WASHINGTON — Twin NASA probes now being readied for a scheduled Sept. 8 launch from Cape Canaveral Air Force Station, Fla., will map the Moon’s interior structure in concert with another pair of spacecraft that recently arrived in lunar orbit after completing their original mission.

The two Gravity Recovery and Interior Laboratory (GRAIL) satellites, built by Lockheed Martin Space Systems of Denver, will reach their destination 90 days after liftoff, bringing to five the number of NASA craft in lunar orbit, said Bill Knopf, NASA’s program executive for GRAIL. The others are two repurposed spacecraft from a mission to study the Earth’s magnetosphere and the Lunar Reconnaissance Orbiter, which launched in 2009.

The GRAIL satellites will fly in tandem orbits separated by a distance of roughly 200 kilometers. Minute changes in the distance between them will indicate changes in the Moon’s gravitational field, said Jim Adams, NASA’s deputy director of planetary science. “The two GRAIL spacecraft have the ability to measure the distance between themselves to within less than the diameter of a red blood cell,” or a few micrometers, Adams said. When a probe “flies over a change in the gravitational field of the Moon, it will speed up a little bit,” widening the gap between the two craft.

Measuring these gaps over the course of GRAIL’s planned 100-day mission will result in “extremely high-resolution maps of the gravity of the Moon … that will enable planetary scientists to develop [models of] internal structures of the Moon,” Adams said.

GRAIL’s measurements will complement those gathered by the ARTEMIS — Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun — spacecraft originally launched as part of the recently completed, five-satellite Themis mission. The ARTEMIS satellites will use Apollo-era techniques to study the Moon’s geology from crust to core.

The first ARTEMIS craft reached lunar orbit June 27; the second arrived July 19. The spacecraft are equipped with identical sets of magnetometers — instruments that can detect magnetic fields — and will be used to make comparative measurements, allowing scientists to “infer changes in the magnetic field that would then infer [the] internal structure” of the Moon, said Adams. “It’s an old technique that will give us some complementary measurements that will go along with the GRAIL mission.”

GRAIL observations will be more detailed than those collected by the Alliant Techsystems-built  ARTEMIS probes, he added.

ARTEMIS is the first NASA mission to orbit at an Earth-Moon Lagrangian point — a spot in space where the gravitational pull of the Earth and the Moon cancel each other out.

GRAIL is the latest in NASA’s Discovery class of planetary missions, which are capped at $450 million in costs and 36 months in development time. Discovery missions are designed by teams of scientists that compete against each other for selection by the space agency.

At $496.2 million, GRAIL’s inflation-adjusted lifecycle cost fits the Discovery mold, said Adams. That cost figure includes GRAIL’s launch aboard a Delta 2 rocket supplied by United Launch Alliance of Denver.

NASA announced in May three finalists for the next Discovery mission, which would launch in 2016. They are: a Mars lander that would study the interior geology of the red planet; a “comet hopper,” which would alight multiple times on a single comet to study the sun’s influence on the icy body; and a spacecraft/floating lander that would explore the hydrocarbon seas of Saturn’s moon Titan. The Discovery mission that preceded GRAIL was Kepler, an observatory launched in 2009 to search for Earth-like planets in the Milky Way.

GRAIL marks another in a line of “lasts” for the Kennedy Space Center on Cape Canaveral: The mission will launch aboard the final Delta 2 medium-class rocket scheduled to lift off from the Florida spaceport. The final space shuttle mission launched July 8 from Kennedy.

The Delta 2, which in recent years has been NASA’s primary workhorse for scientific missions, is no longer in production. United Launch Alliance, a Boeing-Lockheed Martin joint venture, has several unassigned Delta 2 rockets remaining in its inventory but has struggled to find a customer.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.