Glossary:
A and B

A

Advanced Composition Explorer (ACE)
Spacecraft launched August 25, 1997 to study the energy and composition of particles coming from the Sun, interplanetary,, interstellar, and galactic sources. More about ACE...
AccelerationThe increase in velocity over time.More about acceleration...
Alpha particleHelium nucleus: two protons, two neutrons, NO electrons. Alpha particles are ions.
Anomalous cosmic rays (ACRs)ACRs are pick-up ions that are tossed around in and out of the solar wind termination shock until some gain energy and are thrown back inside the heliosphere. The ones that are thrown in are then called ACRs. They are a type of cosmic ray. More about ACRs...
AntimatterMatter composed of the antiparticles of normal matter. A particle and its antiparticle have a number of opposite properties. A proton is electrically positive; an antiproton is electrically negative. When a particle and its antiparticle meet, they annihilate into pure energy. The antiparticle and particle have exactly the same mass and so are affected by gravity in the same way.
APL The Johns Hopkins University Applied Physics Laboratory in Baltimore, MD.
Apollo missions
NASA's Apollo space program ran from 1961 to 1975. The mission goal and accomplishment was to put man on the moon.

AsteroidMetallic, rocky objects without atmospheres that orbit the Sun, but are too small to be called planets. A group of these objects make up the asteroid belt between Mars and Jupiter. Scientist believe that asteroids are pieces of the material that formed the solar system.
AstronomyThe scientific study of the universe around us, everything outside of the Earth's atmosphere.
AstrophysicsThe part of astronomy dealing with the physics and chemistry of astronomical objects and events.
AtmosphereThe mixture of gases that surround a planet, moon or star, held near it by gravity.
AtomA basic unit of matter. It is the smallest particle of an element that still has the characteristics of that element. Every atom has a positively charged central nucleus, surround by a number of negatively charged electrons. More about atoms...
Atomic NumberThe atomic number of an element indicates its place in the periodic table of the elements. It is the number of protons in the nucleus of one of its atoms. Periodic table of the elements...
Atomic Weight Synonym for atomic mass. Atomic weight is the relative mass of an atom of the given element compared to the mass of carbon-12. More about atomic weight...
AUAstronomical Unit. It is a unit of distance equal to the average distance from the Earth to the Sun, about 150 million kilometers (93 million miles).
Aurora(plural = aurorae) The light radiated by ions and atoms in Earth's upper atmosphere, in the region of the Earth's poles. Those that occur during magnetic storms can create extremely impressive spectacles. The Aurora australis, or "Southern Lights" occur near the South Pole while the Aurora borealis, or "Northern Lights" occur near the North Pole. More about auroras...
Axis(plural = axes) In astronomy, the imaginary line through the poles about which a body (planet, star, moon, etc.) rotates.

B

BaryonProtons and neutrons
Black holeA region of space where matter has been collapsed to the point where gravity is so strong that nothing, not even light, entering that area of space can escape. More about black holes...
Bow shockThe supersonic shock wave on the sunward side of the Earth caused by the blast of the solar wind against the Earth's magnetosphere. Solar wind is slowed to subsonic speeds and heated very rapidly as it arrives at the bow shock.