The GRAIL trajectory to the moon has been an interesting one. As Chuck mentioned, the object of this trajectory was reduced fuel requirements, plus it gave them lots of time to make sure everything was on track. The trajectory used an Earth - Sun Lagrange point (EL1) to perform a bit of a bank shot or boomerang maneuver back to the moon. It was launched toward the sun at about escape velocity - as slowly as one can go to get to the Lagrange point, which is about 4 times FURTHER than the moon. The probes came to a very slow (Earth relative) crawl at this point where the gravity of the sun and Earth roughly balance, then a minor maneuver and the combined gravity of the sun, Earth, and moon slowly pulled them back toward Earth on a trajectory to encounter the moon this weekend. A nice graphic is available at http://www.universetoday.com/wp-content/uploads/2011/12/613150main_Lehman-3-... They approached the moon at only ~1200 km/hour, as compared to the blistering ~40000 km/hour for the Apollo missions. This weekend's 40 minute "braking" burns only reduced the speed of the aircraft by about 680km/hour (they reached this speed just a couple seconds after liftoff). So, the probe actually used MORE fuel during launch than was required to get it to the moon, but it required very little fuel to enter lunar orbit. To put into perspective how incredibly long it's taken these probes to get to the moon, if you had jumped in your car on launch day and started driving only 90 miles/hour toward the moon, you'd have arrived just before the spacecraft. Jared On Sun, Jan 1, 2012 at 4:17 PM, <baxman2@q.com> wrote:
The recent NASA lunar probes took three months to reach the Moon from the Earth. I understand escape velocity to be 7 miles per second, and it took the Apollo astronauts three days to get to the Moon. If the NASA probes used the same escape velocity as the Apollo Moon Missions; why did it take three months for the NASA probes to get there, and only three days for the Apollo spacecraft to get there?