All About Artificial Satellite

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Artificial Satellite
I INTRODUCTION
Artificial Satellite, any object purposely placed into orbit around Earth, other planets, or the Sun. Since the launching of the first artificial satellite in 1957, thousands of these “man-made moons” have been rocketed into Earth orbit. Today, artificial satellites play key roles in the communications industry, in military intelligence, and in the scientific study of both Earth and outer space. See also Space Exploration.
II TYPES OF SATELLITES
Engineers have developed many kinds of satellites, each designed to serve a specific purpose or mission. For instance the telecommunications and broadcasting industries use communications satellites to carry radio, television, and telephone signals over long distances without the need for cables or microwave relays. Navigational satellites pinpoint the location of objects on Earth, while weather satellites help meteorologists forecast the weather (see Meteorology). The United States government uses surveillance satellites to monitor military activities (see Remote Sensing). Scientific satellites serve as space-based platforms for observation of Earth, the other planets, the Sun, comets, and galaxies, and are useful in a wide variety of other applications.
A Communications Satellites
Almost all of the earliest satellites included some communications equipment. The National Aeronautics and Space Administration (NASA) launched the first telephone and television satellite, AT&T’s Telstar 1, in 1962. The U.S. Department of Defense launched Syncom 3 in 1964. Syncom 3 was the first communication satellite to use a geostationary orbit—that is, an orbit that keeps the satellite over the same spot above Earth’s equator. Over 300 communications satellites have been launched since 1957. Today satellites in geostationary orbit provide voice, data, and television communications, including the direct broadcast of television to homes around the world.
B Navigation Satellites
Navigation satellites can help locate the position of ships, aircraft, and even automobiles that are equipped with special radio receivers. A navigation satellite sends continuous radio signals to Earth. These signals contain data that a special radio receiver on Earth translates into information about the satellite’s position. The receiver further analyzes the signal to find out how fast and in what direction the satellite is moving and how long the signal took to reach the receiver. From this data, the receiver can calculate its own location. Some navigation satellite systems use signals from several satellites at once to provide even more exact location information.
The U.S. Navy launched the first navigation satellite, Transit 1B, in 1960. The United States ended its support of the Transit system in 1996. The U.S. Air Force operates a system, called the NAVSTAR Global Positioning System (GPS), that consists of 24 satellites. Depending on the type of receiver and the method used, GPS can provide position information with an accuracy from 100 m (about 300 ft) to less than 1 cm (less than about 0.4 in). The Global Orbiting Navigation Satellite System (GLONASS) of the Russian Federation consists of 24 satellites and provides accuracy similar to GPS. In December 2005 the European Union (EU) launched the first of 30 satellites that will make up a civilian satellite navigation system called Galileo. The system will have an accuracy of about 1 m (3.3 ft) and will become operational in 2009. Also known as the Global Navigation Satellite System (GNSS), the European system is designed to be compatible with those of the United States and Russia, allowing receivers around the world to communicate with satellites in any of the three systems. The United States also negotiated an agreement with the EU that will allow it to scramble signals over a battlefield or military target without shutting down the entire system. The two systems will compete economically, however, for commercial applications.
C Weather Satellites
Weather satellites carry cameras and other instruments pointed toward Earth’s atmosphere. They can provide advance warning of severe weather and are a great aid to weather forecasting. NASA launched the first weather satellite, Television Infrared Observation Satellite (TIROS) 1, in 1960. TIROS 1 transmitted almost 23,000 photographs of Earth and its atmosphere. NASA operates the Geostationary Operational Environmental Satellite (GOES) series, which are in geostationary orbit. GOES provides information for weather forecasting, including the tracking of storms. GOES is augmented by Meteosat 3, a European weather satellite also in geostationary orbit. The National Oceanic and Atmospheric Administration (NOAA) operates three satellites that collect data for long-term weather forecasting. These three satellites are not in geostationary orbit; rather, their orbits carry them across the poles at a relatively low altitude.
D Military Satellites
Many military satellites are similar to commercial ones, but they send encrypted data that only a special receiver can decipher. Military surveillance satellites take pictures just as other earth-imaging satellites do, but cameras on military satellites usually have a higher resolution.
