Definitions list
Click one of the letters above to advance the page to terms beginning with that letter.
  • eccentricity
    An orbit’s eccentricity, usually denoted by e, is the eccentricity of the conic section (circle, ellipse, parabola or hyperbola) described by a satellite or celestial body. It is the ratio of the distance between the foci to the length of the major axis. If the eccentricity is zero, the orbit is a circle; if less than one, an ellipse; if equal to one, a parabola; and if greater than one, a hyperbola.
  • ecliptic
    Plane of the Earth’s orbit around the Sun.
  • El Niño
    Climatic phenomenon observed in the Pacific Ocean. During an El Niño event, surface waters off the west coast of America warm abnormally by 4 to 6°C. Such conditions affect the ocean and atmosphere, and may last for a year or more. El Niño conditions return every 2 to 7 years and vary in intensity.
  • electric propulsion
    Mode of jet propulsion that uses electrical energy to accelerate the propulsive fluid. If the fluid is a gas, we use the term electrothermal propulsion; if a flow of ions, ionic propulsion; and if a plasma, plasma propulsion.
    Electric propulsion is sometimes used to stabilize satellites or probes.
  • electromagnetic wave
    Wave characterized by varying electric and magnetic fields. Examples are radio waves and light waves. An electromagnetic wave propagates in a vacuum.
  • elliptical orbit
    Orbit describing an ellipse, that is, with an eccentricity less than 1.
    What is an orbit?
  • endurance test
    Long-duration test to determine an item of equipment’s ability to withstand particular stresses over a given period of time.
  • engine-frame
    Structure supporting an engine.
  • enveloppe
    Part of a balloon filled with the gas that provides buoyancy. It is made of light, flexible and gastight material.
  • environmental test
    Test performed under specified environmental conditions, comprising climatic, mechanical, electrical and electromagnetic tests, or combined tests. An environmental test may be sequential, in which case the equipment is subjected to different sets of environmental conditions in succession.
  • equatorial orbit
    Orbit within the plane of the equator or, in practice, near this plane.
    What is an orbit?
  • equipment bay
    Part of a launch vehicle housing the main electrical equipment used for guidance, positioning, measuring and control.
  • evaluation test
    Test designed to evaluate the performance of an item of equipment.
  • exobiology
    Science that studies the possibility of extraterrestrial life forms existing in the Universe. Synonym: astrobiology.
  • expandable
    Term describing a traditional, non-reusable launch vehicle. It also describes any element of a satellite consumed over time, for example, propellants.
  • extrasolar planet
    Planet orbiting a star other than the Sun.
  • extravehicular
    Term used to describe activities conducted by an astronaut outside a spacecraft (extravehicular activity, synonym: spacewalk).
  • Extravehicular activity (EVA)
    See extravehicular