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Electron Reflectometer
Image courtesy of Lockheed Martin

The ER, unlike the MAG, is a remote instrument: it will measure the magnetic field at the surface of the Moon. Together, the two instruments will able to detect local variations in the Moon's magnetic field that arise from selenological features on the Lunar surface. The key to measuring the magnetic field from 63 miles (100 km) away is a scientific trick that was first used successfully on the Apollo 15 and 16 missions. The Moon, like every other body in the Solar System, is constantly barraged by electrons from the Solar wind. (On Earth, a few of the electrons make it into the upper atmosphere, where they contribute to phenomena such as the Van Allen radiation belts and the Aurora Borealis. Most of them, however, are repelled by the Earth's magnetic field.) Unlike the Earth, the Moon does not have a magnetic field strong enough to repel these tiny charged particles, and so they spiral toward the surface in giant loops, typically several miles wide.

The electrons that descend in a tighter spiral make it to the surface of the Moon and are absorbed there. But if there is magnetic material on the Moon, it will reflect some of the electrons back into space. When the reflected electrons reach the Lunar Prospector spacecraft, the ER will measure their pitch angle. After the pitch angles of many electrons are tabulated, scientists will see an abrupt cut-off above a certain angle -- because all the electrons with a larger pitch angle were absorbed by the Moon. That cut-off tells the researchers just how intense the magnetic field was at the lunar surface.

* The Electron Reflectometer is linked to the Magnetometer by a smaller, 2.5-foot (0.8 meter) boom; the assembly when fully deployed resembles an elbow with a 90-degree bend. The combined weight of the Magnetometer and Electron Reflectometer is about 11 pounds (5 kilograms). Together, the two instruments use 4.5 watts of power, and produce 670 bits of data per second.


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