Astrobiology Glossary R – Space

Term Definition
RADAR (Radio Detection and Ranging) A method of detecting, locating, or tracking an object by using beamed, reflected, and timed radio waves. RADAR also refers to the electronic equipment that uses radio waves to detect, locate, and track objects.
Radial Motion The component of an object’s velocity (speed and direction) as measured along an observer’s line of sight.
Radiation The process by which electromagnetic energy moves through space as vibrations in electric and magnetic fields. This term also refers to radiant energy and other forms of electromagnetic radiation, such as gamma rays and X-rays.
Radiative Process An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light converts the energy of that radiation into electrical potential energy.
Radio Waves Radiation with the longest wavelengths and smallest frequencies and energies in the electromagnetic spectrum.
Radioactivity The spontaneous decay of certain rare, unstable, atomic nuclei into more stable atomic nuclei. A natural by-product of this process is the release of energy.
Reaction Wheel One of four spinning flywheels aboard the Hubble Space Telescope. The flywheels work together to make the observatory rotate either more rapidly or less rapidly toward a new target.
Recessional Velocity The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.
Red Giant Star An old, bright star, much larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of a red giant.
Redshift The lengthening of a light wave from an object that is moving away from an observer. For example, when a galaxy is traveling away from Earth, its light shifts to the red end of the electromagnetic spectrum.
Reflector A type of telescope, also known as a reflecting telescope, that uses one or more polished, curved mirrors to gather light and reflect it to a focal point.
Refractor A telescope, also known as a refracting telescope, that uses a transparent lens to gather light and bend it to a focus.
Regolith The layer of loose rock resting on bedrock (sometimes called mantle rock), found on the Earth, the Moon, or a planet. Regolith is made up of soils, sediments, weathered rock, and hard, near-surface crusts. On the surface of the Moon, regolith is a fine rocky layer of fragmentary debris (or dust) produced mainly by meteoroid collisions.
Relativity A theory of physics that describes the dynamical behavior of matter and energy. The consequences of relativity can be quite strange at very high velocities and very high densities. A direct result of the theory of relativity is the equation E = mc2, which expresses a relationship between mass (m), energy (E), and the speed of light (c).
Revolution The orbital motion of one object around another. The Earth revolves around the Sun in one year. The moon revolves around the Earth in approximately 28 days.
Ribosomal RNA The RNA components of a ribosome. One of these components, the small sub unit ribosomal RNA (also known as 16S ribosomal RNA in prokaryotes or 18S ribosomal RNA in eukaryotes) has been widely used to determine the tree of life.
Ribosome A structure composed of protein and RNA molecules which reads genetic information from messenger RNA and synthesises the corresponding protein.
Ribozyme An RNA molecule which acts as a catalyst. The discovery of RNA catalysis led to a Nobel prize for Sidney Altman and Thomas Cech, and to the RNA World concept.
Right Ascension (RA) A coordinate used by astronomers to locate stars and other celestial objects in the sky. Right ascension is comparable to longitude, but it is measured in hours, minutes, and seconds because the entire sky appears to pass overhead over a period of 24 hours. The zero hour corresponds to the apparent location of the Sun with respect to the stars on the day of the vernal (spring) equinox (approximately March 21).
RNA Ribonucleic acid A molecule which can carry genetic information in a similar way to DNA. In modern cells RNA molecules are important in the process of protein synthesis, in the form of messenger RNA, ribosomal RNA and transfer RNA.
RNA World A hypothetical early stage in the development of life in which RNA molecules provided both the genome and the catalysts, roles which subsequently were taken over by DNA and proteins.
Robotic Optical Transient Search Experiment (ROTSE) A terrestrial telescope that searches for the optical counterparts of gamma-ray bursts. When orbiting satellites detect a gamma-ray burst, ROTSE begins searching for its visible-light afterglow. ROTSE-I (an array of four electronic telephoto cameras) and ROTSE-II (a set of identical telescopes) are located in Los Alamos, New Mexico.
Roche Limit The smallest distance at which two celestial bodies can remain in a stable orbit around each other without one of them being torn apart by tidal forces. The distance depends on the densities of the two bodies and their orbit around each other.
Rotation The spin of an object around its central axis. Earth rotates about its axis every 24 hours. A spinning top rotates about its center shaft.

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