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An orbital propellant depot is a cache of propellant that is placed in orbit around Earth or another body to allow spacecraft or the transfer stage of the spacecraft to be fueled in space. It is one of the types of space resource depot that have been proposed for enabling infrastructure-based exploration.〔 Many different depot concepts exist depending on the type of fuel to be supplied or its location. In-space fuel depots are not necessarily located near or at a space station. Potential users of in-orbit refueling and storage facilities include space agencies, defense ministries and communications satellite or other commercial companies. Satellite servicing depots would extend the lifetime of satellites that have nearly consumed all of their orbital maneuvering fuel and are likely placed in a geosynchronous orbit . The spacecraft would conduct a space rendezvous with the depot, or ''vice versa'', and then transfer propellant to be used for subsequent orbital maneuvers. Intelsat has recently contracted for an initial demonstration mission to refuel several satellites in geosynchronous orbit, beginning in 2015. A low earth orbit (LEO) depot's primary function would be to provide propellant to a transfer stage headed to the moon, Mars, or possibly a geosynchronous orbit. Since all or a fraction of the transfer stage propellant can be off-loaded, the separately launched spacecraft with payload and/or crew could have a larger mass or use a smaller launch vehicle. With the LEO depot, the size of the launch vehicle can be reduced and the flight rate increased, which may reduce the total launch costs since the fixed costs are spread over more flights and fixed costs are usually lower with smaller launch vehicles. A depot could also be placed at Earth-Moon Lagrange point 1 (EML-1) or behind the Moon at EML-2 to reduce costs to travel to the moon or Mars. Placing a depot in Mars orbit has also been suggested.〔 page 13〕 == LEO depot fuels == For rockets and space vehicles, propellants usually take up 2/3 or more of their total mass. Large upper-stage rocket engines generally use cryogenic fuels like liquid hydrogen and liquid oxygen (LOX) because of the large specific impulse possible, but must carefully consider a problem called "boil off". The boil off from only a few days of delay may not allow sufficient fuel for higher orbit injection, potentially resulting in a mission abort. Lunar or Mars missions will require weeks to months to accumulate tens of thousands to hundreds of thousands of kilograms of propellant, so additional equipment may be required on the transfer stage or the depot to mitigate boiloff. Non-cryogenic, earth-storable liquid rocket propellants including RP-1 (kerosene), hydrazine and nitrogen tetroxide (NTO), and mildly cryogenic, space-storable propellants like liquid methane and liquid oxygen, can be kept in liquid form with less boiloff than the cryogenic fuels, but also have lower specific impulse. Additionally, gaseous or supercritical propellants such as those used by ion thrusters include xenon, argon, and bismuth. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Propellant depot」の詳細全文を読む スポンサード リンク
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