WASHINGTON  - The first of two NASA lunar probes on a mission to study the Moon’s inner core so as to better understand the origins of planets went into orbit Saturday, the US space agency said.

The first Gravity Recovery and Interior Laboratory (GRAIL-A) began orbiting the Moon at about 2201 GMT, according to officials at NASA’s Jet Propulsion Laboratory in Pasadena, California. A signal emitted by the probe indicated that it was functioning properly and in place.

GRAIL-B is expected to follow suit on Sunday at around 2205 GMT. “Pop the bubbly & toast the moon! @NASA’s #GRAIL-A spacecraft is in lunar orbit,” the space agency posted on Twitter.

“With GRAIL-A in lunar orbit, we are halfway home,” GRAIL project manager David Lehman said in a statement. GRAIL head researcher Maria Zuber, of the Massachusetts Institute of Technology, had said in a previous statement that the mission “will rewrite the textbooks on the evolution of the Moon.”

The $500-million pair of washing machine-sized satellites were launched on September 10 on a mission to map the Moon’s inner core for the first time.

Beginning in March 2012, the two unmanned spacecraft will send radio signals that allow earth-based scientists to create a high-resolution map.

 of the Moon’s gravitational field, helping them to better understand its sub-surface features and the origins of other bodies in the solar system.

The mission should shed light on the unexplored far side of the Moon and test a hypothesis that there was once a second Moon that fused with ours.

The two spacecraft have taken three months to reach the Moon as opposed to the usual three-day journey taken by the manned Apollo missions. The longer journey allowed scientists to better test the two probes.

The two spacecraft have covered more than 2.5 million miles (four million kilometers) since they were launched in September, according to NASA.

At 2300 GMT, GRAIL-B was 30,018 miles (48,309 kilometers) from the moon and closing at a rate of 896 miles (1,442 kilometers) per hour, NASA said.

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 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.