Science [A-B]
Aberration
Property of an optical system that causes an image to have certain easily recognisable flaws. Aberrations are caused by geometrical factors such as the shapes of surfaces, their spacing, and alignments. Image problems caused by factors such as scratches or contamination are not called aberrations.
Absolute zero
The lowest temperature ever reachable in the Universe: 0 Kelvin (0K), equivalent to minus 273 degrees Celsius (-273 oC). In laboratories on Earth physicists can get very close to that temperature, but it is impossible to achieve the absolute zero.
Absorption
Decrease in intensity of radiation, when it crosses a material medium, as a consequence of an interaction between the radiation and the material medium.
Abundance
Relative number of atoms of a particular element, or isotope of an element, in the chemical composition of a single substance or object.
Accelerometer
Oscillatory mechanical system measuring the acceleration of the body to which it is attached.Accretion (disk, zone)
Process whereby small particles of matter accumulate and create larger bodies under the influence of their mutual gravitational attraction or as a result of chance collisions.
Apogee
The most distant point from Earth on a satellite's orbit.
Arcmin, arcsec
The size of an object in the sky can be measured by the angle that it covers when viewed from Earth. The full circle has 360 degrees. An arcmin is 1/60 of a degree; an arcsec is 1/60 of an arcmin or 1/3600 of a degree. The diameter of the full Moon is about one-half of a degree or 30 arcmin.
Argon
A chemical element, (symbol Ar, atomic number 18).
Ariane (4, 5) rockets
European launcher family (Ariane 4 and Ariane 5) developed by the European Space Agency. Launched from Kourou, French Guiana, flights are commercialised and operated by the Arianespace company.
Asteroid
One of billions of rocky objects, less than 1000 km in diameter, which orbit the Sun. Also known as minor planets. Thought to be planetesimals leftover from the formation of the planets. The first asteroid (Ceres) was discovered by Giuseppe Piazzi in 1801. More than 10 000 asteroids have so far been discovered and given permanent identification numbers. The largest asteroid is 2001 KX76 with a diameter of at least 1200 km.
Asteroid belt
Region between the orbits of Mars and Jupiter which is populated by billions of asteroids.
Astigmatism
Failure of an optical system, such as a lens or a mirror, to image a point source of light as a single point.
Astrometry
The branch of astronomy concerned with measuring the positions of celestial bodies, such as stars and galaxies, and their real and apparent motions.
Attitude
Orientation of the spacecraft's axes relative to Earth.
Astronomical Unit (AU)
1 Astronomical Unit corresponds to the distance separating the Earth from the Sun. 1AU=150 million km.
Astronomy
Study of the space beyond the Earth and of its contents.
Astrophysics
Study of the physical nature of the Universe, its objects and the composition of the space between them.
Atmosphere
Layer of gases surrounding a star or planet.
Aurora
Illumination of the night sky, caused when electrons and protons from space collide with atoms and molecules of air in the Earth's upper atmosphere. Satellite observations usually show them as rings centred on the planet's magnetic poles. Popularly known as the Northern and Southern Lights. Various types of aurora are also found on Jupiter, its moon Ganymede, and Saturn.
Baffle
A device used to prevent stray light.
Baryons
The class of subatomic particles in which protons and neutrons are included. The baryons form the atomic nucleus, along with another class of particles - the mesons. We, and everything we can see around us, are made of baryonic matter.
BepiColombo mission
ESA's cornerstone mission to the planet Mercury due to be launched in 2008.
BeppoSAX satellite
An Italian-Dutch X-ray astronomy satellite launched in 1996.
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