improve the chances for the survival of intelligent life over cosmic timescales, given the many natural and human-related hazards that have been widely noted. If humanity does gain access to a large amount of energy, on the order of the mass-energy of entire planets, it may eventually become feasible to construct Alcubierre drives. These are one of the few methods of superluminal travel which may be possible under current physics. However, it is probable that such a device could never exist, due to the fundamental challenges posed. For more on this see Difficulties of making and using an Alcubierre Drive. Intergalactic travel The distances between galaxies are on the order of a million times farther than those between the stars, and thus intergalactic colonization would involve voyages of millions of years via special self-sustaining methods. Law, governance, and sovereignty Space activity is legally based on the Outer Space Treaty, the main international treaty. But space law has become a larger legal field, which includes other international agreements such as the significantly less ratified Moon Treaty and diverse national laws. The Outer Space Treaty established the basic ramifications for space activity in article one:"The exploration and use of outer space, including the Moon and other celestial bodies, shall be carried out for the benefit and in the interests of all countries, irrespective of their degree of economic or scientific development, and shall be the province of all mankind." And continued in article two by stating:"Outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means." The development of international space law has revolved much around outer space being defined as common heritage of mankind. The Magna Carta of Space presented by William A. Hyman in 1966 framed outer space explicitly not as terra nullius but as res communis, which subsequently influenced the work of the United Nations Committee on the Peaceful Uses of Outer Space. Reasons Survival of human civilization The primary argument calling for space colonization is the long-term survival of human civilization and terrestrial life. By developing alternative locations off Earth, the planet's species, including humans, could live on in the event of natural or human-made disasters on our own planet. On two occasions, theoretical physicist and cosmologist Stephen Hawking argued for space colonization as a means of saving humanity.