receives broadcast signals (such as television programs) from points of origin on Earth and then amplifies, encodes, and retransmits these signals to individual end users scattered throughout some wide area or specific region. Many house-
holds around the world now receive numerous television channels directly from space by means of small (typically less than 0.5-meter-diameter) rooftop satellite dishes that are equipped to decode DBS transmissions.
See alsoCOMMUNICATIONS SATELLITE.
direct conversion
The conversion of thermal energy (heat) or other forms of energy (such as sunlight) directly into elec- trical energy without intermediate conversion into mechanical work—that is, without the use of the moving components as found in a conventional electric generator system. The main approaches for converting heat directly into electricity include thermoelectric conversion, thermionic conversion, and magne- tohydrodynamic conversion. Solar energy is directly converted into electrical energy by means of solar cells (photovoltaic conversion). Batteries and fuel cells directly convert chemical energy into electrical energy.See alsoRADIOISOTOPE THERMOELECTRIC GENERATOR.
directed energy weapon
(DEW) A device that uses a tightly focused beam of very intense energy, either in the form of electromagnetic radiation (e.g., light from a laser) or in the form of elementary atomic particles, to kill its target. The DEW delivers this lethal amount of energy at or near the speed of light. Also called a speed-of-light weapon.See alsoBALLISTIC MISSILE DEFENSE;HIGH ENERGY LASER.
directional antenna
Antenna that radiates or receives radio-frequency (RF) signals more efficiently in some direc- tions than in others. A collection of antennas arranged and selectively pointed for this purpose is called a directionalantenna array.
See also DEEPSPACENETWORK;TELECOMMUNICATIONS; TELEMETRY.
direct motion
This term now has several specialized mean- ings in astronomy and orbital mechanics but traces its origins to antiquity and Greek (geocentric) cosmology. First, the term means the apparent west-to-east motion of a planet or other celestial object as seen by an observer on Earth against the celestial sphere—that is, against the fixed star background. In contrast, an apparent motion from east to west is regarded as a retrograde motion. Second, the term describes the anti- clockwise orbital motion (that is, the direct orbit) of a satel- lite around its primary body when the orbiting body is viewed from the north pole of the primary. In this context a clockwise orbital motion represents a retrograde motion. All the planets in the solar system move in direct orbits around the Sun. Third, the term refers to the anticlockwise rotation of a planet on its axis when that planet is viewed from its north pole. All the major planets in the solar system have direct rotation, except for Venus, Uranus, and Pluto, which undergo retrograde (clockwise) rotation.direct readout
The information technology capability that allows ground stations on Earth to collect and interpret the data messages (telemetry) being transmitted from satellites.See also DEEPSPACENETWORK;TELECOMMUNICATIONS; TELEMETRY.
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An artist’s rendering of a direct broadcast satellite (DBS) and the small (typically less than 0.5-meter-diameter) rooftop satellite dish that is equipped to decode DBS transmissions, bringing numerous television channels and other signals into individual households (Artist’s rendering
Discoverer spacecraft
The public (cover story) name given by the U.S. Air Force to the secret Corona photo recon- naissance satellite program. The Discoverer spacecraft series not only led to operational American reconnaissance satellites (for example, in August 1960 Discoverer 14 successfully imaged the Soviet Union from space and then returned its film capsule from orbit), but also achieved a wide variety of space technology advances and breakthroughs—most of which remained shrouded in secrecy until the mid-1990s. The Advanced Research Projects Agency (ARPA) of the Depart- ment of Defense and the U.S. Air Force managed the Discov- erer Program. The primary goal of this program was to develop a film-return photographic surveillance satellite capa- ble of assessing how rapidly the former Soviet Union was producing long-range bombers and ballistic missiles and of locating where these nuclear-armed strategic weapon systems were being deployed. In the mid-1950s President DWIGHTD. EISENHOWERbecame extremely concerned about the growingthreat of a surprise nuclear attack from the Soviet Union. He needed much better information about military activities inside the Soviet Union, so he decided to pursue the develop- ment of spy satellites that could take high-resolution pho- tographs over the Sino-Soviet bloc. He viewed these spy satellites as a desperately needed replacement for the politi- cally embarrassing U2 spy plane flights over denied Soviet territories.
