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O-stage
Rocket boosters which operate during part of the burning time of the first stage of a launch vehicle to provide additional thrust.
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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.
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OKB
Experimental Construction Bureau.
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Omnidirectional
Capable of transmitting or receiving signals in all directions, as an antenna.
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OMS
Orbital Maneuvering System.
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One-way
Communications mode consisting only of downlink received from a spacecraft.
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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.
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Oort cloud
A large swarm of comets theorized to orbit the sun in the neighborhood of 50,000 AU.
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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.
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Orbit insertion
The placing of a spacecraft into orbit around a planet or moon.
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Orbit trim maneuver
The firing of control rockets to refine a spacecraft's speed and trajectory.
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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.
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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.
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Orbital module
That part of a spacecraft which allows additional volume for crew relaxation and/or experiments. Discarded prior to reentry.
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Orbital period
The time taken by an orbiting body to complete one orbit.
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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.
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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.
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OTM
Orbit Trim Maneuver.
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OTRAG
Orbital Transport- und Raketen- Atktiengesellsschaft.
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OWLT
One-Way Light Time.
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Oxidizer
An agent that releases oxygen for combination with another substance, creating combustion and gas for propulsion. Alternatively oxidants.
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