Editing Interplanetary spaceflight (section)

==Improved technologies and methodologies==
[[File:NASA's SpaceX Europa Clipper Launch (KSC-20241014-PH-AJN01 0022) (cropped).jpg|thumb|A [[Falcon Heavy]] rocket launches [[Europa Clipper]] into a trajectory to Jupiter; mission demands required all parts of the [[Reusable launch vehicle|partially reusable]] launch vehicle to be [[Expendable launch system|expended]].]]
Several technologies have been proposed which both save fuel and provide significantly faster travel than the traditional methodology of using [[#Hohmann transfers|Hohmann transfers]]. Some are still just theoretical, but over time, several of the theoretical approaches have been tested on spaceflight missions.  For example, the [[Deep Space 1]] mission was a successful test of an [[ion drive]].<ref>{{Cite web|url=https://www.jpl.nasa.gov/missions/deep-space-1-ds1/|title=Deep Space 1|website=www.jpl.nasa.gov|access-date=2018-09-12|archive-date=2017-11-17|archive-url=https://web.archive.org/web/20171117225621/https://www.jpl.nasa.gov/missions/deep-space-1-ds1/|url-status=live}}</ref> These improved technologies typically focus on one or more of:
* [[Space propulsion]] systems with much better fuel economy. Such systems would make it possible to travel much faster while keeping the fuel cost within acceptable limits.
* Using solar energy and [[in-situ resource utilization]] to avoid or minimize the expensive task of shipping components and fuel up from the Earth's surface, against the Earth's gravity (see "Using non-terrestrial resources", below).
* Novel methodologies of using energy at different locations or in different ways that can shorten transport time or reduce [[cost]] per unit mass of [[space transport]]

Besides making travel faster or cost less, such improvements could also allow greater design "safety margins" by reducing the imperative to make spacecraft lighter.

===Improved rocket concepts===
{{main|Spacecraft propulsion}}
All rocket concepts are limited by the [[Tsiolkovsky rocket equation]], which sets the characteristic velocity available as a function of exhaust velocity and mass ratio, of initial (''M''<sub>0</sub>, including fuel) to final (''M''<sub>1</sub>, fuel depleted) mass. The main consequence is that mission velocities of more than a few times the velocity of the rocket motor exhaust (with respect to the vehicle) rapidly become impractical, as the [[dry mass]] (mass of payload and rocket without fuel) falls to below 10% of the entire rocket's [[wet mass]] (mass of rocket with fuel).

====Nuclear thermal and solar thermal rockets====
[[File:Nuclear thermal rocket en.svg|thumb|250px|Sketch of nuclear thermal rocket]]
In a [[nuclear thermal rocket]] or [[solar thermal rocket]] a working fluid, usually [[hydrogen]], is heated to a high temperature, and then expands through a [[nozzle|rocket nozzle]] to create [[thrust]]. The energy replaces the chemical energy of the reactive chemicals in a traditional [[rocket engine]]. Due to the low [[molecular mass]] and hence high thermal velocity of hydrogen these engines are at least twice as fuel efficient as chemical engines, even after including the weight of the reactor.{{Citation needed|date=April 2007}}

The US [[United States Atomic Energy Commission|Atomic Energy Commission]] and NASA tested a few designs from 1959 to 1968. The NASA designs were conceived as replacements for the upper stages of the [[Saturn V]] launch vehicle, but the tests revealed reliability problems, mainly caused by the vibration and heating involved in running the engines at such high thrust levels. Political and environmental considerations make it unlikely such an engine will be used in the foreseeable future, since nuclear thermal rockets would be most useful at or near the Earth's surface and the consequences of a malfunction could be disastrous. Fission-based thermal rocket concepts produce lower exhaust velocities than the electric and plasma concepts described below, and are therefore less attractive solutions. For applications requiring high thrust-to-weight ratio, such as planetary escape, nuclear thermal is potentially more attractive.<ref>{{cite web |url=https://x-energy.com/why/nuclear-and-space/nuclear-thermal-propulsion |title=Nuclear Thermal Propulsion |author=<!--Not stated--> |website=X-Energy |access-date=2024-02-07 |quote=One of the main benefits of nuclear thermal propulsion is its efficiency. A nuclear thermal rocket can achieve more than twice the efficiency compared to a conventional chemical rocket because it's propellant is brought to a far higher temperature than can be achieved in a conventional combustion chamber. |archive-date=2024-02-07 |archive-url=https://web.archive.org/web/20240207175946/https://x-energy.com/why/nuclear-and-space/nuclear-thermal-propulsion |url-status=live }}</ref>

