A form of matter which has not been directly observed but whose existence has been deduced by it's gravitational effects.

Conversion of observed values into useful, ordered and simplified information.

Direct Current.

The action of air drag upon an artificial satellite causing it to spiral back into the atmosphere, eventually to disintegrate or burn up.

Negative acceleration, slowing.

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.

The relief of pressure. Explosive decompression would occur if the cabin of a spacecraft was punctured in space.

Difference or change in velocity.

To extract information from a modulated carrier wave.

Amount of matter per unit volume.

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.

The point at which an orbiting object or spacecraft, moving from north to south, crosses the plane of the equator.

The rocket used to power a spacecraft as it makes a controlled landing on the surface of a planet or moon.

That part of a spacecraft that descends from orbit to the surface of a planet or moon.

Deutsche Gesellschaft fur Luft-und Raumfahrt (German Company for Air and Space Travel).

An electronic device for solving numerically a variety of problems.

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.

Electrical current flowing in one direction and substantially constant in value.

Instruments that interact with phenomena in their immediate vicinity, and register characteristics of them.

A reflector for radio waves, usually a paraboloid.

The technique of connecting two or more spacecraft in space.

Department of Defense (USA).

Degrees Of Freedom.

A phenomenon in which waves appear to compress as their source approaches the observer or stretch out as the source recedes from the observer.

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.

A dose normally applied to biological effects and including scaling factors to account for the more severe effects of certain kinds of radiation.

The radio signal transmitted from a spacecraft to Earth.

The resistance offered by a gas or liquid to a body moving through it.

A small parachute used to slow and stabilize a spacecraft returning to the atmosphere, usually preceding deployment of a main landing parachute.

Deep Space Network.

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).

A device for measuring the velocity, mass, charge, flight direction and number of dust particles striking the instrument.

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.

A unit of force equal to the force required to accelerate a 1 g mass 1 cm per square second.