlight and solar wind encountered in Apollo EVAs. There will thus inevitably be a close relationship between the study of the physics of this environment and the study of its possible uses; it should be possible to use the same equipment for both scientific experiments and communication tests. It is also important that any solutions adopted do not interfere with astronomy from the far-side of the Moon in largely unexplored frequency regime with f . 100 MHz [1]. Table 1 briefly describes the complicated and timevariable plasma environment near the lunar surface. The Moon is in the complicated plasma dynamics of the Earth’s magnetotail roughly 25% of the time, and the remainder of the time will be in supersonic solar wind [2]. In the lunar wake the e− density is substantially decreased. Thermal velocities of electrons in the solar wind are higher than the bulk wind velocity, while the thermal ion velocity is substantially below the bulk velocity. Non-neutral plasmas are thus likely to form in shadowed lunar craters [3], and possibly also in the lunar wake, in both cases on a scale that does not allow for laboratory simulation on Earth. These non-neutral electron clouds are likely to prevent grounding of astronauts and their equipment by the local plasma [4] and must be better characterized to understand this safety hazard. The low frequency radio range can be conveniently separated into a regime between 30 and 100 MHz, where radio waves will not be affected by the nearMoon plasma and should be able substantially penetrate the regolith, allowing for direct communication “through-the-rocks,” a medium-wave regime around 1 MHz where a long distance day-time ground-way propagation using the layer of photo-emitted electrons (e−) caused by the solar UV flux should be possible, and a very long-wave band below 30 kHz where night-time reflections off the lunar wake should be possible; this very long-wave band could possibly be used for both low-bit-rate night-time communications and to monitor the entire wake environment.