====Electric propulsion====
[[File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|A xenon ion engine being tested at [[NASA|NASA's]] [[Jet Propulsion Laboratory]], 1999]]
[[Spacecraft electric propulsion|Electric propulsion]] systems use an external source such as a [[nuclear reactor]] or [[solar cell]]s to generate [[electricity]], which is then used to accelerate a chemically inert propellant to speeds far higher than achieved in a chemical rocket. Such drives produce feeble thrust, and are therefore unsuitable for quick maneuvers or for launching from the surface of a planet. But they are so economical in their use of
[[working mass]] that they can keep firing continuously for days or weeks, while chemical rockets use up reaction mass so quickly that they can only fire for seconds or minutes. Even a trip to the Moon is long enough for an electric propulsion system to outrun a chemical rocket – the [[Apollo program|Apollo]] missions took 3 days in each direction.

NASA's [[Deep Space 1|Deep Space One]] was a very successful test of a prototype [[ion drive]], which fired for a total of 678 days and enabled the probe to run down Comet Borrelly, a feat which would have been impossible for a chemical rocket. ''[[Dawn (spacecraft)|Dawn]]'', the first NASA operational (i.e., non-technology demonstration) mission to use an ion drive for its primary propulsion, successfully orbited the large [[main-belt asteroid]]s [[1 Ceres]] and [[4 Vesta]].  A more ambitious, nuclear-powered version was intended for a Jupiter mission without human crew, the [[Jupiter Icy Moons Orbiter]] (JIMO), originally planned for launch sometime in the next decade. Due to a shift in priorities at NASA that favored human crewed space missions, the project lost funding in 2005.  A similar mission is currently under discussion as the US component of a joint NASA/ESA program for the exploration of [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]].

A NASA multi-center Technology Applications Assessment Team led from the [[Johnson Spaceflight Center]], has as of January 2011 described "Nautilus-X", a concept study for a multi-mission space exploration vehicle useful for missions beyond [[low Earth orbit]] (LEO), of up to 24 months duration for a crew of up to six.<ref>[https://archive.today/20120918055537/http://www.spaceref.com/news/viewsr.html?pid=36068 Nautilus-X] – NASA's Multi-mission Space Exploration Vehicle Concept</ref><ref>{{cite web|url=https://nss.org/wp-content/uploads/NautilusX-Multi-Mission-Space-Exploration-Vehicle.pdf |title=NAUTILUS-X: NASA/JSC Multi-Mission Space Exploration Vehicle|date=January 26, 2011|website=National Space Society|access-date=15 March 2025}}</ref> Although [[Nautilus-X]] is adaptable to a variety of mission-specific propulsion units of various low-thrust, high [[specific impulse]] (I<sub>sp</sub>) designs, nuclear ion-electric drive is shown for illustrative purposes.  It is intended for integration and checkout at the [[International Space Station]] (ISS), and would be suitable for deep-space missions from the ISS to and beyond the Moon, including [[Lagrangian point|Earth/Moon L1]], [[Lagrangian point|Sun/Earth L2]], [[Near-Earth object|near-Earth asteroidal]], and Mars orbital destinations.  It incorporates a reduced-g centrifuge providing artificial gravity for crew health to ameliorate the effects of long-term 0g exposure, and the capability to mitigate the space radiation environment.<ref>[http://moonandback.com/2011/02/21/nasa-team-produces-nautilus-x-a-fascinating-spacecraft/  "NASA Team Produces NAUTILUS-X, A Fascinating Spacecraft"] {{Webarchive|url=https://web.archive.org/web/20130526114516/http://moonandback.com/2011/02/21/nasa-team-produces-nautilus-x-a-fascinating-spacecraft/ |date=2013-05-26 }} February 21, 2011</ref>

