Glossary of Space Exploration Terminology

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O
O-stage
Rocket boosters which operate during part of the burning time of the first stage of a launch vehicle to provide additional thrust.
Occultation
The passage of a celestial body across a line between an observer and another celestial object; and the progressive blocking of light, radio waves, or other radiation from a celestial source during such a passage.
OKB
Experimental Construction Bureau.
Omnidirectional
Capable of transmitting or receiving signals in all directions, as an antenna.
OMS
Orbital Maneuvering System.
One-way
Communications mode consisting only of downlink received from a spacecraft.
One-way light time
The elapsed time it takes for light, or a radio signal, to reach a spacecraft or other body from Earth, or vice versa.
Oort cloud
A large swarm of comets theorized to orbit the sun in the neighborhood of 50,000 AU.
Orbit
The path of a body acted upon by the force of gravity. Under the influence of a single attracting body, all orbital paths trace out simple conic sections. Although all ballistic or free-fall trajectories follow an orbital path, the word orbit is more usually associated with the continuous path of a body which does not impact with its primary.
Orbit insertion
The placing of a spacecraft into orbit around a planet or moon.
Orbit trim maneuver
The firing of control rockets to refine a spacecraft's speed and trajectory.
Orbital elements
Six quantities used to mathematically describe an orbit; i.e. semi-major axis, eccentricity, inclination, argument of periapsis, time of periapsis passage and longitude of ascending node.
Orbital mechanics
The study of the motions of artificial satellites and space vehicles moving under the influence of forces such as gravity, drag, and thrust. Also called flight mechanics.
Orbital module
That part of a spacecraft which allows additional volume for crew relaxation and/or experiments. Discarded prior to reentry.
Orbital period
The time taken by an orbiting body to complete one orbit.
Orbital velocity
The velocity necessary to overcome the gravitational attraction of the Earth and so keep a satellite in orbit, about 17,450 mph (28,080 km/hr) close to the Earth.
Orbiter spacecraft
A spacecraft designed to travel to a distant planet or moon and enter orbit. It must carry a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion.
OTM
Orbit Trim Maneuver.
OTRAG
Orbital Transport- und Raketen- Atktiengesellsschaft.
OWLT
One-Way Light Time.
Oxidizer
An agent that releases oxygen for combination with another substance, creating combustion and gas for propulsion. Alternatively oxidants.

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Rocketry & Space Technology

Glossary courtesy of Robert Braeunig's
Rocketry & Space Technology website.