Editing Launch window (section)

==Launch period==

To go to another planet using the simple low-energy [[Hohmann transfer orbit]], if [[Orbital eccentricity|eccentricity]] of orbits is not a factor, launch periods are periodic according to the [[synodic period]]; for example, in the case of Mars, the period is 780 days (2.1 years). In more complex cases, including the use of [[gravitational slingshot]]s, launch periods are irregular. Sometimes rare opportunities arise, such as when ''[[Voyager 2]]'' took advantage of [[Planetary Grand Tour|a planetary alignment]] occurring once in 175 years to visit [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]].  When such an opportunity is missed, another target may be selected. For instance, [[European Space Agency|ESA]]'s ''[[Rosetta (spacecraft)|Rosetta]]'' mission 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]]).

Launch periods are often{{cn|reason=you need multiple reliable sources to say often|date=June 2020}} calculated from [[porkchop plot]]s, which show the [[delta-v]] needed to achieve the mission plotted against the launch time.<ref>{{cite web|title="Porkchop" is the First Menu Item on a Trip to Mars|url=https://mars.jpl.nasa.gov/spotlight/porkchopAll.html|website=NASA's Mars Exploration Program|publisher=NASA JPL|access-date=3 May 2018}}</ref>