Glossary - N

Nadir
1: the direction directly beneath an observer on the celestial sphere; opposite of the zenith
(astronomy)

Nadir
2: the direction from a spacecraft directly down towards the centre of a planet or other celestial body
(space technology)

Narrowcast
a transmission to a small user population
(communications)

Navstar
satellites used in the GPS system
(satellite navigation)

Nebula
a large bright cloud of gas or dust in interstellar space
(astronomy)

Neutron star
a star, with a mass of about 1.4 solar masses and a radius of about 10 km, formed from the collapsed core of a massive supergiant star, and composed entirely of neutrons
(astronomy, astrophysics)

Node
the point where an orbit crosses a reference plane
(orbital geometry)

Noise
unwanted signals, generally thermal in origin, which are present with the wanted signal
(communications)

Non-ionising radiation
radiation which does not ionise the material it is passing through; low energy electromagnetic radiation including radio waves, microwaves, infrared radiation, visible light and ultraviolet radiation
(physics)

Non-invasive mount
a type of mount used for small antennas which rests on, for example, the roof of a building but is not fastened to the building, achieving its stability by its weight
(earth station technology)

Non-preemptible service
a service which may not be displaced by another service
(satellite communications)

Nova
a white dwarf star which increases dramatically in brightness and then fades in brightness over a period of several months or years; generally the white dwarf has a larger, close companion star from which it takes material to form a shallow atmosphere, eventually the atmosphere around the white dwarf becomes unstable and a runaway fusion reaction starts which produces a large amount of energy resulting in the rapid brightening of the star
(astronomy, astrophysics)

Nucleus
the central, solid part of a comet composed of rock, dust and frozen gases; generally less than 20 km across
(astronomy, planetary science)

Nutation
the rocking or nodding variation in the axis of rotation of a spinning object such as a gyroscope, planet or moon
(orbital geometry, space technology)