====Fission powered rockets====
The electric propulsion missions already flown, or currently scheduled, have used [[solar electric]] power, limiting their capability to operate far from the Sun, and also limiting their peak acceleration due to the mass of the electric power source.  Nuclear-electric or plasma engines, operating for long periods at low thrust and powered by fission reactors, can reach speeds much greater than chemically powered vehicles.

====Fusion rockets====
[[Fusion rocket]]s,  powered by [[nuclear fusion]] reactions, would "burn" such light element fuels as deuterium, tritium, or <sup>3</sup>He.  Because fusion yields about 1% of the mass of the nuclear fuel as released energy, it is energetically more favorable than fission, which releases only about 0.1% of the fuel's mass-energy. However, either fission or fusion technologies can in principle achieve velocities far higher than needed for Solar System exploration, and fusion energy still awaits practical demonstration on Earth.

One proposal using a fusion rocket was [[Project Daedalus]].  Another fairly detailed vehicle system, designed and optimized for crewed Solar System exploration, "Discovery II",<ref>[https://web.archive.org/web/20110610051632/http://gltrs.grc.nasa.gov/reports/2005/TM-2005-213559.pdf PDF] C. R. Williams et al., 'Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion', 2001, 52 pages, NASA Glenn Research Center</ref> based on the D<sup>3</sup>He reaction but using hydrogen as reaction mass, has been described by a team from NASA's [[Glenn Research Center]].  It achieves characteristic velocities of >300&nbsp;km/s with an acceleration of ~1.7•10<sup>−3</sup> ''g'', with a ship initial mass of ~1700 metric tons, and payload fraction above 10%.

Fusion rockets are considered to be a likely source of interplanetary transport for a [[planetary civilization]].<ref>{{Cite web|title=The Physics of Interstellar Travel : Official Website of Dr. Michio Kaku|url=https://mkaku.org/home/articles/the-physics-of-interstellar-travel/|access-date=2021-09-27|archive-date=2019-07-08|archive-url=https://web.archive.org/web/20190708003829/http://mkaku.org/home/?page_id=250|url-status=live}}</ref>

====Exotic propulsion====
See the [[spacecraft propulsion]] article for a discussion of a number of other technologies that could, in the medium to longer term, be the basis of interplanetary missions. Unlike the situation with [[interstellar travel]], the barriers to fast interplanetary travel involve engineering and economics rather than any basic physics.

===Solar sails===
{{main|Solar sail}}
[[File:Solarsail msfc.jpg|left|thumb|NASA illustration of a solar-sail propelled spacecraft]]
Solar sails rely on the fact that light reflected from a surface exerts pressure on the surface. The [[radiation pressure]] is small and decreases by the square of the distance from the Sun, but unlike rockets, solar sails require no fuel. Although the thrust is small, it continues as long as the Sun shines and the sail is deployed.<ref>{{cite web | url=http://www.ugcs.caltech.edu/~diedrich/cgi/search.cgi?solar+sail | title=Abstracts of NASA articles on solar sails | url-status=dead | archive-url=https://web.archive.org/web/20080311000832/http://www.ugcs.caltech.edu/~diedrich/cgi/search.cgi?solar+sail | archive-date=2008-03-11 }}</ref>

The original concept relied only on radiation from the Sun – for example in [[Arthur C. Clarke]]'s 1965 story "[[Sunjammer]]". More recent light sail designs propose to boost the thrust by aiming ground-based [[laser]]s or [[maser]]s at the sail. Ground-based [[laser]]s or [[maser]]s can also help a light-sail spacecraft to ''decelerate'': the sail splits into an outer and inner section, the outer section is pushed forward and its shape is changed mechanically to focus reflected radiation on the inner portion, and the radiation focused on the inner section acts as a brake.

