Story highlights NASA's GRAIL-B space probe slipped into lunar orbit Sunday afternoon

Its companion probe arrived Saturday after a nearly four-month voyage

The second of two new research probes successfully entered the moon's orbit Sunday afternoon, a day after its companion, NASA announced.

The unmanned Gravity Recovery and Interior Laboratory-B (GRAIL-B) fired its main engines at 5:05 p.m. and slipped into lunar orbit 39 minutes later, controllers at the space agency's Jet Propulsion Laboratory reported. Its companion GRAIL-A probe arrived on Saturday.

"Cheers in JPL mission control as everything is looking good for #GRAIL-B," the Pasadena, California, laboratory announced on its Twitter page. "It's going to be a great 2012!"

NASA says the twin orbiters will study the moon "in unprecedented detail" from a near pole-to-pole orbit about 34 miles (55 km) high. For an expected 82 days, their instruments will chart the lunar gravitational field, giving scientists a peek into what goes on beneath the surface -- and possibly an insight into how the Earth and its neighbors formed.

Both probes were launched aboard a single rocket from Florida's Cape Canaveral on September 10.

The Apollo manned missions of the 1960s and 1970s took only three days to cross the 250,000-mile distance from Earth to the moon. But the GRAIL probes took more than three months and covered more than 2.5 million miles "reshaping and merging their orbits" so that they could be better positioned and coordinated to study the moon, NASA said.

Maria Zuber, principal investigator for GRAIL, said the moon remains mysterious in many ways. Scientists theorize that the moon formed when a Mars-sized object crashed into Earth, but it remains unknown why the near side of the moon is so different from the far side.

The basins are flooded with volcanic material on one side, and the other side has mountainous highlands.

"We think the answer is locked in the interior," Zuber said at a NASA news briefing Wednesday.