of the Moon and Earth. The far side is also a good location for a large radio telescope because it is well shielded from the Earth. Due to the lack of atmosphere, the location is also suitable for an array of optical telescopes, similar to the Very Large Telescope in Chile. Scientists have estimated that the highest concentrations of helium-3 can be found in the maria on the far side, as well as near side areas containing concentrations of the titanium-based mineral ilmenite. On the near side the Earth and its magnetic field partially shield the surface from the solar wind during each orbit. But the far side is fully exposed, and thus should receive a somewhat greater proportion of the ion stream. Lunar lava tubes Lunar lava tubes are a potential location for constructing a lunar base. Any intact lava tube on the Moon could serve as a shelter from the severe environment of the lunar surface, with its frequent meteorite impacts, high-energy ultra-violet radiation and energetic particles, and extreme diurnal temperature variations. Lava tubes provide ideal positions for shelter because of their access to nearby resources. They also have proven themselves to be reliable structures, having withstood the test of time for billions of years. An underground habitat would escape the extreme temperatures on the Moon's surface. The day period (about 354 hours) has an average temperature of about , although it can rise as high as . The night period (also 354 hours) has an average temperature of about . Underground, both day and night periods would be around , and humans could install ordinary heaters for warmth. One such lava tube was discovered in early 2009. Habitat construction There have been numerous proposals regarding lunar habitats. The designs have evolved throughout the years as knowledge about the Moon has grown, and as the technological possibilities have changed. The proposed habitats range from the actual spacecraft landers or their used fuel tanks, to inflatable modules of various shapes. Some hazards of the lunar environment such as sharp temperature shifts, lack of atmosphere or magnetic field (which means higher levels of radiation and micrometeoroids) and long nights, were unknown early on. Proposals have shifted as these hazards were recognized and taken into consideration. Underground habitat Some suggest building the lunar habitats underground, which would give protection from radiation and micrometeoroids. This would also greatly reduce the risk of