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[silence]

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[music] Narrator: If you were to go looking for water on

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the Moon, how would you find a good place to start digging? NASA's Lunar Reconnaissance

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Orbiter has a pretty unique answer: Count the neutrons coming from the Moon!

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LRO's LEND instrument, or Lunar Explorer Neutron Detector,

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is specifically designed to do just this. But how does counting neutrons help you find water?

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The answer lies in hydrogen--the smallest atom--and how those neutrons

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interact with it. So where do those neutrons come from, and what do they do?

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The Moon is constantly bombarded by cosmic rays. Sometimes, these

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rays hit atoms in the Moon's soil and knock off a neutron or two. These neutrons then

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go bouncing across other atoms before heading off into space. In soil that doesn't

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contain much hydrogen, the neutrons are much smaller than the surrounding atoms they crash into,

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so they don't slow down too much. However, in soil containing larger amounts of hydrogen,

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some of the neutrons hit the similarly-sized hydrogen atoms, which absorb a good

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deal of the neutrons' energy and slow them down. By measuring the varying amounts of slow

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and fast neutrons, LEND can effectively measure the likely amount of hydrogen in different

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areas of the Moon. And that being said, scientists think that where there's hydrogen,

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there's a good chance there's water. So LEND will keep counting neutrons,

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and LRO will keep looking for water on the Moon.

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[music, beeping]