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| In the context of [[spaceflight]], a '''launch window''' is a time period during which a particular vehicle ([[rocket]], [[Space Shuttle]], etc.) must be launched in order to reach its intended target. If the rocket is not launched within the "window", it has to wait for the next window.<ref>[http://www.nasa.gov/audience/forstudents/brainbites/nonflash/bb_home_launchwindow.html What is a launch window?]</ref>
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| For trips into largely arbitrary [[Earth orbit]]s, almost any time will do. But if the [[spacecraft]] intends to [[space rendezvous|rendezvous]] with a space station (such as the [[International Space Station]]) or another vehicle already in an orbit, the launch must be carefully timed to occur around the times that the target vehicle's [[orbital plane (astronomy)|orbital plane]] intersects the launch site.
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| For launches above [[low Earth orbit]] (LEO), the actual launch time can be somewhat flexible if a [[parking orbit]] is used, because the inclination and time the spacecraft initially spends in the parking orbit can be varied. See the launch window used by the ''[[Mars Global Surveyor]]'' spacecraft to the planet [[Mars]] at [http://mars.jpl.nasa.gov/mgs/overvu/delta/window/window.html].
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| To go to another planet using the simple low-energy [[Hohmann transfer orbit]], if eccentricity of orbits is not a factor, launch windows are periodic according to the [[synodic period]]; for example, in the case of Mars the period is 2.135 years, i.e. 780 days. In more complex cases, including the use of [[gravitational slingshot]]s, launch windows are irregular. Sometimes rare opportunities arise such as when ''[[Voyager 2]]'' took advantage of a 175 year planetary alignment (launch window) to visit [[Jupiter]], [[Saturn]], [[Uranus]] and [[Neptune]]. When such an opportunity is missed, another target may be selected. For example, the [[Rosetta (spacecraft)|Rosetta]] mission of [[European Space Agency|ESA]] was originally intended for comet [[46P/Wirtanen]], but a launcher problem delayed it and a new target had to be selected (comet [[67P/Churyumov-Gerasimenko]]).
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| Launch windows are often calculated from [[porkchop plot]]s that show the [[delta-v]] needed to achieve the mission, plotted against the launch time.
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| The term "launch window" is often used metaphorically, particularly in [[marketing]], to describe the launch of a product, the "window" describing a key opportunity for the product to be released. For example, a [[film]] may be scheduled for release in the summer – a popular time for films – but if there is a problem in development, it might miss that launch window and make less of a profit.{{citation needed (lead)|date=August 2012}}
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| ==Specific issues==
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| [[Space Shuttle]] missions to the [[International Space Station]] were restricted by [[beta angle]] cutout. Beta angle (<math>\beta</math>) is defined as the angle between the orbit plane and the vector from the Sun.<ref name="knk">{{cite web|url=http://www.tak2000.com/data/planets/earth.htm|title= Earth's Thermal Environment|last=K&K Associates|year=2008|work=Thermal Environments JPL D-8160|publisher=K&K Associates|accessdate=June 20, 2009}}</ref> Due to the relationship between an orbiting object's beta angle (in this case, the ISS) and the percent of its orbit that is spent in sunlight, solar power generation and thermal control are affected by that beta angle.<ref name="mcc">{{cite web|url=http://spaceflight.nasa.gov/feedback/expert/answer/mcc/sts-113/11_23_20_01_179.html|title=MCC Answers|author=Derek Hassman, NASA Flight Director|date=December 1, 2002|publisher=NASA|accessdate=June 20, 2009}}</ref> Shuttle launches to the ISS were normally only attempted when the ISS is in an orbit with a beta angle of less than 60 degrees.<ref name="mcc"/>
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| == See also ==
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| *[[Beta angle]]
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| *[[Collision avoidance (spacecraft)]]
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| *[[Delta-v budget]]
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| *[[Interplanetary travel]]
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| *[[Oberth effect]]
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| {{Wiktionary|window of opportunity}}
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| == References ==
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| {{Reflist}}
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| {{Orbits}}
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| {{DEFAULTSORT:Launch Window}}
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| [[Category:Spaceflight concepts]]
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