The GRAIL spacecraft are now orbiting the moon -- GRAIL-A arrived on Saturday, GRAIL -B arrived on New Years Day.
Both of the Gravity Recovery and Interior Laboratory (GRAIL) probes reached lunar orbit as planned.
The mission will measure the moon's gravitational field, ultimately helping NASA understand how planets form.
GRAIL may also answer the question as to whether a second, smaller moon used to orbit our planet.
The second of two NASA lunar probes on a mission to study the moon's inner core went into orbit as planned, the US space agency said.
The second Gravity Recovery and Interior Laboratory (GRAIL-B) will join GRAIL-A which reached lunar orbit on Saturday, according to officials at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Scientists hope the two probes will allow them to better understand the origins of the planets.
"NASA greets the new year with a new mission of exploration," NASA Administrator Charles Bolden said in a statement.
"The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet," he said.
"We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown."
The $500-million pair of washing machine-sized satellites were launched on Sept. 10 on a mission to map the moon's inner core for the first time.
The spacecraft are in a near-polar elliptical orbit, traveling around the moon every 11.5 hours, NASA said. In the coming weeks, that orbit time will be reduced to just under two hours.
Beginning in March, the two unmanned spacecraft will send radio signals that allow 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.
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.