NASA ARTEMIS Spacecraft Prepares for Lunar Orbit

They've almost arrived. It took one and a half years, over 90 orbit maneuvers, and - wonderfully - many gravitational boosts and only the barest bit of fuel to move two spacecraft from their orbit around Earth to their new home around the moon.

Along their travels, the spacecraft have been through orbits never before attempted and made lovely curlicue leaps from one orbit to the next. This summer, the two ARTEMIS spacecraft - which began their lives as part of the five-craft THEMIS mission studying Earth's aurora - will begin to orbit the moon instead. THEMIS is an acronym for the Time History of Events and Macroscale Interaction during Substorms spacecraft.

The view from above of the ARTEMIS orbits as they make the transition from the kidney-shaped Lissajous orbits on either side of the moon to orbiting around the moon. 

Credit: NASA/Goddard Space Flight Center

Even with NASA's decades of orbital mechanics experience, this journey was no easy feat.

The trip required several maneuvers never before attempted, including several months when each craft moved in a kidney-shaped path on each side of the moon around, well, nothing but a gravitational point in space marked by no physical planet or object.

"No one has ever tried this orbit before, it's an Earth-moon libration orbit," says David Folta a flight dynamics engineer at NASA's Goddard Space Flight Center in Greenbelt, Md. "It's a very unstable orbit that requires daily attention and constant adjustments."

The journey for ARTEMIS - short for Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun - began in 2009, after THEMIS had completed some two years of science data collection on the magnetic environment around Earth, the aurora, and how these are affected by the sun.

The spacecraft are solar-powered, but orbits for the two outermost THEMIS spacecraft had slipped over time and were going to be subjected to regular eight-hour periods of darkness. These spacecraft could withstand up to three hours without sunlight, but this much darkness would soon leave the batteries completely discharged.