Although most articles about light sails focus on [[interstellar travel]], there have been several proposals for their use within the Solar System.

Currently, the only spacecraft to use a solar sail as the main method of propulsion is [[IKAROS]] which was launched by [[JAXA]] on May 21, 2010. It has since been successfully deployed, and shown to be producing acceleration as expected.  Many ordinary spacecraft and satellites also use solar collectors, temperature-control panels and Sun shades as light sails, to make minor corrections to their attitude and orbit without using fuel. A few have even had small purpose-built solar sails for this use (for example Eurostar E3000 [[geostationary]] communications satellites built by [[EADS Astrium]]).

===Cyclers===
It is possible to put stations or spacecraft on orbits that cycle between different planets, for example a [[Mars cycler]] would synchronously cycle between Mars and Earth, with very little propellant usage to maintain the trajectory. Cyclers are conceptually a good idea, because massive radiation shields, life support and other equipment only need to be put onto the cycler trajectory once. A cycler could combine several roles: habitat (for example it could spin to produce an "artificial gravity" effect), or a mothership (providing life support for the crews of smaller spacecraft which hitch a ride on it).<ref>{{cite magazine | url=http://www.popularmechanics.com/science/air_space/2076326.html?page=1 | title=Buzz Aldrin's Roadmap To Mars | date=2005 | last1=Aldrin | first1=B | last2=Noland | first2=D | magazine=Popular Mechanics | url-status=dead | archive-url=https://web.archive.org/web/20061211195430/http://www.popularmechanics.com/science/air_space/2076326.html?page=1 | archive-date=2006-12-11 }}</ref> Cyclers could also possibly make excellent cargo ships for resupply of a colony.

===Space elevator===
{{main|Space elevator}}
A space elevator is a theoretical structure that would transport material from a planet's surface into orbit.<ref>{{cite web|url=http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html |title=The Space Elevator Comes Closer to Reality |last=David |first=D |publisher=space.com |date=2002 |url-status=dead |archive-url=https://web.archive.org/web/20101104104658/http://www.space.com/businesstechnology/technology/space_elevator_020327-1.html |archive-date=2010-11-04 }}</ref> The idea is that, once the expensive job of building the elevator is complete, an indefinite number of loads can be transported into orbit at minimal cost. Even the simplest designs avoid the [[vicious circle]] of rocket launches from the surface, wherein the fuel needed to travel the last 10% of the distance into orbit must be lifted all the way from the surface, requiring even more fuel, and so on. More sophisticated space elevator designs reduce the energy cost per trip by using [[counterweight]]s, and the most ambitious schemes aim to balance loads going up and down and thus make the energy cost close to zero. Space elevators have also sometimes been referred to as "[[Space elevator|beanstalks]]", "space bridges", "space lifts", "space ladders" and "orbital towers".<ref>{{Cite journal|last=Edwards|first=Bradley C.|date=2004|title=A Space Elevator Based Exploration Strategy|journal=AIP Conference Proceedings|volume=699|pages=854–862|doi=10.1063/1.1649650|bibcode=2004AIPC..699..854E}}</ref>

A terrestrial space elevator is beyond our current technology, although a [[lunar space elevator]] could theoretically be built using existing materials.