The U.S. military operates a variety of satellite systems. The Defense Satellite Communications System (DSCS) consists of five spacecraft in geostationary orbit that transmit voice, data, and television signals between military sites. The Defense Support Program (DSP) uses satellites that are intended to give early warning of missile launches. DSP was used during the Persian Gulf War (1991) to warn of Iraqi Scud missile launches.
Some military satellites provide data that is available to the public. For instance, the satellites of the Defense Meteorological Satellite Program (DMSP) collect and disseminate global weather information. The military also maintains the Global Positioning System (GPS), described earlier, which provides navigation information that anyone with a GPS receiver can use.
E Scientific Satellites
Earth-orbiting satellites can provide data to map Earth, determine the size and shape of Earth, and study the dynamics of the oceans and the atmosphere. Scientists also use satellites to observe the Sun, the Moon, other planets and their moons, comets, stars, and galaxies. The Hubble Space Telescope is a general-purpose observatory launched in 1990. Some scientific satellites orbit bodies other than Earth. The Mars Global Surveyor, for example, orbits the planet Mars.
III SATELLITE LAUNCHES
Placing a satellite into orbit requires a tremendous amount of energy, which must come from the launch vehicle, or device that launches the satellite. The satellite needs to reach an altitude of at least 200 km (120 mi) and a speed of over 29,000 km/h (18,000 mph) to lift into orbit successfully. Satellites receive this combination of potential energy (altitude) and kinetic energy (speed) from multistage rockets burning chemical fuels.
The first stage of a multistage rocket consists of rocket engines that provide a huge amount of force, or thrust. The first stage lifts the entire launch vehicle—with its load of fuel, the rocket body, and the satellite—off the launch pad and into the first part of the flight. After its engines use all their fuel, the first stage portion of the rocket separates from the rest of the launch vehicle and falls to Earth. The second stage then ignites, providing the energy necessary to lift the satellite into orbit. It, too, then separates from the satellite and any remaining rocket stages.
The rest of the launch depends on the satellite’s mission. For example, if the mission requires a geostationary orbit, which can be achieved only at a distance of about 35,000 km (22,000 mi) above Earth, a third rocket stage provides the thrust to lift the satellite to its final orbital altitude. After the satellite has reached the final altitude, another rocket engine fires and gives the satellite a circular orbit. All rocket-engine burns occur at a precise moment and last for a precise amount of time so that the satellite achieves its proper position in space.
In 1990 the United States began launching some satellites from aircraft flying at high altitudes. This method still requires a rocket-powered launch vehicle, but because the vehicle does not have to overcome friction with the thick atmosphere found at low altitudes, much less fuel is needed. However, the size of the rocket is limited by the size and strength of the aircraft, so only smaller satellites can be launched this way.
Another method of launching satellites is to have astronauts launch them from the U.S. space shuttle. The space shuttle can carry large satellites and, because the shuttle is already in orbit when the satellite is launched, the astronauts can verify that the satellite has survived the rigors of launch. The space shuttle can also bring satellites back to Earth for repair, or astronauts can repair satellites in space.
The Single Stage to Orbit (SSTO) is a launch vehicle that may lower the cost of launching satellites by decreasing the number of launch stages needed and increasing the reusability of launch vehicles. The SSTO would be a piloted vehicle like the space shuttle, but it would be designed to launch satellites more inexpensively and efficiently than the space shuttle can.
IV OPERATIONS IN SPACE
Because satellites must survive the launch and must operate in the harsh environment of space, they require unique and durable technologies. Satellites have to carry their own power source because they cannot receive power from Earth. They must remain pointed in a specific direction, or orientation, to accomplish their mission. Satellites need to maintain proper temperature in the face of direct rays from the Sun and in the cold blackness of space. They must also survive high levels of radiation and collisions with micrometeoroids (see Meteoroid). Most satellites have onboard computers that help with satellite operations and with the satellite’s mission.
