The THEMIS (Time History of Events and Macroscale Interactions during Substorms) satellites of NASA were launched in 2007 to investigate the magnetosphere of our planet. The five probes orbited at different distances from the Earth, P1 and P2 being the two farthest. The program was greatly successful, providing new discoveries about the dynamics of the magnetosphere, but by 2009 the future of P1 and P2 looked more and more grim. They passed through he Earth's shadow regularly, relying on their batteries as designed. However, as their orbit gradually changed, they spent more and more time in darkness and it was uncertain how long would they last. But then a solution emerged: the two, fully functioning satellites with ample supply of fuel will be transferred to the Moon.

Original orbits of the five THEMIS satellites. The two farthest were transferred to the Moon.

A new mission also requires a new name: THEMIS turned into ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun). But, more importantly, new science was expected: at the Moon, the two probes will spend most of their time outside the magnetosphere where the solar wind dominates, plus they will cross the Earth's magnetic tail regularly too. Although interplanetary probes measure solar wind as well, such detailed measurements, especially at the Moon, were unprecedented. Meanwhile, the mission of the remaining three THEMIS satellites was extended as well.

Orbital path of ARTEMIS P1. It actually approached the Moon from the outside, entering a Lissajous-orbit around the L₂ point. It then transferred to L₁, and from there it finally entered lunar orbit.

To save as much from the valuable fuel as possible, the duo didn't reached the Moon on the usual, direct course. They even set off towards two specific point in space instead of the Moon. There are five points in the gravitational field of two massive bodies, called Lagrange points where a third (with negligible mass) can stay for an extended time. We use them in the Sun-Earth system, Sun-observing spacecrafts are around the L₁ point while various telescopes like Herschel and Planck orbit the L₂ point. But Lagrange points of the Earth-Moon system were never explored yet.

The ARTEMIS pair started their long journey to the L₁ and L₂ points in mid-2009. Though the points aren't far, the trip took a long time because instead of simply burning fuel, celestial mechanics was mainly used to alter the orbits: the cooperation of just a small push at the right moment with the thrusters, a few flybys close to the Moon and ever-changing orbits due to various perturbations brought the spacecrafts to the desired places. Then they spent six months orbiting the Lagrangian points to examine the solar wind and the Earth's magnetic tail before departing again. Finally, at the end of June, two years after the first maneuver, P1 entered orbit around the Moon and P2 will follow it this Sunday (July 17^th). There they will explore the interactions of the solar wind and the unshielded lunar surface. This is, once again, a nice example of recycling, repurposing great spacecrafts.