===Skyhook===
{{main|Skyhook (structure)}}
[[File:Mature non-rotating Skyhook.png|thumb|[https://skyhooksandspaceelevators.wordpress.com Non-rotating skyhook] first proposed by E. Sarmont in 1990]]
A skyhook is a theoretical class of orbiting [[tether propulsion]] intended to lift payloads to high altitudes and speeds.<ref>{{cite journal | last1 = Moravec | first1 = H. | year = 1977 | title = A non-synchronous orbital skyhook | journal = Journal of the Astronautical Sciences | volume = 25 | issue = 4| pages = 307–322 | bibcode = 1977JAnSc..25..307M }}</ref><ref>{{cite journal | last1 = Colombo | first1 = G. | last2 = Gaposchkin | first2 = E. M. | last3 = Grossi | first3 = M. D. | last4 = Weiffenbach | first4 = G. C. | year = 1975 | title = The sky-hook: a shuttle-borne tool for low-orbital-altitude research | journal = Meccanica | volume = 10 | issue = 1| pages = 3–20 | doi=10.1007/bf02148280| s2cid = 123134965 }}</ref><ref>M. L. Cosmo and E. C. Lorenzini, Tethers in Space Handbook, NASA Marshall Space Flight Center, Huntsville, Ala, USA, 3rd edition, 1997.</ref><ref>L. Johnson, B. Gilchrist, R. D. Estes, and E. Lorenzini, "Overview of future NASA tether applications," ''Advances in Space Research'', vol. 24, no. 8, pp. 1055–1063, 1999.</ref><ref>E. M. Levin, "Dynamic Analysis of Space Tether Missions", ''American Astronautical Society'', Washington, DC, USA, 2007.</ref> Proposals for skyhooks include designs that employ tethers spinning at hypersonic speed for catching high speed payloads or high altitude aircraft and placing them in orbit.<ref name=hastol>[https://www.tethers.com/papers/HASTOLAIAAPaper.pdf Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System: Interim Study Results] {{webarchive|url=https://web.archive.org/web/20160427114416/http://www.tethers.com/papers/HASTOLAIAAPaper.pdf |date=2016-04-27 }}</ref> In addition, it has been suggested that the rotating skyhook is "not engineeringly feasible using presently available materials".<ref name="Boeing.2000">{{cite conference |first1=Thomas J. |last1=Bogar |first2=Michal E. |last2=Bangham |first3=Robert L. |last3=Forward |first4=Mark J. |last4=Lewis |contribution=Hypersonic Airplane Space Tether Orbital Launch System |title=Research Grant No. 07600-018l Phase I Final Report |publisher=NASA Institute for Advanced Concepts |date=7 January 2000 |contribution-url=http://www.niac.usra.edu/files/studies/final_report/355Bogar.pdf |access-date=2014-03-20 |archive-date=2013-08-21 |archive-url=https://web.archive.org/web/20130821132225/http://www.niac.usra.edu/files/studies/final_report/355Bogar.pdf |url-status=live }}</ref><ref name="io9.why">{{cite web |last=Dvorsky |first=G. |title=Why we'll probably never build a space elevator |work=io9.com |date=13 February 2013 |url=http://io9.com/5984371/why-well-probably-never-build-a-space-elevator |access-date=13 August 2014 |archive-date=10 August 2014 |archive-url=https://web.archive.org/web/20140810232702/http://io9.com/5984371/why-well-probably-never-build-a-space-elevator |url-status=live }}</ref><ref name="pop.15185070">{{Cite journal |last=Feltman |first=R. |title=Why Don't We Have Space Elevators? |journal=Popular Mechanics |date=7 March 2013 |url=http://www.popularmechanics.com/science/space/nasa/why-dont-we-have-space-elevators-15185070 |access-date=13 August 2014 |archive-date=6 August 2014 |archive-url=https://web.archive.org/web/20140806075236/http://www.popularmechanics.com/science/space/nasa/why-dont-we-have-space-elevators-15185070 |url-status=live }}</ref><ref name="Huff.3353697">{{cite web |first=Jillian |last=Scharr |title=Space Elevators On Hold At Least Until Stronger Materials Are Available, Experts Say |work=Huffington Post |date=29 May 2013 |url=http://www.huffingtonpost.com/2013/05/29/space-elevators-stronger-materials_n_3353697.html |access-date=13 August 2014 |archive-date=2 March 2014 |archive-url=https://web.archive.org/web/20140302234815/http://www.huffingtonpost.com/2013/05/29/space-elevators-stronger-materials_n_3353697.html |url-status=live }}</ref><ref name="extreme.176625">{{cite news |last=Templeton |first=Graham |url=http://www.extremetech.com/extreme/176625-60000-miles-up-geostationary-space-elevator-could-be-built-by-2035-says-new-study |title=60,000 miles up: Space elevator could be built by 2035, says new study |work=Extreme Tech |date=6 March 2014 |access-date=2014-04-19 |archive-date=2014-04-12 |archive-url=https://web.archive.org/web/20140412055111/http://www.extremetech.com/extreme/176625-60000-miles-up-geostationary-space-elevator-could-be-built-by-2035-says-new-study |url-status=live }}</ref>