A Power
A satellite provides its own power for the duration of its mission, which can extend to ten years or more. The most common source of power for Earth-orbiting satellites is a combination of solar cells (see Solar Energy) with a battery backup. Solar cells need to be large enough to provide the power that the satellite requires. For example, the solar array of the complex Hubble Space Telescope is about 290 sq m (about 3,120 sq ft) in area and generates about 5,500 watts of electricity, while the solar array of a smaller Global Positioning System satellite is about 4.6 sq m (about 50 sq ft) in area and generates about 700 watts of electricity. Solar cells are often mounted on winglike panels that unfold from the body of the satellite after it reaches its final orbit. Batteries provide power before the solar panels are deployed and when sunlight does not reach the solar panels.
B Orientation
A satellite’s orientation is the direction each of its sides faces. The satellite keeps the solar panels pointed toward the Sun. In addition, the satellite’s antennas and sensors point toward Earth or toward the object the satellite is observing. For example, communications and weather satellites have antennas and cameras pointed earthward, while space telescopes are pointed toward the astronomical objects that scientists wish to study. Methods of maintaining orientation include small rocket engines, known as attitude thrusters; large spinning wheels that turn the satellite; and magnets that interact with Earth’s magnetic field to correctly orient the satellite. Attitude thrusters can make large changes to orientation quickly, but they are not the best solution when the stability of the turn is critical. Attitude thrusters also require fuel, so the lifetime of the satellite depends on a limited supply of fuel for the thrusters. A spinning wheel on a satellite acts as a gyroscope. The rotational motion of the wheel makes the satellite stay in one orientation, and changing the rotational motion will cause the satellite to turn. Spinning wheels and magnets are slower than thrusters but are excellent for attitude stability and require only electric power.
C Heat Dissipation
As it orbits Earth, a satellite encounters intense heat and intense cold as it alternately faces or is hidden from the Sun. The electronic equipment on the satellite also creates heat that can cause damage. On Earth, convection, conduction, or radiation of heat can transfer heat (see Heat Transfer). With no air flowing over the satellite to transfer heat by convection and no body to which the satellite can conduct heat, the satellite must radiate heat to control temperature. Often satellites use radiators in the form of louvered panels, including panels that open and close to adjust the amount of radiating surface area. To prevent the direct rays of the Sun from causing hot spots, the satellite may spin or rotate to distribute the Sun’s heat more evenly.
D Cosmic Radiation and Micrometeoroid Protection
Satellites have to endure the effects of radiation and of continuous, damaging micrometeoroid hits, especially during long-term missions. Earth’s atmosphere blocks most cosmic radiation from affecting microprocessors in computers on the ground. A satellite, however, needs shielding for its computers. Radiation from space also causes some materials to become brittle, so parts of satellites break more easily after long exposure to the electromagnetic radiation of space. Solar panels gradually produce less and less power because of damage from radiation effects and from the impact of micrometeoroids.
V REENTRY AND SATELLITE DISPOSAL
Satellites reach the end of their useful lives when they reenter Earth’s atmosphere or their instruments fail. Many satellites eventually fall out of orbit and burn up as they reenter the atmosphere. Others continue to orbit as “space junk” long after their instruments have ceased working. Sometimes the onboard rockets are purposely fired to slow a satellite and cause it to reenter Earth’s atmosphere. This technique is usually limited to satellites with equipment packages intended for recovery. Such satellites have shields that enable them to withstand the intense heat of reentry.
A Orbit Decay and Reentry
A satellite that orbits within a few hundred miles of Earth’s surface experiences friction from the thin atmosphere that exists at those altitudes. Eventually the satellite’s altitude will decrease until atmospheric friction causes the satellite to plunge earthward out of orbit. The lifetime of a satellite depends on its orbit, the satellite’s orientation in its orbit, and the size, shape, and weight of the satellite. A large, light satellite will probably reenter Earth’s atmosphere sooner than a small, heavy satellite that orbited at the same altitude, because the large satellite has more surface area and experiences more atmospheric friction. At an orbital altitude of 200 km (120 mi), a satellite will likely last from a week to three months. At 300 km (190 mi), a satellite may stay in orbit for two years or more. Satellites that orbit above 1,000 km (620 mi) will stay aloft for thousands of years.
