Glossary of Space Exploration Terminology

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D
Dark matter
A form of matter which has not been directly observed but whose existence has been deduced by it's gravitational effects.
Data reduction
Conversion of observed values into useful, ordered and simplified information.
DC
Direct Current.
Decay
The action of air drag upon an artificial satellite causing it to spiral back into the atmosphere, eventually to disintegrate or burn up.
Deceleration
Negative acceleration, slowing.
Declination
One of the coordinates, measured in degrees, used to designate the location of an object on the celestial sphere. Declination is a north-south value similar to latitude on Earth.
Decompression
The relief of pressure. Explosive decompression would occur if the cabin of a spacecraft was punctured in space.
Delta V
Difference or change in velocity.
Demodulation
To extract information from a modulated carrier wave.
Density
Amount of matter per unit volume.
Density Wave
A kind of wave induced in a flat plane of a resisting medium (such as the rings of Saturn) by gravitational forces, often assuming the form of a tightly wound spiral.
Descending node
The point at which an orbiting object or spacecraft, moving from north to south, crosses the plane of the equator.
Descent engine
The rocket used to power a spacecraft as it makes a controlled landing on the surface of a planet or moon.
Descent module
That part of a spacecraft that descends from orbit to the surface of a planet or moon.
DGLR
Deutsche Gesellschaft fur Luft-und Raumfahrt (German Company for Air and Space Travel).
Digital computer
An electronic device for solving numerically a variety of problems.
Dipole
A compact source of magnetic force, with two magnetic poles. A bar magnet, coil or current loop, if their size is small, create a dipole field. The Earth's field, as a first approximation, also resembles that of a dipole.
Direct current
Electrical current flowing in one direction and substantially constant in value.
Direct sensing
Instruments that interact with phenomena in their immediate vicinity, and register characteristics of them.
Dish
A reflector for radio waves, usually a paraboloid.
Docking
The technique of connecting two or more spacecraft in space.
DoD
Department of Defense (USA).
DOF
Degrees Of Freedom.
Doppler effect
A phenomenon in which waves appear to compress as their source approaches the observer or stretch out as the source recedes from the observer.
Dose
A quantity of radiation delivered at a position. In the context of space energetic particle radiation effects, it usually refers to the energy absorbed locally per unit mass as a result of radiation exposure.
Dose equivalent
A dose normally applied to biological effects and including scaling factors to account for the more severe effects of certain kinds of radiation.
Downlink
The radio signal transmitted from a spacecraft to Earth.
Drag
The resistance offered by a gas or liquid to a body moving through it.
Drogue
A small parachute used to slow and stabilize a spacecraft returning to the atmosphere, usually preceding deployment of a main landing parachute.
DSN
Deep Space Network.
Dust
Particulates which have a direct relation to a specific solar system body and which are usually found close to the surface of this body (e.g. Lunar, Martian or Cometary dust).
Dust detector
A device for measuring the velocity, mass, charge, flight direction and number of dust particles striking the instrument.
Dynamo process
The generation of an electric currents by the flow of an electrically conducting fluid through a magnetic field. For instance, the magnetic field originating inside the Earth is believed to come from a dynamo process involving the flow of molten iron in the Earth's hot core. The energy required by the current is obtained from the motion of the flow.
Dyne
A unit of force equal to the force required to accelerate a 1 g mass 1 cm per square second.

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Rocketry & Space Technology

Glossary courtesy of Robert Braeunig's
Rocketry & Space Technology website.