pole. Since lunar regolith contains structural metals like iron and aluminum, solar panels could be mounted high up on locally-built towers that might rotate to follow the Sun. A third possibility would be to leave the panels in orbit, and beam the power down as microwaves. The solar energy converters need not be silicon solar panels. It may be more advantageous to use the larger temperature difference between Sun and shade to run heat engine generators. Concentrated sunlight could also be relayed via mirrors and used in Stirling engines or solar trough generators, or it could be used directly for lighting, agriculture and process heat. The focused heat might also be employed in materials processing to extract various elements from lunar surface materials. Energy storage Fuel cells on the Space Shuttle have operated reliably for up to 17 Earth days at a time. On the Moon, they would only be needed for 354 hours (14 days) – the length of the lunar night. Fuel cells produce water directly as a waste product. Current fuel cell technology is more advanced than the Shuttle's cells – PEM (Proton Exchange Membrane) cells produce considerably less heat (though their waste heat would likely be useful during the lunar night) and are lighter, not to mention the reduced mass of the smaller heat-dissipating radiators. This makes PEMs more economical to launch from Earth than the shuttle's cells. PEMs have not yet been proven in space. Combining fuel cells with electrolysis would provide a "perpetual" source of electricity – solar energy could be used to provide power during the lunar day, and fuel cells at night. During the lunar day, solar energy would also be used to electrolyze the water created in the fuel cells – although there would be small losses of gases that would have to be replaced. Even if lunar facilities could provide themselves access to a near-continuous source of solar energy, they would still need to maintain fuel cells or an alternate energy storage system to sustain themselves during lunar eclipses and emergency situations. Locations Soviet astronomer Vladislav V. Shevchenko proposed in 1988 the following three criteria that a lunar outpost should meet: good conditions for transport operations; a great number of different types of natural objects and features on the Moon of scientific interest; and natural resources, such as oxygen. The abundance of certain minerals, such as iron oxide, varies dramatically