Science Glossary

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Van Allen radiation belt

See Radiation belt (van Allen).


Space which is entirely devoid of matter.


One of the terrestrial planets, Venus is the second planet out from the Sun, orbiting at a distance of 0.7 AU. It is about the same size as the Earth and has slightly lower gravity. Venus has a dense atmosphere consisting mainly of carbon dioxide and nitrogen. Atmospheric pressure at the surface is very high, about 92 times that on Earth. Average surface temperature is 737 K. Venus rotates about its axis in a clockwise or retrograde direction, compared with most other Solar System bodies which have anti-clockwise orbits. See Retrograde.


Periods at which a celestial object, either natural or artificial, is visible from a point on Earth.

Volt (V)

Unit of electric potential in the SI unit system. It measures the difference in electric potential that causes a current of 1 ampere to flow through a conductor of 1 ohm.

Voyager satellite

The name of two NASA spacecraft designed to study the outer planets (Jupiter, Saturn, Uranus and Neptune) of our Solar System.

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