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.
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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.
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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.
"It greatly simplifies the mission," GRAIL lead scientist Maria Zuber, with the Massachusetts Institute of Technology, told Discovery News.
The extra time in space also gives the GRAIL spacecraft time to vent gases that could impact their science measurements. To map the moon's gravity, scientists will be making measurements as precise as a couple of tenths of a micron per second. A micron is about the size of a red blood cell.
"We essentially measure the distance between two points, but we have to do it very, very well," Zuber said.
Once the moon's gravity fields are mapped, the data will be compared with topographical features, such as mountains and impact basins. The goal of the mission is to figure out the composition of the moon from crust to core and use that information to reconstruct the evolution of the moon.
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Understanding how the moon formed provides a window into what terrestrial planets, including Earth, were like shortly after they formed, Zuber added.
"We can infer something about where we came from," she said.
GRAIL is closely based on a similar Earth-gravity mapping mission called Grace, which also made use of a Lagrange Point.
"The word is getting out about it," Zuber said, referring to Lagrange Point navigation and staging. "It's good place to study the sun, and it's pretty good place for astrophysical missions that want to minimize interference from Earth."
The GRAIL satellites were hoisted on top of a Delta 2 rocket on Thursday in preparation for a Sept. 8 launch from Cape Canaveral, Fla.