A roomful of scientists and engineers monitoring the New Year's Eve maneuver clapped after the Grail-A probe signaled that it was circling the moon.
The celebration was brief. Mission team members looked ahead to New Year's Day when twin Grail-B was scheduled to perform the same move.
The duo will study the moon's gravity to determine what's inside straight down to the core.
The $496 million mission is managed by the NASA Jet Propulsion Laboratory.