Flying to the moon takes about three days, but all that speed comes with a heavy price in fuel. To lighten the load, NASA’s newest lunar probes will take an indirect route to the moon, via a type of celestial way station where the sun’s gravity and Earth’s gravity balances out.
The primary goal of the Gravity Recovery and Interior Laboratory mission, nicknamed GRAIL, is to precisely map the moon’s gravity so scientists can figure out what is inside the moon and how it formed. Along the way, however, GRAIL will demonstrate an alternative type of celestial navigation that takes advantage of one of several naturally occurring locations in space where orbital motion and gravitational forces balance out. These locations are called Lagrange points, named after mathematician Joseph Louis Lagrange, who made their discovery in 1772.
Staged from Lagrange Point 1, or L1, which is located 1.5 million kilometers (932,0570 miles) inside Earth’s orbit, part way between the sun and the Earth, the GRAIL satellites will take three to four months to reach the moon, said Ralph Roncoli, the GRAIL mission designer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. But the satellites will arrive with less relative speed than spacecraft making the trip in three days, which means less fuel will be needed to brake and drop into lunar orbit. Coming from L1 also means the twins can arrive on different days.