Editing Interplanetary spaceflight (section)

===Using extraterrestrial resources===
{{main|In-situ resource utilization}}
[[File:Mars Ice Home concept.jpg|thumb|left|300px|Langley's Mars Ice Dome design from 2016 for a Mars base would use in-situ water to make a sort of space-[[igloo]].{{clarify|date=May 2020}}]]
Current space vehicles attempt to launch with all their fuel (propellants and energy supplies) on board that they will need for their entire journey, and current space structures are lifted from the Earth's surface. [[In-Situ Resource Utilization|Non-terrestrial sources of energy and materials]] are mostly a lot further away, but most would not require lifting out of a strong gravity field and therefore should be much cheaper to use in space in the long term.

The most important non-terrestrial resource is energy, because it can be used to transform non-terrestrial materials into useful forms (some of which may also produce energy). At least two fundamental non-terrestrial energy sources have been proposed: solar-powered energy generation (unhampered by clouds), either directly by [[solar cell]]s or indirectly by focusing solar radiation on boilers which produce steam to drive generators; and [[electrodynamic tether]]s which generate electricity from the powerful magnetic fields of some planets (Jupiter has a very powerful magnetic field).

Water ice would be very useful and is widespread on the moons of Jupiter and Saturn:
* The low gravity of these moons would make them a cheaper source of water for space stations and planetary bases than lifting it up from Earth's surface.
* Non-terrestrial power supplies could be used to [[electrolysis|electrolyse]] water ice into oxygen and hydrogen for use in [[bipropellant rocket]] engines.
* [[Nuclear thermal rocket]]s or [[Solar thermal rocket]]s could use it as [[reaction mass]]. Hydrogen has also been proposed for use in these engines and would provide much greater [[specific impulse]] (thrust per kilogram of reaction mass), but it has been claimed that water will beat hydrogen in cost/performance terms despite its much lower specific impulse by orders of magnitude.<ref>{{Cite web |url=http://www.neofuel.com/moonice1000/ |title=Origin of How Steam Rockets can Reduce Space Transport Cost by Orders of Magnitude |access-date=2007-02-16 |archive-date=2017-11-16 |archive-url=https://web.archive.org/web/20171116002743/http://www.neofuel.com/moonice1000/ |url-status=live }}</ref><ref>{{Cite web |url=http://www.neofuel.com/ |title="Neofuel" -interplanetary travel using off-earth resources |access-date=2006-10-08 |archive-date=2006-11-16 |archive-url=https://web.archive.org/web/20061116064420/http://www.neofuel.com/ |url-status=live }}</ref>
* A spacecraft with an adequate water supply could carry the water under the hull, which could provide a considerable additional safety margin for the vessel and its occupants:
** The water would absorb and conduct solar energy, thus acting as a [[heat shield]]. A vessel traveling in the inner Solar System could maintain a constant heading relative to the Sun without overheating the side of the spacecraft facing the Sun, provided the water under the hull was constantly circulated to evenly distribute the solar heat throughout the hull;
** The water would provide some additional protection against ionizing radiation;
** The water would act as an insulator against the extreme cold assuming it was kept heated, whether by the Sun when traveling in the inner Solar System or by an on board power source when traveling further away from the Sun;
** The water would provide some additional protection against micrometeoroid impacts, provided the hull was compartmentalized so as to ensure any leak could be isolated to a small section of the hull.

Oxygen is a common constituent of the [[Moon]]'s crust, and is probably abundant in most other bodies in the Solar System. Non-terrestrial oxygen would be valuable as a source of water ice only if an adequate source of [[hydrogen]] can be found.{{clarify|date=April 2014}}<!-- if ice is found, then both O2 and H2 would be available, given the energy to melt and then electrolyze it -->  Possible uses include:

* In the [[life support system]]s of space ships, space stations and planetary bases.
* In rocket engines. Even if the other propellant has to be lifted from Earth, using non-terrestrial oxygen could reduce propellant launch costs by up to 2/3 for hydrocarbon fuel, or 85% for hydrogen. The savings are so high because oxygen accounts for the majority of the mass in most [[rocket propellant]] combinations.

Unfortunately hydrogen, along with other volatiles like carbon and nitrogen, are much less abundant than oxygen in the inner Solar System.

Scientists expect to find a vast range of [[organic compound]]s in some of the planets, moons and comets of the [[outer Solar System]], and the range of possible uses is even wider. For example, [[methane]] can be used as a fuel (burned with non-terrestrial oxygen), or as a feedstock for [[petrochemical]] processes such as making [[plastic]]s. And [[ammonia]] could be a valuable feedstock for producing [[fertilizer]]s to be used in the vegetable gardens of orbital and planetary bases, reducing the need to lift food to them from Earth.

Even unprocessed rock may be useful as rocket propellant if [[mass drivers]] are employed.