The Discoverer Program was part of the publicly visible portion of the secret Corona satellite program. In addition to providing photographic information for exploitation by the American intelligence community, imagery data from the
Corona satellites were also used to produce maps and charts
for the Department of Defense and other U.S. government mapping programs. At the time, however, the true national security objectives of this important space program were not revealed to the public. Instead, the U.S. Air Force presented the various Discoverer spacecraft launches as being part of an overall research program to orbit large satellites and to test various satellite subsystems. The public news releases also described how different Discoverer spacecraft were helping to investigate the communications and environmen- tal aspects of placing humans in space. To support this cover story, some Discoverer missions even carried biological pack- ages that were returned to Earth from orbit. In all, 38 Dis- coverer satellites were launched from the beginning of the program through February 1962, when the U.S. Air Force quietly ended all public announcements concerning the Dis- coverer Program. However, the Discoverer satellite recon- naissance program, launched from Vandenberg Air Force Base in California, continued in secret until 1972 as Corona. From August 1960 until the 145th and final launch on May 15, 1972, the Corona (Discoverer) Program provided the leaders of the United States with imagery data collected dur- ing many important spy satellite missions. Finally, as pub- licly disclosed in 1995, the American intelligence community also designated the Corona spacecraft by the codename Key-
hole (KH)—for example, personnel within the Central Intel-
ligence Agency (CIA) called the Corona 14 (Discoverer 14) spacecraft a Keyhole 1 (or KH-1) spacecraft, and imagery products from these spy satellites could be viewed only by
intelligence personnel who possessed a special high-security clearance.
The U.S. government declassified numerous Discoverer Program documents in 1995, and so this entry also contains some interesting technical details about selected missions. Without question, the collection of intelligence from Earth- orbiting spacecraft provided national leaders the crucial information they needed to help defuse cold war–era crises and to prevent even the most prickly of superpower disagree- ments (such as the Cuban missile crisis in 1962) from escalat- ing into a civilization-ending nuclear war.
Discoverer 2 was launched by a Thor Agena A rocket
from Vandenberg Air Force Base, California, on April 13, 1959, which placed the spacecraft into a 239-km (perigee) by 346-m (apogee) polar orbit around Earth at an inclination of 89.9 degrees and with an orbital period of 90.4 minutes. The cylindrical satellite was designed to gather spacecraft engi- neering data. The mission also attempted to eject an instru- ment package from orbit for recovery on Earth. The spacecraft was three-axis stabilized and was commanded from Earth. After 17 orbits, on April 14, 1959, a reentry vehicle was ejected. The reentry vehicle separated into two sections. The first section consisted of the protection equip- ment, retrorocket, and main structure; the other section was the reentry capsule itself. U.S. Air Force engineers had planned for the capsule to reenter over the vicinity of Hawaii to support a midair or ocean recovery. However, a timer mal- function caused premature capsule ejection, and it experi- enced reentry over Earth’s north polar region. This test capsule was never recovered. The main instrumentation pay- load remained in orbit and carried out vehicular performance and communications tests.
The Discoverer 2 spacecraft was 1.5 m in diameter, 5.85 m long, and had a mass after second-stage separation, includ- ing propellants, of roughly 3,800 kg. The mass excluding propellants was 743 kg, which included 111 kg for the instrumentation payload and 88 kg for the reentry vehicle. The capsule section of the reentry vehicle was 0.84 m in diameter, 0.69 m long, and held a parachute, test life-support systems, cosmic-ray film packs to determine the intensity and composition of cosmic radiation (presumably as a test for storage of future photographic film), and a tracking beacon. The capsule was designed to be recovered by a specially equipped aircraft during parachute descent but was also designed to float to permit recovery from the ocean. The main spacecraft contained a telemetry transmitter and a tracking beacon. The telemetry transmitter could send more than 100 measurements of the spacecraft performance, including 28 environmental, 34 guidance and control, 18 sec- ond-stage performance, 15 communications, and nine reentry capsule parameters. Nickel-cadmium (NiCd) batteries provid- ed electrical power for all instruments. Orientation was pro- vided by a cold nitrogen gas jet stream system. The spacecraft’s attitude control system also included a scanner for pitch attitude and an inertial reference package for yaw and roll data.
