Nasa's twin spacecraft are now circling the moon on a mission to measure lunar gravity.
The latest probe slipped into orbit on New Year's Day, joining its twin, which arrived a day earlier.
The Grail spacecraft - short for Gravity Recovery and Interior Laboratory - will spend the next two months spiralling closer to the surface.
In March, they will begin mapping the moon's uneven gravity field from orbit to understand its interior down to the core.
The probes were launched in September and took a roundabout trip to the moon.