Washington: Aimed at unlocking the mysteries of the Moon, the first of the two NASA aircraft entered the lunar orbit, the American space agency has said.

NASA's Gravity Recovery And Interior Laboratory (GRAIL)-A spacecraft successfully completed its planned main engine burn and began orbiting the Moon on Friday.

A signal emitted by the probe indicated that it was functioning properly and in place, according to officials at NASA's Jet Propulsion Laboratory in Pasadena, California.

The spacecraft is in a 56-mile (90-kilometer) by 5,197-mile (8,363-kilometer) orbit around the moon that takes approximately 11.5 hours to complete.

GRAIL-A's mirror twin, GRAIL-B is slated to enter the lunar orbit on Saturday.

NASA said once both spacecraft are confirmed in orbit and operating, science work will begin in March.

The spacecraft will transmit radio signals precisely defining the distance between them as they orbit the moon in formation.

As they fly over areas of greater and lesser gravity caused by both visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly, it said.

NASA said scientists will translate this information into a high-resolution map of the moon's gravitational field.

"The data will allow scientists to understand what goes on below the lunar surface," it said.

"This information will increase knowledge of how Earth and its rocky neighbours in the inner solar system developed into the diverse worlds we see today," NASA said.