Definitions list
Click one of the letters above to advance the page to terms beginning with that letter.
  • day
    Time taken by a celestial body to complete one full revolution about its axis. It varies according to the body: 24 hours on Earth, 176 Earth days on Mercury, and 10 hours on Jupiter and Saturn.
  • decubitus / bedrest
    Horizontal or near-horizontal lying position. E.g. antiorthostatic decubitus. See orthostatic.
  • deorbiting
    Braking manoeuvre used to land an orbiting spacecraft on a celestial body.
  • Digital elevation model (DEM)
    Three-dimensional representation of terrain relief using a grid of coded elevation values (for example, spaced every 20 metres).
  • docking
    Operation through which two spacecraft meet and are physically joined in space.
  • Doppler effect
    Phenomenon that occurs when a source of vibration (sound, for example) or electromagnetic radiation (light, radio waves) at a given frequency is in motion relative to an observer, which results in a shift of the frequency reaching the observer. The Doppler effect is thus used to determine the speed of a moving object emitting sound or light, by measuring the difference between the frequency received and the frequency leaving the source.
  • Drag compensation
    Technique used to compensate for all non-gravitational forces acting on a satellite. Such forces are chiefly due to drag, caused by atmospheric friction against the walls of the satellite.
  • drop shaft
    Vertical underground facility used to perform experiments in weightless conditions.
  • drop tower
    Facility in which experiments are subjected to free-fall for a few seconds to create conditions of weightlessness.
  • dynamic topography
    Variation in the level of the oceans, from a few centimetres to several tens of centimetres, due to ocean circulation. Actual sea level is governed by the combined effects of the dynamic topography and the geoid, which is the static component.