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O

Occultation

Complete or partial eclipsing of one astronomical object by another.

Oort cloud

Swarm of billions of comets thought to surround the Solar System between 2000 and 20 000 AU from the Sun. First proposed by E. Öpik in 1932 and later developed by J. Oort in the 1950s. Its existence is based on studies of long-period comet orbits, which seem to have their aphelia in this zone.

Open Universe

See Critical density.

Opposition

The point at which a planet that is further away from the Sun than the Earth, lines up with the Sun and Earth. Opposition is a good time for observation because the planet is near its closest point to Earth.

Optical light ('visible' light)

The light that can be detected by the human eye. It has a wavelength between 4000 and 7000 angstroms (or between 0.00004 and 0.00007 cm). See also Electromagnetic radiation.

Optics

Elements (glasses, mirror, telescope...) used to see or visualise something.

Orbit

The path through space of one celestial body or spacecraft about another.

Orbit acquisition

Reception of the telemetry containing the information about the orbital parameters of the spacecraft.

Orbiter

A spacecraft orbiting around a planet or other celestial body to act as a telecommunications relay or from which remote sensing observations can be made.

Organics

Carbon-based material.

Orientation

Position in space relative to a reference point.

Ozone layer

A layer in the Earth's atmosphere at 15-30 km altitude in which ozone is at higher concentration than at lower or higher altitudes. The ozone is created by a series of processes beginning with the splitting up of the oxygen molecule to single oxygen atoms. The ozone layer protects the Earth from UV radiation harmful to life.

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