The twin spacecraft, known as the Gravity Recovery And Interior Laboratory, or GRAIL-A and GRAIL-B, will explore the moon from crust to core in unprecedented detail, NASA said.
"NASA greets the new year with a new mission of exploration," NASA Administrator Charles Bolden said in a news release.
"The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet. We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown."
GRAIL-B reached lunar orbit at 2:43 PST Sunday, while GRAIL-A completed its burn Saturday at 2 p.m. PST.
The spacecraft are to transmit radio signals precisely measuring the distance between them. As they fly over areas of greater and lesser gravity caused by visible features such as mountains and craters, as well as masses hidden beneath the lunar surface, the distance between the spacecraft will change slightly.
The information will be translated by scientists into a high-resolution map of the moon's gravitational field.That will help scientists understand what goes on beneath the surface and increasing knowledge of how Earth and its rocky neighbors in the inner solar system developed into their current forms, NASA said.