N

NADIR The nadir is the point directly below an observer.

NAIAD Naiad is a tiny moon of Neptune. It was discovered using NASA's Voyager 2 mission in 1989. It orbits 48,230 km from the center of Neptune and is about 58 km in diameter.

NANOMETER A nanometer is unit of measure equal to 10 angstroms, which is one billionth (1 x 10-9) of a meter.

NANOROVER A nanorover (also called small science vehicle or ssv) is a small rover designed to be sent to explore small celestial bodies, like asteroids. The book-sized Japanese nanorover MUSES-CN (MUSES stands for the Mu Space Engineering Spacecraft) will be sent to explore asteroid 4660 Nereus in Jan. 2002 and arrive on Nereus on September 9, 2003; it will be the smallest rover ever to fly on a space mission. It will analyse rock samples, send color video to Earth, collect rock samples, and will eventually return to Earth (in 2006).

NASA NASA is the National Aeronautics and Space Administration.

NEA NEA stands for Near-Earth Asteroid. NEA's are asteroids whose orbits bring them within 1.3 AU (121 million miles/195 million kilometers) of the Sun are called Near-Earth Asteroids (NEA) or Earth-Approaching asteroids. These asteroids probably came from the main asteroid belt, but were jolted from the belt by collisions or interactions with other planets' gravitational fields. 433 Eros, pictured above, is a NEA.

NEAP TIDES Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.

NEAR PROJECT NEAR (Near-Earth Asteroid Rendezvous) is a spacecraft designed to fly by and examine near-earth asteroids. It was launched on February 17, 1996; its goal was to rendezvous with asteroid 433 Eros (pictured above). In 2001, NASA renamed the spacecraft NEAR Shoemaker to honor the planetary science pioneer Dr. Eugene M. Shoemaker, who died in 1997. It orbited Eros from Feb. 14, 2000, until Feb. 28, 2001, when it landed on Eros. After orbiting the asteroid Eros (433) for 3 years, NEAR Shoemaker landed on Eros on Feb 12, 2001 (this was the first spacecraft to land on an asteroid). After transmitting radio data, the last contact with NEAR Shoemaker was on Feb. 28, 2001 (it is powered by solar power).

NEAT NEAT (Near-Earth Asteroid Tracking) is a NASA/JPL system that tracks near-earth asteroids. NEAT uses the 1.2-meter- diameter (48-inch) Palomar telescope to track asteroids that come close to the Earth.

NEBULA A nebula is a huge, diffuse cloud of gas and dust in intergalactic space. The gas in nebulae (the plural of nebula) is mostly hydrogen gas (H2). The plural of nebula is nebulae or nebulas.

NEMESIS Nemesis is a hypothetical companion dark star to our Sun. Once every 30 million years, this dark star would pass through the Oort cloud, triggering comets that perhaps cause periodic mass extinctions on Earth.

NEO NEO stands for Near-Earth Object. NEO's are comets and asteroids that have ventured close to the Earth's orbit.

NEPTUNE Neptune is the eighth planet from the sun. It is a cold, gaseous giant with a hazy atmosphere and is orbited by eight moons and three narrow, faint rings.

NEREID Neptune's outermost moon is Nereid. It is the smaller of Neptune's 2 larger moons. It has a VERY elliptical orbit, going as close as 867,000 miles from Neptune and as far as 6 million miles from it; it may be a captured asteroid. It takes almost one Earth year for Nereid to orbit Neptune in this extreme orbit. Nereid's diameter is about 200 miles (320 km). It was discovered by G. Kuiper in 1949.

NEREUS 4660 Nereus is a Near-Earth asteroid that is about 0.6 to 1.25 miles (1-2 km) in diameter. It has an orbital period of 1.82 yrs and a semimajor axis of 1.490 AU. Nereus may be an extinct comet. MUSES-CN is a Japanese nanorover, a small, book-sized rover designed to be sent to explore the asteroid Nereus. MUSES will be launched from Japan in Jan. 2002 and arrive on Nereus on September 9, 2003; it will be the smallest rover ever to fly on a space mission. It will analyse rock samples, send color video to Earth, collect rock samples, and will eventually return to Earth (in 2006).

