NASA on Saturday launched a $500 million pair of washing-machine-sized satellites on a mission to map the Moon's inner core for the first time.
The twin spacecraft took off from Cape Canaveral Air Force Station in Florida on a three-month journey to the Moon at 9:08 am (1308 GMT) aboard a Delta II rocket.
"Liftoff of the Delta II with GRAIL, on a journey to the center of the moon," NASA commentator George Diller said upon blast-off of the GRAIL mission, which stands for Gravity Recovery and Interior Laboratory.
High upper level winds delayed the first launch attempt on Thursday, and also briefly set back Saturday's launch.
The duo will travel to the Moon for more than three months, arriving into a polar lunar orbit one after the other around New Year's Day.
With one spacecraft trailing the other, the plan is for the two to use gravity tools to map the terrain beneath, revealing the contents of the inner core of the Moon, about which little is known.
The mission should also shed light on the unexplored far side of the Moon, and perhaps tell scientists whether there was once a second Moon that fused with ours.
"GRAIL will be the first mission to determine the internal structure of the Moon," program scientist Bobby Fogel told reporters this week.
"We have used gravity science before to try to gain some insight as to what is going on inside the Moon, however these have been very primitive attempts.
"If those previous attempts could be likened to a magnifying glass, GRAIL by contrast would be a high-powered microscope."
Scientists believe that the Moon was formed when a planet-sized object crashed into the Earth, throwing off a load of material that eventually became what we now recognize as our planet's airless, desolate satellite.
How it heated up over time, creating a magma ocean that later crystallized, remains a mystery, despite 109 past missions to study the Moon since 1959, and the fact that 12 humans have walked on its surface.
A recent hypothesis that there may have been two Moons that slowly merged into each other can also be tested with this mission, said principal investigator Maria Zuber.
"If we want to reconstruct the evolution of the Moon over time, we certainly need to reconstruct the temperature structure of the Moon right now," she said.
Little is known for certain about what lies inside the Moon. The widely held belief that there is a small solid iron core surrounded by a liquid iron core is unproven, said Zuber.
"It is actually quite possible that deep inside the Moon the core could be titanium oxide, which is a material that would have fallen out or would have crystallized out of the magma ocean and sunk to the deep interior of the Moon," she said.
Once the GRAIL twins enter the orbit of the Moon, they will line up with each other and "essentially chase each other around in a polar orbit as the Moon rotates slowly underneath them," said Zuber.
They will hover about 34 miles (55 kilometers) above the lunar surface, with the distance between them ranging from 37 to 140 miles (60 to 225 kilometers), collecting measurements of the terrain beneath.
The duo will accomplish the mission's primary aim of understanding the Moon's inner character by performing a series of low-altitude gravity field measurements using what is known as a Ka-band ranging instrument.
The mission itself is relatively short in duration, just 90 days once the two spacecraft reach orbit.
About 40 days after their work is done, the pair will plunge into the lunar surface, NASA said. Scientific analysis of their data is expected to continue for a year.
The project is part of NASA's Discovery program, which has launched 10 spacecraft since 1992 to study the solar system.
Last month, NASA launched its billion-dollar solar-powered spacecraft Juno on a five-year journey to Jupiter aiming to discover what makes up the solar system's biggest planet.
After GRAIL, the US space agency plans to launch its Mars Science Laboratory in November on a nearly two-year journey to the red planet.