How well does this technique work for altering trajectory to achieve a polar orbit, either around the Moon or the Sun? Seems that with such a low relative velocity at a/the Lagrange point it would be easier to alter the trajectory to do just that. Gotta go - I'll look for answers tomorrow. Kim -----Original Message----- From: utah-astronomy-bounces+kimharch=cut.net@mailman.xmission.com [mailto:utah-astronomy-bounces+kimharch=cut.net@mailman.xmission.com] On Behalf Of Jared Smith Sent: Sunday, January 01, 2012 6:09 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Time it takes to get to the Moon 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- 673.jpg 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