Editing Spacecraft propulsion (section)

==Operating domains==
Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel, and interstellar travel.

===Orbital===
{{Main|Orbital mechanics}}
Artificial satellites are first [[Rocket launch|launched]] into the desired altitude by conventional liquid/solid propelled rockets, after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form of [[Spacecraft attitude control|attitude control]] so that they are correctly pointed with respect to the [[Earth]], the [[Sun]], and possibly some [[astronomy|astronomical]] object of interest.<ref>{{cite news |author1=Hess, M. |author2=Martin, K. K. |author3=Rachul, L. J. | title=Thrusters Precisely Guide EO-1 Satellite in Space First | publisher=NASA | date=February 7, 2002 | url=http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20071206154134/http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm <!-- Bot retrieved archive --> |archive-date = 2007-12-06}}</ref> They are also subject to [[Atmospheric drag|drag]] from the thin [[Earth's atmosphere|atmosphere]], so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections ([[orbital stationkeeping|orbital station-keeping]]).<ref>{{cite web  |last=Phillips  |first=Tony  |date=May 30, 2000  |url=https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-url=https://web.archive.org/web/20000619105529/https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-date=June 19, 2000  |title=Solar S'Mores  |publisher=NASA  |access-date=2007-07-30 
}}</ref> Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.<ref>{{cite web | last=Olsen | first=Carrie | date=September 21, 1995 | url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html | title=Hohmann Transfer & Plane Changes | publisher=NASA | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20070715042552/http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html <!-- Bot retrieved archive --> |archive-date = 2007-07-15}}</ref> A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit.<ref>{{Cite web |title=Satellite communication – Orbit, Signals, Relay {{!}} Britannica |url=https://www.britannica.com/technology/satellite-communication/How-satellites-work |access-date=2024-04-20 |website=www.britannica.com |language=en}}</ref>

===Interplanetary===
{{Main|Interplanetary spaceflight}}
For [[interplanetary travel]], a spacecraft can use its engines to leave Earth's orbit. It is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (see [[New Horizons]]). Once it has done so, it must make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.<ref>{{cite web|author=Staff |date=April 24, 2007 |url=http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |title=Interplanetary Cruise |publisher=NASA |work=2001 Mars Odyssey |access-date=2007-07-30 |archive-url=https://web.archive.org/web/20070802071234/http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |archive-date=August 2, 2007 }}</ref> In between these adjustments, the spacecraft typically moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with a [[Hohmann transfer orbit]]: the spacecraft begins in a roughly circular orbit around the Sun. A short period of [[thrust]] in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.<ref>{{cite news | first=Dave | last=Doody | title=Chapter 4. Interplanetary Trajectories | work=Basics of Space Flight | publisher=NASA JPL | date=February 7, 2002 | url=http://www2.jpl.nasa.gov/basics/bsf4-1.html | access-date=2007-07-30 | archive-url=https://web.archive.org/web/20070717143018/http://www2.jpl.nasa.gov/basics/bsf4-1.html | archive-date=July 17, 2007 }}</ref> Special methods such as [[aerobraking]] or aerocapture are sometimes used for this final orbital adjustment.<ref>{{cite conference |last=Hoffman |first=S. |date=August 20–22, 1984 |title=A comparison of aerobraking and aerocapture vehicles for interplanetary missions |url=http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |conference= |location=Seattle, Washington |publisher=American Institute of Aeronautics and Astronautics |pages= |archive-url=https://web.archive.org/web/20070927230504/http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |archive-date=September 27, 2007 |access-date=2007-07-31 |book-title=AIAA and AAS, Astrodynamics Conference}}</ref>

[[File:Ssunsail.jpg|thumb|right|Artist's concept of a solar sail]]
Some spacecraft propulsion methods such as [[solar sail]]s provide very low but inexhaustible thrust;<ref>{{cite web|author=Anonymous |year=2007 |url=http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |title=Basic Facts on Cosmos 1 and Solar Sailing |publisher=The Planetary Society |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20070703052531/http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |archive-date=July 3, 2007 }}</ref> an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun, or constantly thrusting along its direction of motion to increase its distance from the Sun.{{citation needed|date = July 2023}} The concept has been successfully tested by the Japanese [[IKAROS]] solar sail spacecraft.<ref name="d545">{{cite web | last=Malik | first=Tariq | title=Japanese solar sail successfully rides sunlight | website=NBC News | date=2010-07-13 | url=https://www.nbcnews.com/id/wbna38222268 | access-date=2024-09-27}}</ref>

===Interstellar===
{{Main|Interstellar travel}}
Because interstellar distances are great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers.<ref>{{cite web | last=Rahls | first=Chuck | date=December 7, 2005 | url=http://www.physorg.com/news8817.html | title=Interstellar Spaceflight: Is It Possible? | publisher=Physorg.com | access-date=2007-07-31 }}</ref> No spacecraft capable of short duration (compared to human lifetime) [[interstellar travel]] has yet been built, but many hypothetical designs have been discussed.