hostile missile in flight.
See alsoBALLISTIC MISSILE DEFENSE; SPRINT.
antimissile missile 33
An early U.S. antimissile missile called the Sprint being tested at the White Sands Missile Range in 1967 (Courtesy of the U.S. Army/White
antiparticle
Every elementary particle has a corresponding (or hypothetical) antiparticle, which has equal mass but opposite electrical charge (or other property, as in the case of the neutron and antineutron). The antiparticle of the electron is the positron; of the proton, the antiproton; and so on. However, the photon is its own antiparticle. When a particle and its corresponding antiparticle collide, their masses are converted into energy in a process called annihilation. For example, when an electron and positron collide, the two par- ticles disappear and are converted into annihilation radiation consisting of two gamma ray photons, each with a character- istic energy level of 0.511 million electron volts (MeV).See also ANNIHILATION RADIATION; ANTIMATTER; ELE-
MENTARY PARTICLE(S); GAMMA RAYS.
antiradiation missile
(ARM) A missile that homes pas- sively on a radiation source, such as an enemy radar signal.antisatellite spacecraft
(ASAT) A spacecraft designed todestroy other satellites in space. An ASAT spacecraft could be deployed in space disguised as a peaceful satellite that quietly lurks as a secret hunter/killer satellite, awaiting instructions to track and attack its prey. In-orbit testing of ASAT systems could litter near-Earth space with large quantities of space debris.
antislosh baffle
A device provided in a propellant tank of a liquid fuel rocket to dampen unwanted liquid motion, or sloshing, during flight. This device can take many forms, including a flat ring, truncated cone, and vane.See alsoROCKET.
antisubmarine rocket
(ASROC) A surface ship–launched,rocket-propelled, nuclear depth charge or homing torpedo.
antivortex baffles
Assemblies installed in the propellant tanks of liquid fuel rockets to prevent gas from entering the rocket engine(s). The baffles minimize the rotating action as the propellants flow out the bottom of the tanks. Without these baffles, the rotating propellants would create a vortex similar to a whirlpool in a bathtub drain.See alsoROCKET.
apareon
That point on a Mars-centered orbit at which a satellite or spacecraft is at its greatest distance from the plan- et. The term is analogous to the term APOGEE.apastron
1. The point in a body’s orbit around a star at which the object is at a maximum distance from the star. 2. Also, that point in the orbit of one member of a binary star system at which the stars are farthest apart. Compare withPERIASTRON.
aperture
1. The opening in front of a telescope, camera, or other optical instrument through which light passes. 2. The diameter of the objective of a telescope or other optical instrument. 3. Concerning a unidirectional antenna, that por- tion of a plane surface near the antenna, perpendicular to the direction of maximum radiation, through which the major portion of the electromagnetic radiation passes.aperture synthesis
A resolution-improving technique in radio astronomy that uses a variable aperture radio interfer- ometer to mimic the full dish size of a huge radio telescope.See also RADAR ASTRONOMY; RADAR IMAGING; RADIO ASTRONOMY.
apex
The direction in the sky (celestial sphere) toward which the Sun and its system of planets appear to be moving relative to the local stars. Also called the solar apex, it is located in the constellation of Hercules.aphelion
The point in an object’s orbit around the Sun that is most distant from the Sun. Compare withPERIHELION.Aphrodite Terra
A large, fractured highland region near the equator of Venus.See also BETAREGIO; ISHTARTERRA; VENUS.
apoapsis
That point in an orbit farthest from the orbited central body or center of attraction.See alsoORBITS OF OBJECTS IN SPACE.
apocynthion
That point in the lunar orbit of a spacecraft or satellite launched from Earth that is farthest from the Moon. Compare withAPOLUNE.apogee
1. In general, the point at which a missile trajectory or a satellite orbit is farthest from the center of the gravita- tional field of the controlling body or bodies. 2. Specifically, the point that is farthest from Earth in a geocentric orbit. The term is applied to both the orbit of the Moon around Earth as well as to the orbits of artificial satellites around Earth. At apogee, the velocity of the orbiting object is at a minimum.34 antiparticle
This artist’s rendering depicts the on-orbit testing of a Soviet antisatellite weapon system. In the 1980s the Soviet Union tested and operated an orbital antisatellite weapon, a killer spacecraft designed to destroy space targets by orbiting near them and then releasing a multipellet blast. The cloud of high-speed pellets would destroy the target spacecraft.
(Courtesy of the U.S. Department of Defense/Defense Intelligence Agency; artist Ronald C. Wittmann, 1986)
To enlarge or “circularize” the orbit, a spacecraft’s thruster (i.e., the apogee motor) is fired at apogee, giving the space vehicle and its payload necessary increase in velocity. Oppo- site of PERIGEE. 3. The highest altitude reached by a sounding
rocket launched from Earth’s surface.
See alsoORBITS OF OBJECTS IN SPACE.
apogee motor
A solid propellant rocket motor that is attached to a spacecraft and fired when the deployed space- craft is at the apogee of an initial (relatively low-altitude) parking orbit around Earth. This firing establishes a new orbit farther from Earth or permits the spacecraft to achieve escape velocity. Also called apogee kick motor and apogee rocket.Apollo group
A collection of near-Earth asteroids that have perihelion distances of 1.017 astronomical units or less, taking them across the orbit of Earth around the Sun. This group acquired its name from the first asteroid to be discovered, Apollo, in 1932 by the German astronomer KARLREINMUTH.See alsoASTEROID;NEAR-EARTH ASTEROID.