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radar
A device to determine the distance and direction of movement of an object. Radio waves are sent out and reflected back from an object.

radiation
Energy that is emitted in the form of electromagnetic waves. The electronics of spacecraft camera system are sensitive to radiation. Radiation can affect the images a spacecraft sends - similar to "snow" on a television with poor reception - which is referred to as "noise."

raw data
Original data recieved from a spacecraft before it has been processed.

reddening
The phenomenon of the trailing hemisphere of a planetary body being darker at shorter wavelengths ("redder") than the leading hemisphere. This effect may be due to magnetospheric bombardment acting preferentially on the trailing hemisphere and impact gardening on the leading hemisphere. Of the Galilean satellites, Europa displays this effect most prominently, and Ganymede to a lesser extent.

refraction
The bending of light due to a change in its velocity as it passes the boundary between two materials (e.g. a pencil inserted into a glass of water will look bent due to refraction.)

refractive index (index of refraction)
The ratio of the velocity of light in a vacuum to the velocity of light with in a material.

regio
A region.

regolith
The layer of rocky or icy debris and dust made by meteoritic impact that forms the uppermost surface of planets, satellites and asteroids.

relative age
The age of one thing, such as a geologic unit, in comparison to another. Relative ages are usually determined by crosscutting relationships and the number of impact craters on planetary surfaces.

Relativity, Theory of
Formulated by Albert Einstein in 1905, the famous Specail Theory of Relativity describes the motions of bodies in strong gravitational fields or near the speed of light than newtonian mechanics. Relative to the observer, both are altered near the speed of light: distances appear to stretch; clocks tick more slowly. All experiments done to date agree with relativity's predictions to a high degree of accuracy.

resolution
The amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details.

resonance
A state in which one orbiting object is subject to periodic gravitational perturbations by another.

resurfacing
Creation of a new surface on a planetary body by volcanic or tectonic processes.

reticulum
reticular (net-like) pattern

retrograde
Rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole.

retrograde motion
The apparent backward (westward) motion of planets as seen against the background of stars.

revolution
The motion of one body around another (e.g. the motion of the planets in their orbit around the Sun).

rift valley
an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike.

rima
A fissure.

rotation
The turning or spinning of a body about an axis running through it.

rupes
A line of cliffs produced by faulting or erosion. Also known as scarp.