Moon's Positioin During Interplanetary Spacecraft Launches

[RJ Emery]

When rockets are launched from Cape Canaveral for destinations beyond earth's orbit, I am curious to know the position of the moon at such times.

The moon has a gravitational sphere of influence, and so I would expect launches are scheduled when the moon is opposite the space center to minimize the moon's influence. Is that not the case?

[Chris Peterson]

When rockets are launched from Cape Canaveral for destinations beyond earth's orbit, I am curious to know the position of the moon at such times.

The moon has a gravitational sphere of influence, and so I would expect launches are scheduled when the moon is opposite the space center to minimize the moon's influence. Is that not the case?

The effect of the Moon is small unless the spacecraft passes very close to it. And any perturbation can be compensated for by very small adjustments to the trajectory. I doubt that it is necessary to adjust the launch window to wait for the Moon to move.

[neufer]

When rockets are launched from Cape Canaveral for destinations beyond earth's orbit, I am curious to know the position of the moon at such times. The moon has a gravitational sphere of influence, and so I would expect launches are scheduled when the moon is opposite the space center to minimize the moon's influence. Is that not the case?

The effect of the Moon is small unless the spacecraft passes very close to it. And any perturbation can be compensated for by very small adjustments to the trajectory. I doubt that it is necessary to adjust the launch window to wait for the Moon to move.

<<The STEREO satellites used multiple gravitational assists from the Moon to significantly decrease the amount of fuel needed to put those two satellites into heliocentric orbits. The first flyby resulted in STEREO ahead (STEREO-A) being ejected from the Earth-Moon system with a semi-major axis slightly less than that of the Earth-Moon system. STEREO-A has a period of 346 days.

STEREO behind (STEREO-B) didn't pass as close to the Moon as did STEREO-A. It didn't quite escape. It instead went into a large orbit and later re-encountered the Moon. This second flyby did eject STEREO-B from the Earth-Moon system, but with a slightly larger semi-major axis than that of the Earth-Moon system. STEREO-B has a period of 388 days.

Lunar flybys can be useful if the intent is to stay in the vicinity of the Earth-Moon system. A number of near Earth vehicles have used the Moon's gravity to reduce propellant needs. The WIND and GEOTAIL spacecraft took advantage of the "double lunar swingby".

Another proposed use is to reduce fuel needed to place a vehicle in geostationary orbit. Putting a vehicle into a geostationary orbit is a surprisingly expensive operation, particularly if the launch isn't from an equatorial site. The delta-V using a traditional geostationary transfer orbit followed by a circularization and plane change burn at geosynchronous altitude is greater than escape delta-V. Instead transferring into a highly eccentric orbit that reaches the Moon's altitude at apogee enable using the Moon to affect the plane change and to raise perigee to geostationary. See Ramanan and Adimurthy, Precise Lunar Gravity Assist Transfers to Geostationary Orbits.

One reason the Moon isn't used much for interplanetary missions is because things don't line up properly, and in the rare cases when they do, the opportunities are rather short-lived.>>