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On-orbit satellite servicing
On-orbit satellite servicing is the refueling, repair, and upgrading of satellites in orbit, from the 1984 SolarMax repair and Hubble missions to commercial robotic craft like MEV-1.
OPIR (satellite program)
OPIR, or Next Generation Overhead Persistent Infrared, is a United States Space Force program of missile-warning satellites built by Lockheed Martin and Northrop Grumman that will replace SBIRS.
Opportunity (rover)
Opportunity, nicknamed Oppy, was a NASA Mars rover that landed in 2004 and found evidence of ancient water, operating for nearly 15 years against a planned 90-day mission.
OPS-SAT
OPS-SAT was a 3U CubeSat of the European Space Agency launched in December 2019 as an in-orbit laboratory for testing mission control techniques, ending with atmospheric reentry in May 2024.
Optical ground station
An optical ground station is a ground-based facility with a telescope, laser transmitters, and sensitive receivers that communicates with spacecraft using free-space laser links instead of radio frequencies.
Optical solar reflector
An optical solar reflector, also called a thermal control mirror, is a passive spacecraft thermal control element that reflects most sunlight while radiating heat away in the infrared.
ORBCOMM
ORBCOMM is a satellite communications company providing global machine-to-machine and IoT data messaging services through a constellation of low Earth orbit satellites combined with cellular connectivity.
Orbital eccentricity
Orbital eccentricity, written e, is a dimensionless orbital element measuring how much an orbit deviates from a perfect circle: 0 is circular, and values near 1 are elongated.
Orbital elements
Orbital elements, also called Keplerian or classical orbital elements, are the six parameters defining an orbit's size, shape, orientation, and the orbiting body's position, standard in satellite catalogs.
Orbital Express
Orbital Express was a DARPA mission launched in 2007 that demonstrated autonomous robotic satellite refueling, completing seven rendezvous and dockings and 14 refuelings in orbit.
Orbital inclination
Orbital inclination is the angle between an orbit's plane and a reference plane, one of six Keplerian elements: 0° rides the equator, 90° passes over the poles.
Orbital inclination change
An orbital inclination change, also called a plane change, is a maneuver that rotates a spacecraft's orbital plane and is among the most expensive in orbital mechanics.
Orbital Maneuvering System
The Orbital Maneuvering System (OMS) was the Space Shuttle orbiter's primary in-space propulsion system, two AJ10 rocket engines that performed orbital insertion, orbit changes, rendezvous, and the de-orbit burn.
Orbital mechanics
Orbital mechanics, also known as astrodynamics or orbital dynamics, is the branch of mechanics that studies the motion of spacecraft and planets under gravity to plan space missions.
Orbital rendezvous
Orbital rendezvous is the set of maneuvers by which a chaser spacecraft matches a target's position and velocity in orbit, a required step for docking and space station missions.
Orbital Sciences Corporation
Orbital Sciences Corporation was an American aerospace company founded in 1982 and based in Dulles, Virginia, that built the Pegasus rocket, the Antares rocket, and the Cygnus cargo spacecraft before merging into Orbital ATK in 2015.
Orbital spaceflight
Orbital spaceflight, or orbital flight, is flight in which a spacecraft travels fast enough to fall freely around a celestial body instead of back to its surface.
Orbital station-keeping
Orbital station-keeping is the propulsive maneuvers that keep a spacecraft within its designated orbital position or window despite perturbations such as drag, gravity irregularities, and solar radiation pressure.
OrbView-2
OrbView-2, originally named SeaStar, was a commercial Earth-observation satellite launched by Orbital Sciences in 1997 whose SeaWiFS instrument measured ocean chlorophyll for more than 13 years.
Orion (spacecraft)
Orion is NASA's deep-space crewed spacecraft, built by Lockheed Martin, carrying four astronauts to the Moon on Artemis missions; its first crewed flight, Artemis II, flew around the Moon in 2026.
Orville Carlisle
Orville H. Carlisle (1917–1988) was an American shoe store owner and pyrotechnics hobbyist in Norfolk, Nebraska whose Rock-A-Chute rockets and single-use motor founded model rocketry.
Osculating orbit
An osculating orbit is the Keplerian ellipse or hyperbola a body would follow at a given instant if perturbing forces stopped, sharing the body's actual position and velocity.
OSIRIS-APEX
OSIRIS-APEX is a NASA asteroid mission using the former OSIRIS-REx spacecraft to study changes to Apophis caused by its close Earth approach in April 2029.
OSIRIS-REx
OSIRIS-REx, also called the Osiris mission, was a NASA New Frontiers spacecraft that collected rocks from asteroid Bennu and returned them to Earth in 2023.
Overview effect
The overview effect is a cognitive shift reported by astronauts viewing Earth from space, first named by Frank White, who explained it in his 1987 book.
Oxygen Generation System
The Oxygen Generation System (OGS) is the International Space Station's primary source of breathable oxygen, electrolyzing reclaimed water in a 28-cell stack and running nearly continuously since 2007.
P80
The P80, also written P80FW or P-80, is a European solid-fuel rocket motor that served as the first stage of the ESA Vega launcher from 2012 to 2024.
PAM-6
PAM-6 (Private Astronaut Mission 6), also known as Vast ISS Mission 1, is a planned crewed flight to the International Space Station operated by Vast, launching no earlier than 2027.
Parc du Radôme
Parc du Radôme, Pleumeur-Bodou, Brittany: leisure and science park around the Radôme, a 50 m inflatable dome sheltering the 340-tonne PB1 antenna that received the first live transatlantic TV via Telstar on 11 July 1962; hosts Cité des Télécoms, Planétarium de Bretagne, Village Gaulois.
Parker Solar Probe
Parker Solar Probe, formerly Solar Probe Plus, is a NASA spacecraft launched in 2018 to fly through the Sun's corona and study its heating and the solar wind.