The method GRAIL will employ to map the Moon's gravity is modeled after the method the GRACE mission has been using since 2002 to map Earth's gravity: carefully measure changes in the separation distance of two spacecraft in the same orbit, caused by variations in the gravitational field. However, GRACE's strategy for placing the two spacecraft into the same carefully designed orbit was not transferable to GRAIL.
GRACE kept the two spacecraft coupled together as the launch vehicle inserted them into their gravity-mapping orbit, and then allowed them to separate. But that procedure would have increased the cost of the GRAIL mission beyond acceptable levels, so mission designers conceived a challenging alternative. Once launched into space, the two GRAIL spacecraft will take similar but separate routes to the Moon and enter lunar orbit independently. Over the course of about two months, through a series of about 20 maneuvers, their orbits will be made to converge as they are reshaped into their ultimate gravity-mapping orbit. This is the first time such a procedure has ever been attempted.
The two GRAIL spacecraft will be launched side-by-side atop a single Delta II vehicle sometime between September 8 and October 19, 2011. NASA's Deep Space Network (DSN) complex at Goldstone, California will track the spacecraft during their initial activities in space, including deployment of the solar arrays.
The two spacecraft will take a long, counterintuitive but energy-saving route to the Moon by way of a point in space of relative gravitational stability called "Sun-Earth Lagrange point 1" (EL1). Regardless of launch date, their trajectories will be manipulated so that GRAIL-A will arrive at the Moon on December 31, 2011 and GRAIL-B will arrive about 25 hours later, on January 1, 2012. (All dates are in the Pacific time zone.)
Why will they take three to four months to make a trip that the Apollo astronauts made in three days? The extra travel time has a number of benefits for this mission.
After the two spacecraft are captured into lunar orbit, they'll spend about two months reshaping their orbits from highly elliptical, taking 11.5 hours per orbit, to nearly circular with an orbital period of less than 2 hours. Both spacecraft will ultimately trace the same orbital path, with one spacecraft following the other in a route that takes them nearly over the Moon's poles as the Moon slowly rotates beneath them.
The science phase, when the gravity-mapping operation will take place, will last 82 days beginning March 8, 2012.
That's the maximum amount of time that the position of the Sun relative to the two spacecraft in orbit about the Moon will permit them to keep their Ka-band antennas pointed at each other (to measure the distance between them) while simultaneously keeping their solar panels facing the Sun.
The GRAIL scientists want the spacecraft to vary from close together at a low altitude to farther apart at a higher altitude and back again. GRAIL's mission designers have arranged for nature to do the work of varying the orbits.
The initial conditions of the science-phase orbit were selected so that the natural perturbations of the lunar gravity field will cause the orbit altitude (at its lowest point) to increase from approximately 15 km at the start of the science phase to over 50 km and then decrease back down again.
During the science phase, two small maneuvers will be used to adjust the separation distance between the two orbiters. The first occurs at the beginning of the science phase and the second is near the end of the first mapping cycle (the first of three times when the Moon will have made one complete rotation beneath the orbiting spacecraft). During this period, the separation distance will change from about 85 km apart to about 225 km apart, and then back down again to about 65 km.