CAPE CANAVERAL, Fla. (AP) — Four decades after landing men on the moon, NASA is returning to Earth’s orbiting companion, this time with a set of robotic twins that will measure lunar gravity while chasing one another in circles.

By creating the most precise lunar gravity map ever, scientists hope to figure out what’s beneath the lunar surface, all the way to the core. The orbiting probes also will help pinpoint the best landing sites for future explorers, whether human or mechanical.

Near-identical twins Grail-A and Grail-B — short for Gravity Recovery and Interior Laboratory — are due to blast off Thursday aboard an unmanned rocket.

Although launched together, the two washing machine-size spacecraft will separate an hour into the flight and travel independently to the moon.

It will be a long, roundabout trip — three to four months — because of the small Delta II rocket used to boost the spacecraft. NASA’s Apollo astronauts used the mighty Saturn V rocket, which covered the approximately 240,000 miles to the moon in a mere three days.

NASA’s Grail twins will travel more than 2 million miles to get to the moon under this slower but more economical plan.