Glossary of Space Exploration Terminology

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R
Radar
System or technique for detecting the position, motion, and nature of a remote object by means of radio waves reflected from its surface.
Radian
Unit of angular measurement equal to the angle at the center of a circle subtended by an arc equal in length to the radius. Equals about 57.296 degrees.
Radiation
Energy in the form of electromagnetic waves or particles.
Radiation belt
The region of high-energy particles trapped in the Earth's magnetic field, also known as the Van Allen belts.
Radio
The least energetic form of electromagnetic radiation, having the lowest frequency and the longest wavelength.
Radio astronomy
The science of astronomy using radio waves instead of light waves.
Radio guidance
A system which is dependent on outside signals for information.
Radioisotope thermoelectric generator
A device that converts the heat produced by the radioactive decay of plutonium-238 into electricity by an array of thermocouples made of silicon-germanium junctions. The Pu-238 is contained within a crash resistant housing.
Radioisotopes
Atomic particles which decay by natural radioactivity.
Radiometry
The detection and measurement of radiant electromagnetic energy, usually in the infrared.
Rankine
A temperature scale, having a degree equal to the Fahrenheit degree but having a zero point at absolute zero. The freezing point of water is at 459.69oR.
RCS
Reaction Control System.
RD
Reaktivnyi Dvigatel. Russian for reaction motor.
RE
Unit of distance equal to the radius of the Earth, 6371.2 km.
Reaction control system
System of thrusters used to control spacecraft attitude.
Reaction wheels
Electrically-powered wheels mounted in three orthogonal axes aboard a spacecraft. To rotate the vehicle in one direction, you spin up the proper wheel in the opposite direction. To rotate the vehicle back, you slow down the wheel.
Readout
The action of a radio transmitter sending data either at the same time as data are acquired or by playback from an electronic memory.
Receiver
An electronic device that receives incoming radio signals and converts them to perceptible forms.
Red dwarf
A small star, on the order of 100 times the mass of Jupiter.
Redundancy
The duplication of certain critical components in a space vehicle.
Reentry
The descent into Earth's atmosphere from space.
Reentry interface
An altitude 400,000 feet; the point at which reentering spacecraft are considered to enter the Earth's atmosphere.
Refraction
The deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another.
Regenerative cooling
Circulation of a propellant through a jacket around the combustion chamber in order to cool the chamber wall, the propellant subsequently being injected into the combustion chamber.
Relay
An electrical switch employing an armature to open and close circuits.
Rem
Roentgen Equivalent Man. A measure of nuclear radiation causing biological damage.
Remote sensing
Instruments that record characteristics of objects at a distance, sometimes forming an image by gathering, focusing, and recording reflected light from the Sun, or reflected radio waves emitted by the spacecraft.
Rendezvous
A place of meeting at a given time, for example, a spaceship with a space station.
Resolution
Ability to distinguish visual detail, usually expressed in terms of the size (in kilometers) of the smallest features that can be distinguished.
Resonance
A relationship in which the orbital period of one body is related to that of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
Retrograde
Orbital motion in the direction opposite to the primary's rotation.
Retrorocket
A rocket fired to reduce the speed of a spacecraft.
Revolution
Orbital motion about a primary.
RF
Radio Frequency.
Right ascension
With declination, one of the coordinates used to designate the location of an object on the celestial sphere. Right ascension is measured in hours, minutes, and seconds and is similar to longitude on Earth.
Ring current
A very spread-out electric current circling around the Earth, carried by trapped ions and electrons.
RJ-1
A hydrocarbon rocket fuel (a refined kerosene).
RLV
Reusable Launch Vehicle.
Rocket
A missile or vehicle propelled by the combustion of a fuel and a contained oxygen supply. The forward thrust of a rocket results when exhaust products are ejected from the tail.
Rocketdyne
A division of Rockwell International concerned with the design and development of rocket engines (USA).
Roll
The rotational movement of a vehicle about a longitudinal (X) axis.
Rotation
Rotary motion about an axis.
Round-trip light time
The elapsed time is takes for light, or a radio signal, to travel from Earth, be received and immediately transmitted or reflected, and return to the starting point.
RP-1
A hydrocarbon rocket fuel (a refined kerosene).
RTG
Radioisotope Thermoelectric Generator.
RTLT
Round-Trip Light Time.
RV
Reentry Vehicle.

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Rocketry & Space Technology

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