The Discoverer 2 mission successfully gathered data on propulsion, communications, orbital performance, and stabi- lization. All equipment functioned as programmed—except
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the timing device. Telemetry functioned until April 14, 1959, and the main tracking beacon functioned until April 21, 1959. Discoverer 2 was the first satellite to be stabilized in orbit in all three axes, to be maneuvered on command from Earth, to separate a reentry vehicle on command, and to send its reentry vehicle back to Earth.
Discoverer 13 was an Earth-orbiting satellite designed to
test spacecraft engineering techniques and to attempt deceler- ation, reentry through the atmosphere, and recovery from the sea of an instrument package. The U.S. Air Force launched this Discoverer series spacecraft from Vandenberg Air Force Base on August 10, 1960, using a Thor Agena A rocket vehi- cle. This precursor spy satellite went into a 258-km by 683- km polar orbit that had a period of 94 minutes and an inclination of approximately 83 degrees. The cylindrical Agena A stage that placed the spacecraft into orbit carried a telemetry system, a tape recorder, receivers for command sig- nals from the ground, a horizon scanner, and a 55-kg recov- ery capsule, which contained biological specimens. The capsule was a bowl-shaped configuration 0.55 m in diameter and 0.68 m deep. A conical afterbody increased the total length to about 1 meter. A retrorocket, mounted at the end of the afterbody, decelerated the capsule out of orbit. An 18-kg monitoring system in the capsule reported on selected events, such as firing of the retrorocket, jettisoning of the heat shield, and others. The recovery capsule was retrieved on August 11, 1960. This achievement represented the first successful recov- ery of an object ejected from an orbiting satellite. The Agena upper stage rocket reentered the atmosphere and burned up on November 14, 1960.
The Discoverer 13 spacecraft was also known as Corona
13—the first successful reconnaissance satellite mission.
However, Discoverer 13 carried only diagnostic equipment rather than an actual camera/film capsule payload into orbit. On August 18, 1960, a technical sibling, called Discoverer 14 (or Corona 14), carried an actual camera/film capsule pay- load into orbit, imaged large portions of the Soviet Union, and then successfully ejected the film capsule for midair recovery over the Pacific Ocean near Hawaii. With these two successful Discoverer spacecraft missions, the era of the reconnaissance satellite began in August 1960.
See also NATIONAL RECONNAISSANCE OFFICE; RECON- NAISSANCE SATELLITE; UNITEDSTATESAIRFORCE.
dish
Aerospace jargon used to describe a parabolic radio or radar antenna, whose shape is roughly that of a large soup bowl.See also DEEP SPACE NETWORK; RADIO ASTRONOMY; RADIO TELESCOPE.
disk
1. (astronomy) The visible surface of the Sun (or any other celestial body) seen in the sky or through a telescope. 2. (of a galaxy) Flattened, wheel-shaped region of stars, gas, and dust that lies outside the central region (nucleus) of a galaxy.Disney, Walter (Walt) Elias
(1901–1966) American Enter-tainment Industry Visionary, Space Travel Advocate Walt Disney was the American entertainment genius who popular- ized the concept of space travel in the early 1950s, especially
through a widely acclaimed three-part television series. Because of Disney’s commitment to excellence, other space visionaries such as WERNHER VONBRAUNwere able to inspire millions of Americans with credible images of space explo- ration in the mid-1950s.
Born on December 5, 1901, in Chicago, Illinois, this leg- endary American motion picture animator and producer also introduced millions of people to the excitement of space trav- el. While a young boy on his family’s farm near Marceline, Missouri, Disney began his cartooning career by sketching farm animals. When the family returned to Chicago in 1917, Disney attended high school, but evening art classes proved to be his real interest. Then, without graduating from high school, Disney volunteered to serve as a Red Cross ambu- lance driver during World War I.
In 1919 Disney began producing advertising films in Kansas City, Missouri, and eventually turned to animation. Enjoying only limited success in Kansas City, he made an eventful decision in 1923 that would change the entertain- ment industry forever. This decision involved a move to Hol- lywood, California, and the formation of a business partnership with his brother, Roy. In 1928 Walt Disney pro- duced the first animated cartoon to use synchronized sound.
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In the mid-1950s entertainment industry genius Walt Disney (left) collaborated with rocket scientist Wernher von Braun (right) in the development of a well-animated three-part television series that popularized the dream of space travel for millions of Americans.