NEUTRINO A neutrino is an uncharged particle with virtually no mass. Neutrinos are produced in some nuclear reactions in stars.

NEUTRON A neutron is an uncharged atomic nuclear particle. It has a mass slightly greater than a proton. In beta decay, a neutron decays into a proton, an electron, and an anti-neutrino.

NEUTRON STAR A neutron star is a very small, super-dense star which is composed mostly of tightly-packed neutrons. This hard-to-see body has a thin atmosphere of superhot hydrogen plasma and a crust. It has a diameter of about 5-10 miles (5-16 km) and a density of roughly 10 15 gm/cm3. Neutron stars are formed from supernova explosions.

NEW GENERAL CATALOG The New General Catalog (NGC) is a list of over 13,000 deep-sky celestial objects. It was developed in 1888. For example, the Great Nebula in Orion is NCG 1976 (and M42).

NEW MOON The new moon is the phase of the moon when the moon is not visible from Earth, because the side of the moon that is facing us is not being lit by the sun.

NEWTON, ISAAC Isaac Newton (1642-1727) was an English mathematician and physicist who invented calculus (simultaneously, but independently of Leibniz), formulated the laws of gravitation, investigated the nature of light (he discovered that sunlight is made of light of different colors), and the laws of motion: 1. An object in uniform motion tends to remain in that state of motion unless an external force is applied to it (the Law of Inertia). 2. A force causes a change in the velocity (acceleration) of an object (F=ma). 3. For every action there is an equal and opposite reaction. Newton also improved the design of the refracting telescope (using an objective mirror, instead of a lens), and it is now called a Newtonian telescope.

NGC The NGC (New General Catalog) is a list of over 13,000 deep-sky celestial objects. It was developed in 1888. For example, the Great Nebula in Orion is NCG 1976 (and M42). NGC4414 (pictured above) is a spiral galaxy 60 million light-years away.

NOCTILUCENT CLOUD Noctilucent clouds (abbreviated "NLC") are very high-altitude clouds that are visible at night in June and July from the latitudes 50 to 65 degrees. These clouds are at roughly 82-85 km altitude, a dry part of the atmosphere. Noctilucent means "night shine." Noctilucent clouds are probably made of small ice-coated particles. Not very much is currently known about these clouds. Noctilucent clouds were named in 1884.

NORTHERN LIGHTS Northern lights is another name for auroras that appear in the Northern Hemisphere. Auroras are beautiful undulating sheets of light in the near-polar sky. They are caused by gases that become excited after being hit by solar particles. Most auroras are 100 to 250 km above the ground.

NORTH STAR The north star is a star that is located almost due north and is useful for navigation. Polaris is currently the pole star of the Northern Hemisphere.

NOVA A nova is a white dwarf star that suddenly increases in brightness by several magnitudes. It fades very slowly.

NUCLEAR BULGE The nuclear bulge is the central, spherical part of a spiral galaxy. It is surrounded by a disk-shaped mass of stars with spiral arms.

NUCLEAR FISSION Nuclear fission is a reaction in which an atom's nucleus is broken apart, releasing a tremendous amount of energy. Nuclear fission is used in many power plants on Earth (and is used in atomic weapons). Nuclear fission is not an important reaction in the Sun (for nuclear fission, the nuclear reaction that occurs in the Sun, click here).

NUCLEAR FUSION Nuclear fusion is an atomic reaction in which many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). The result of this process is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy by the equation E=mc2). Suns are powered by nuclear fusion, mostly converting hydrogen into helium. In this process, 4 hydrogen nuclei combine to form a single helium nucleus; about 0.3% of the original mass is converted into energy.

NUCLEOSYNTHESIS Nucleosynthesis is the production of new elements via nuclear reactions. Nucleosynthesis takes place in stars. It also took place soon after the Big Bang.

NUCLEUS The nucleus of an atom is its center, which contain most of the atom's mass; in the nucleus is everything but the orbiting electrons.

NUCLEUS The nucleus is the frozen center of a comet's head. The nucleus contains most of the comet's mass.