A day after a NASA spacecraft safely entered orbit around the moon, its twin was poised to do the same, AP reported. The duo are on a mission to study what's inside the moon by mapping its uneven gravity field. On New Year's Eve, a probe named Grail-A fired its engine and was captured into lunar orbit. NASA hopes for a similar flawless performance from Grail-B on New Year's Day. Once in orbit, the near-identical probes will spend two months tweaking their positions until they are circling 34 miles (55 kilometers) above the surface. They will begin collecting data in March.