Editing Interplanetary spaceflight (section)

===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/>