Editing Interplanetary spaceflight (section)

===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.