===Launch vehicle and spacecraft reusability===
The [[SpaceX Starship]] is designed to be fully and rapidly reusable, making use of the [[SpaceX reusable rocket|SpaceX reusable technology]] that was developed during 2011–2018 for [[Falcon 9]] and [[Falcon Heavy]] launch vehicles.<ref name=nsf20160927a>{{cite news |last=Bergin |first=Chris |url=https://www.nasaspaceflight.com/2016/09/spacex-reveals-mars-game-changer-colonization-plan/ |title=SpaceX reveals ITS Mars game changer via colonization plan |work=[[NASASpaceFlight.com]] |date=2016-09-27 |access-date=2016-09-27 |archive-date=2019-07-13 |archive-url=https://web.archive.org/web/20190713031720/https://www.nasaspaceflight.com/2016/09/spacex-reveals-mars-game-changer-colonization-plan/ |url-status=live }}</ref><ref name=nsf20140307>{{cite news |last=Belluscio |first=Alejandro G. |title=SpaceX advances drive for Mars rocket via Raptor power |url=http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ |access-date=2014-03-07 |newspaper=NASAspaceflight.com |date=2014-03-07 |archive-date=2015-09-11 |archive-url=https://web.archive.org/web/20150911235533/http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ |url-status=live }}</ref>

SpaceX CEO [[Elon Musk]] estimates that the reusability capability alone, on both the launch vehicle and the spacecraft associated with the Starship will reduce overall system costs per tonne delivered to Mars by at least two [[orders of magnitude]] over what NASA had previously achieved.<ref name="spacex-itsvideo201609-09:20">
{{cite AV media |people=Elon Musk |date=27 September 2016 |title=Making Humans a Multiplanetary Species |medium=video |url=https://www.youtube.com/watch?v=H7Uyfqi_TE8 | archive-url=https://ghostarchive.org/varchive/youtube/20211211/H7Uyfqi_TE8| archive-date=2021-12-11 | url-status=live|access-date=10 October 2016 |time=9:20–10:10 |location=Guadalajara, Mexico |publisher=SpaceX |quote=''So it is a bit tricky. Because we have to figure out how to improve the cost of the trips to Mars by five million percent ... translates to an improvement of approximately 4 1/2 orders of magnitude. These are the key elements that are needed in order to achieve a 4 1/2 order of magnitude improvement.  Most of the improvement would come from full reusability—somewhere between 2 and 2 1/2 orders of magnitude—and then the other 2 orders of magnitude would come from refilling in orbit, propellant production on Mars, and choosing the right propellant.'' }}{{cbignore}}</ref><ref name=spacex-itspresentation201609>
{{cite web |url=http://www.spacex.com/sites/spacex/files/mars_presentation.pdf |publisher=[[SpaceX]] |title=Making Humans a Multiplanetary Species |date=2016-09-27 |archive-url=https://web.archive.org/web/20160928040332/http://www.spacex.com/sites/spacex/files/mars_presentation.pdf |archive-date=2016-09-28 |access-date=2016-09-29}}</ref>

