Index

A

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

Y

Z

 

A

Accretion
Accumulation of dust and gas into larger bodies.
Albedo
Reflectivity of an object; ratio of reflected light to incident light.
Albedo feature
A dark or light marking on the surface of an object that might not be a geological or topographical feature.
Allocthonous
(1) Material that is formed or introduced from somewhere other than the place it is presently found. (2) Fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.
Alpha Centauri
The closest bright star to our solar system.
Angstrom
A unit of length = 1.0E-08cm.
Antipodal Point
The point that is directly on the opposite side of the planet; e.g., the Earth's north pole is antipodal to its south pole.
Antimatter Particles
Antimatter is any substance that, when combined with an equal amount of matter, results in the complete and direct conversion of all substance to energy. Antimatter is composed of antiparticles. Each particle of matter has a corresponding antiparticle of antimatter.
The antiparticle of an electron is known as a positron. It has the same mass as an electron, but opposite electric charge (positive rather than negative). The antiparticle of the proton is called an antiproton. It has the same mass as a proton, but opposite charge (negative rather than positive). The antiparticle of the neutron is an antineutron. It has the same mass as a neutron, and, like the neutron, has no electric charge. However, the relationship between two properties called spin and magnetic moment is different in the antineutron than in the neutron.

Antiparticles have been found in cosmic rays, and are also produced in particle accelerators (also known as atom smashers). An antiparticle never lasts for very long, because shortly after it forms, it encounters one of its equivalent matter particles, and both are annihilated, yielding pure energy.

Artificially produced antimatter has practical applications, especially in medical science, where it is involved in the operation of positron emission tomography (PET) scanning equipment. Antimatter has been suggested as a possible propellant for spacecraft, a source of energy for public consumption, and as material for a doomsday bomb that would vaporize the earth. However, all these potential applications require the isolation of large amounts of antimatter, and the technology for this does not yet exist.
Antipattern
An antipattern is a frequently used, but largely ineffective solution to a problem. The term was originally used to refer to a pattern gone wrong. Just as a viable pattern describes the way from a problem to a valid solution, an antipattern describes the way from a problem to a poor solution. Furthermore, by adding more difficulties to the ones that originally existed, an antipattern may leave you in a worse position than before you started. Several writers have published books or papers on how to recognize and avoid adopting an antipattern, especially in developing computer programming.
The term amelioration pattern (to ameliorate means to improve, and especially to improve a bad situation) was coined for a pattern that describes how to go from a bad solution to a better one.
Aphelion
The point in its orbit where a planet is farthest from the Sun.
Apoapsis
The point in orbit farthest from the planet.
Apogee
The point in orbit farthest from the Earth.
Ash
The fine-grained material produced by a pyroclastic eruption. An ash particle is defined to have a diameter of less than 2 millimetres.
Asteroid Number
Asteroids are assigned a serial number when they are discovered; it has no particular meaning except that asteroid N+1 was discovered after asteroid N.
Astronomical Unit (AU)
The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometres (92,960,116 miles).
Atom
An atom is a particle of matter that uniquely defines a chemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged. The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons. A proton is positively charged. The number of protons in the nucleus of an atom is the atomic number for the chemical element. A proton has a rest mass, denoted mp, of approximately 1.673 x 10-27 kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted mn, of approximately 1.675 x 10-27 kg. The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator.
An early model of the atom was developed by the physicist Ernest Rutherford in 1912. He was the first to suggest that atoms are like miniature solar systems, except that the attractive force is not caused by gravity, but by opposing electrical charges. In the so-called Rutherford atom, electrons orbit the nucleus in circular paths. Niels Bohr revised Rutherford's theory in 1913. In the Bohr atom, the negatively charged electrons orbit the nucleus at specific median distances. These distances are represented by spheres, called shells, surrounding the nucleus. Electrons can move from shell to shell. When an electron absorbs enough energy, it moves to a larger, or higher, shell. When it loses a certain amount of energy, it falls to a smaller, or lower, shell.
The total mass of an atom, including the protons, neutrons and electrons, is the atomic mass or atomic weight. Electrons contribute only a tiny part of this mass. For most practical purposes, the atomic weight can be thought of as the number of protons plus the number of neutrons. Because the number of neutrons in an atom can vary, there can be several different atomic weights for most elements.
Atoms having the same number of protons, but different numbers of neutrons, represent the same element, but are known as different isotopes of that element. The isotope for an element is specified by the sum of the number of protons and neutrons. Examples of different isotopes of an element are carbon 12 (the most common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon).
Protons and electrons have equal and opposite charge, and normally an atom has equal numbers of both. Thus, atoms are usually neutral. An ion is an atom with extra electrons or with a deficiency of electrons, resulting in its being electrically charged. An ion with extra electrons is negatively charged and is called an anion; an ion deficient in electrons is positively charged and is called a cation.
Atmosphere
One atmosphere is 14.7 pounds per square inch (105 Newtons per square meter); the average atmospheric pressure at sea level on Earth.
Atmospheric Bubble
Used in interstellar travel for protection against hostile atmospheres.
Aurora
A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.
Aurora Borealis
The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.

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