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NASA
Acronym for the National Aeronautics and Space Administration.

nephelometer
An instrument that uses measurements of scatterred light from a laser beam to detect and study cloud particles.

Neptune
Neptune is the eighth planet from the sun. Named for the ancient Roman god of the sea, it is the fourth largest planet in our solar system.
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neutrino
A fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars. They are very hard to detect since the vast majority of them pass completely through the Earth without interacting.

nitrogen
A colorless and odorless gas that is makes up the largest part of the Earth's atmosphere.

nuclear fusion
Anuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy therefore ultimately of (almost) all energy on Earth.

nucleus
A core. In the case of a comet, it is the central, solid part made of ice and dust.