Definitions list
Click one of the letters above to advance the page to terms beginning with that letter.
  • g
    From the Latin gravis, meaning “heavy”. Unit of acceleration due to Earth’s gravity. Its average value is 9.8 m/s². Also used as a unit of acceleration, defined by the relation 1 g = 9.8 m/s²
  • galaxy
    A system of stars. There are billions of galaxies in the Universe. Our galaxy, the Milky Way, contains more than 100 billion stars.
  • gamma astronomy
    Branch of astronomy that studies gamma radiation of natural cosmic origin.
  • gamma-astronomie
    Synonyme d’astronomie Gamma
  • gas giant
    The four largest planets in the Solar System—Jupiter, Saturn, Uranus and Neptune—are gas giants. They are composed mainly of hydrogen and helium, and have a thick atmosphere, a solid core, a system of rings and numerous natural satellites.
  • geoid
    Model of the unperturbed sea level—that is, in the absence of the effects of tides, winds, currents and other factors—that continues in continental areas as an imaginary sea-level surface, defined mathematically.
    It is a very irregular and undulating surface that reflects variations in gravity. Large irregularities can be noted over long distances (departures of 100 metres over several thousand kilometres), due to uneven densities within the Earth’s crust, mantle and core. Smaller and less visible irregularities, of the order of 1 metre, are due to the relief of the ocean floor, which causes variations in the mass and density of the subsurface, generating more or less gravity and thus retaining a larger or smaller layer of water.
  • geophysics
    Study of the physical properties of Earth and its environment. There are two branches of geophysics, one focusing on the study of the planet’s exterior (atmosphere and Sun-Earth relationship), and one on its interior (volcanology and seismology).
  • geostationary orbit
    See geostationary
    What is an orbit?
  • Geostationary transfer orbit (GTO)
    Temporary orbit reached by a satellite before being boosted into geostationary orbit. Geostationary transfer orbits are elliptical with a perigee as low as 200 km and an apogee as high as 36,000 km.
    What is an orbit?
  • geostationnary
    Term describing a satellite that appears stationary to an observer on Earth. It also designates the orbit of such satellites, which is equatorial, circular and has an average period equal to the Earth’s rotational period (23 h 56 min.). Geostationary satellites orbit at an altitude of 35,786 km.
    What is an orbit?
  • geosynchronous
    Describes an Earth-orbiting satellite with an orbital period equal to the Earth’s rotational period (23 h 56 min.). A geostationary satellite can thus be described as geosynchronous. The term also designates the orbit of such satellites.
    What is an orbit?
  • geosynchronous orbit
    See geosynchronous
    What is an orbit?
  • gondola
    Part of a balloon flight train that houses equipment inside a single structure. The flight train of a stratospheric balloon comprises two gondolas: the ballooncraft and the scientific gondola.
  • graveyard orbit
    Orbit to which a satellite is moved on reaching the end of its service life.
    What is an orbit?
  • gravitate
    To undergo the effects of gravitation. All celestial bodies—natural and man-made satellites, planets, comets and asteroids—gravitate.
  • gravitation
    Universal force of attraction acting between all matter. Gravitation and the force that it exerts, called gravity, are what propel man-made satellites and space probes.
  • gravity
    Gravitational attraction exerted by a celestial body on another mass.
  • gravity assist
    Manoeuvre designed to take advantage of a celestial object’s gravitational pull to modify the velocity and direction of a probe or spacecraft passing within its vicinity. Many interplanetary probes use gravity assists to save fuel.
    How interplanetary trajectories work
  • ground segment
    Teams, equipment and software on the ground that control a satellite and receive and process telemetry.
  • ground track
    Intangible track on the Earth’s surface formed by the succession of points overflown by a satellite.
  • géocentric (Earth centred) theory
    Theory that the Earth lies at the centre of the “world”, with the celestial bodies—Sun, Moon, planets and stars—orbiting around it. The geocentric theory, expounded by the Greeks, was questioned in the 16th century by Copernicus.