When Is the Best Time to Launch a Rocket to the Moon?
There is virtually no “best time” to launch a rocket to the moon, because, in terms of space travel, the journey is so short. The average distance is 238,000 miles, with a maximum variation of only 25,000 miles. A space shot, whether to the moon or some other object, is made by a rocket already in orbit.
To take a rocket out of orbit and put it on course for the moon calls for a boost in speed to 25,000 miles (40,000 kilometers) an hour. This is necessary to overcome the pull of the earth’s gravity. The moment when the engines are re-fired to start the rocket on its journey does not depend on the position of the launching base in relation to the rocket or the moon.
In contrast, the great variations in the distances between the earth and Mars or Venus make the launching of a rocket to the planets impractical for periods of nine to eighteen months.
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 this time period, it has to wait for the next window.
For trips into largely arbitrary Earth orbits, no specific launch time is required. But if the spacecraft intends to rendezvous with an object already in orbit, the launch must be carefully timed to occur around the times that the target vehicle’s orbital plane intersects the launch site.
Earth observation satellites are often launched into sun-synchronous orbits which are near-polar. For these orbits, the launch window occurs at the time of day when the launch site location is aligned with the plane of the required orbit. To launch at another time would require an orbital plane change maneuver which would require a large amount of propellant.
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. 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, (780 days).
In more complex cases, including the use of gravitational slingshots, 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 ESA’s 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 windows are often calculated from porkchop plots, which show the delta-v needed to achieve the mission plotted against the launch time.