===Staging propellants===
When launching interplanetary probes from the surface of Earth, carrying all energy needed for the long-duration mission, payload quantities are necessarily extremely limited, due to the basis mass limitations described theoretically by the [[rocket equation]].  One alternative to transport more mass on interplanetary trajectories is to use up nearly all of the [[upper stage]] propellant on launch, and then refill propellants in Earth orbit before firing the rocket to [[escape velocity]] for a [[heliocentric orbit|heliocentric]] trajectory.  These propellants could be stored on orbit at a [[propellant depot]], or carried to orbit in a [[propellant tanker]] to be directly transferred to the interplanetary spacecraft.  For returning mass to Earth, a related option is to mine raw materials from a solar system celestial object, refine, process, and store the reaction products (propellant) on the Solar System body until such time as a vehicle needs to be loaded for launch.

====On-orbit tanker transfers====
As of 2019, SpaceX is developing a system in which a reusable first stage vehicle would transport a crewed interplanetary spacecraft to Earth orbit, detach, return to its launch pad where a tanker spacecraft would be mounted atop it, then both fueled, then launched again to rendezvous with the waiting crewed spacecraft. The tanker would then transfer its fuel to the human crewed spacecraft for use on its interplanetary voyage.  The [[SpaceX Starship]] is a [[stainless steel]]-structure spacecraft propelled by six [[Raptor (rocket engine family)|Raptor engines]] operating on [[subcooling|densified]] methane/oxygen propellants.  It is {{convert|55|m|ft|sp=us|abbr=on|adj=on}}-long, {{convert|9|m|ft|sp=us|abbr=on|adj=on}}-diameter at its widest point, and is capable of transporting up to {{convert|100|tonne|lbs}} of cargo and passengers per trip to Mars, with on-orbit propellant refill before the interplanetary part of the journey.<ref name=spacex-itspresentation201609/><ref name=nsf20160927a/><ref name=ars20160918>{{cite news |last=Berger |first=Eric |url=https://arstechnica.com/science/2016/09/spacexs-interplanetary-transport-system-will-go-well-beyond-mars/ |title=Elon Musk scales up his ambitions, considering going "well beyond" Mars |work=[[Ars Technica]] |date=2016-09-18 |access-date=2016-09-19 |archive-date=2016-09-20 |archive-url=https://web.archive.org/web/20160920000810/http://arstechnica.com/science/2016/09/spacexs-interplanetary-transport-system-will-go-well-beyond-mars/ |url-status=live }}</ref>

====Propellant plant on a celestial body====
As an example of a funded project currently{{when|date=October 2019}} under development, a key part of the [[system]] SpaceX has designed for [[Mars]] in order to radically decrease the cost of spaceflight to interplanetary destinations is the placement and operation of a [[physical plant]] on Mars to handle production and storage of the propellant components necessary to launch and fly the Starships back to Earth, or perhaps to increase the mass that can be transported onward to destinations in the [[outer Solar System]].<ref name=spacex-itspresentation201609/>

The first Starship to Mars will carry a small propellant plant as a part of its cargo load.  The plant will be expanded over multiple [[synodic period|synods]] as more equipment arrives, is installed, and placed into mostly-[[autonomous robot|autonomous production]].<ref name=spacex-itspresentation201609/>

The [[SpaceX Mars propellant plant|SpaceX propellant plant]] will take advantage of the large supplies of [[carbon dioxide]] and [[Water on Mars|water resources]] on Mars, mining the water (H<sub>2</sub>O) from subsurface [[ice]] and collecting CO<sub>2</sub> from the [[Atmosphere of Mars|atmosphere]].  A [[chemical plant]] will process the raw materials by means of [[electrolysis]] and the [[Sabatier reaction|Sabatier process]] to produce [[oxygen]] (O<sub>2</sub>) and [[methane]] (CH<sub>4</sub>), and then [[Vacuum distillation|liquefy]] it to facilitate long-term storage and ultimate use.<ref name=spacex-itspresentation201609/>

===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.