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 Hydrogen originates from the Greek words "hydro" and "genes" meaning "water" and "generator". Hydrogen is the simplest and most abundant element in the universe, making up an estimated 90% of the mass of the universe. However, hydrogen (H) does not exist by itself on earth, but is combined with other elements such as oxygen (H2O or water) or carbon such as methane (CH4) or petroleum. Hydrogen is classified as a secondary source of energy, or an energy carrier that can be used to store, move and deliver energy in a practical way. It has many applications and can be used for transportation, heating and power generation, making it a versatile form of energy carrier. Moreover, it has the highest energy content per unit of weight (52,000 British Thermal Units per pound) of any known fuel, which makes it an extremely efficient source of energy. NASA has used hydrogen in the space program since the 1960s, because it is extremely efficient, and the only by-product is water, which is used as drinking water by the crew, and heat, which can be used for other applications. Hydrogen is now being pursued as the optimal alternative fuel for more "down to earth" applications, because it is a clean source of energy that is efficient and reliable.
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