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A quick change of plans-on to the Moon

Twin NASA robot spacecraft that were mothballed in orbit just one year ago are now on their way to the Moon. In an unusual, creative move-that has transformed two of a former five-member near-Earth satellite fleet into lunar explorers-NASA has dusted off a defunct geomagnetic mission that still has plenty of cryogenic fuel to burn with lots of solar energy to power onboard batteries.

The former THEMIS P1 and P2, which have been called 'dead spacecraft walking,' by Vassilis Angelopoulos of UCLA and principal investigator, are now called ARTEMIS P1 and P2, the Greek goddess of the Moon.

The new mission acronym stands for Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun.

"Not so long ago, we thought they were goners. Now they are beginning a whole new adventure," Angelopoulos said. "The story began in 2007 when NASA launched a fleet of five spacecraft into Earth's magnetosphere to study the physics of geomagnetic storms. Collectively, they were called THEMIS, short for Time History of Events and Macroscale Interactions during Substorms. P1 and P2 were the outermost members of the quintet."

THEMIS findings in 2008 allowed researchers to solve several longstanding mysteries of the Northern Lights.

"The two spacecraft were running out of power and freezing to death," Angelopoulos told reporters at a NASA news conference. "We had to do something to save them. The team brainstormed a solution: Because the mission had gone so well, the spacecraft still had an ample supply of fuel-enough to go to the Moon. We could do some great science from lunar orbit. NASA approved our trip plans... P1 and P2 headed away from the shadows of Earth."

The reborn THEMIS to ARTEMIS mission is now underway; it's first task is to investigate the Moon's mysterious plasma cavity. Much like a turbulent wake left behind by a Jetski pilot hot-dogging across a tranquil mountain lake, the Moon's plasma wake is formed by the solar wind, but we don't understand much about how it is agitated by our natural satellite. The Moon passes through this stream of energetic stellar particles stirring up a hornet's nest of particles in its wake. When they arrives near the Moon, the ARTEMIS probes will check out Earth's magneto-tail and see how it impacts the Moon-literally. So, within a few weeks, NASA mission planners will begin to skillfully move the ARTEMIS twins to within 100 km of the lunar surface. There, both spacecraft will observe how the solar wind batters the Moon's bleak surface. There's not much of a lunar magnetic field to speak of; unlike on Earth, there's nothing to protect the Moon's surface. A better understanding of what's going on will help future lunarbound astronauts.

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