 This extended and circuitous trajectory enabled the mission to reduce fuel requirements, protect instruments and reduce the velocity of the two spacecraft at lunar arrival to help achieve the extremely low orbits with separation between the spacecraft (arriving 25 hours apart) of .The very tight tolerances in the flight plan left little room for error correction leading to a launch window lasting one second and providing only two launch opportunities per day.The primary science phase of GRAIL lasted for 88 days, from 7 March 2012 to 29 May 2012.