One of two probes, launched to survey the moon, successfully dropped into orbit.
"Everything went just as we hoped. The burn was spot-on," chief scientist Maria Zuber of the Massachusetts Institute of Technology said in a post-mission interview with The Associated Press.
Mission control at the NASA Jet Propulsion Laboratory erupted in cheers and applause after receiving confirmation that the probe was healthy and circling the moon.
The Grail probes, short for Gravity Recovery and Interior Laboratory, have been cruising independently toward the moon since September. They are the size of washing machines.
The second spacecraft is now being monitored, and is expected to enter lunar orbit Sunday.
Grail is the 110th mission to target the moon since the dawn of the Space Age including the six Apollo moon landings that put 12 astronauts on the surface. Despite the attention the moon has received, scientists don't know everything about Earth's nearest neighbor.
The probes are expected to help researchers get a better understanding of why the moon has its slightly lopsided shape and how it was formed.
The Associated Press contributed to this report.
