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(tone)

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(Music) Narrator: Launched in 2007,

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NASA's five THEMIS spacecraft have now successfully completed their two-year mission to

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determine the cause of geomagnetic substorms.

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Because they are continuing to work perfectly, NASA is redirecting

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the outermost two spacecraft to special orbits at and around the Moon.

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This new mission, which is called ARTEMIS, uses some very complex maneuvers

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over two years to get both spacecraft into position.

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Although the spacecraft sometimes move far

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outside the orbit of the Moon, their every move is carefully orchestrated to ease

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them into position while using very little of their precious fuel.

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ARTEMIS 1, in red, had the larger orbit to begin with,

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so it requires a much more dynamic path to harness its energy.

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(Music)

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For a few months, the spacecraft don't even orbit the Earth or the Moon.

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Instead, they circle the two locations where the the gravity of the Earth and the Moon

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cancel out exactly. These spots, just inside and just beyond

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lunar orbit are known as Lagrange points. NASA has

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several satellites orbiting one such point between the Earth and the Sun.

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As the Moon orbits the Earth, it passes in and out of the Earth's

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magnetic field and the million-mile per hour stream of particles emitted by the Sun

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known as the solar wind. While in these regions, the two ARTEMIS

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spacecraft will seek evidence turbulence, particle acceleration

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and magnetic reconnection, three fundamental phenomena that control

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the nature of the solar wind's interaction with the Earth's magnetosphere.

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Finally, in 2011,

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the ARTEMIS settle into a lunar orbit.

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(Music)

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Employing their full complement of instruments and unique vantage points,

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the spacecraft will study the vacuum the Moon carves out in the solar wind and the processes

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that eventually fill this lunar wake.

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(Music)

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Nearer the Moon, they will observe the effects of surface electric fields,

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ions sputtered off the lunar surface, and determine the internal structure of the

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Moon from variations in its magnetic field.

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These observations will improve

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understanding of the Sun-Earth connection called space weather, and its interaction

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with our nearest neighbor in space.