B Disposal of Satellites
The space around Earth seems boundless, but space operations tend to take place in a limited number of preferred types of orbits. The U.S. Air Force tracks satellites and other objects within these orbits so that other satellites and piloted vehicles can avoid collisions with the objects. Radio interference between satellites can also present spacing problems. Many satellites share a limited area, called the geostationary corridor, where a satellite’s orbit takes it around Earth at the same rate that Earth rotates. Satellites in this area have to maintain certain separation distances, so that the radio signals sent to one satellite do not interfere with the signals sent to nearby satellites. A final rocket thrust is sometimes used to put old satellites into less-desirable orbits to make room for newer satellites.
VI SATELLITE ORBITS
The defining characteristics of an orbit are its shape, its altitude, and the angle it makes with Earth’s equator. A satellite’s controllers choose an orbit with a particular combination of shape, altitude, and angle that will best serve the satellite’s mission. Most orbits are circular, but some satellites use elliptical orbits—that is, orbits in which the satellite’s distance from Earth is not constant. The altitude of an orbit determines how long the satellite takes to circle Earth and how much of the planet is visible to the satellite at one time. Satellites pass over different ranges of Earth’s latitude depending on the angle of their orbits with respect to the equator. Some satellites orbit along the equator. Satellites that pass over high northern and southern latitudes have orbits that form a large angle to the equator. Some satellites move clockwise around Earth as seen from the North Pole, but most satellites move counterclockwise around Earth.
A Geostationary Equatorial Orbit
Satellites in geostationary equatorial orbit (GEO) orbit Earth around the equator at a very specific altitude that allows them to complete one orbit in the same amount of time that it takes Earth to rotate once. As a result, these satellites stay above one point on Earth’s equator at all times. The altitude of GEO is about 5.6 times the radius of Earth, or about 35,800 km (about 22,200 mi).
Direct-broadcast television satellites are in GEO. A few satellites in GEO can provide coverage for the entire Earth, and antennas do not need to track the satellite to receive a signal. Earth-surveillance missions, including military surveillance and weather tracking missions, also use GEO.
B Low Earth Orbit
A satellite in low Earth orbit (LEO) orbits at an altitude of 2,000 km (1,200 mi) or less. Almost every satellite enters a LEO after it is launched. If a satellite’s mission requires an orbit other than LEO, it uses rockets to move into its final orbit.
A low Earth orbit minimizes the amount of fuel needed. In addition, a satellite in LEO can obtain clearer surveillance images and can avoid the Van Allen radiation belts, which contain harmful high-energy particles. It needs less powerful signals to communicate with Earth than satellites with higher orbits. A signal to or from a low Earth orbit also reaches its destination more quickly, making LEO satellites especially good for transmitting data.
C Medium Earth Orbit
Medium Earth orbit (MEO) satellites orbit at an altitude about 10,000 km (about 6,000 mi) and balance the benefits and problems between LEO and GEO. The most common uses of MEO are by navigation and communication satellites. The U.S. navigation system NAVSTAR Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), and Odyssey, a private U.S. communications satellite program, all use MEO.
D Polar Orbits
Satellites in polar orbits orbit around Earth at right angles to the equator over both the North and South poles. Polar orbits can occur at any altitude, but most satellites in polar orbits use LEOs. Two polar satellites belonging to the U.S. National Oceanic and Atmospheric Administration provide weather information for all areas of the world every six hours. The satellites also map ozone levels (see Ozone Layer) in the atmosphere, including the level over the poles. Landsat is a U.S. government remote-sensing satellite system that operates in polar orbit. Scientists often use Landsat to view agricultural phenomena such as deforestation and crop blight. Transit, the first satellite-based navigation system, used polar orbits in order to support navigation around the world, especially for submarines in the polar regions.
E Sun-Synchronous Orbits
A satellite in a Sun-synchronous orbit always passes over a certain point of Earth when the Sun is at the same position in Earth’s sky. A Sun-synchronous satellite has a retrograde orbit (it moves clockwise around Earth), orbits in a low Earth orbit, and orbits at a specific angle with respect to Earth’s equator (about 98°). The satellite crosses each latitude about 1° east of where it crossed the latitude the previous day. Thus, the satellite stays synchronized with the location of the Sun relative to Earth. Sun-synchronous orbits are useful for satellites photographing Earth, because the Sun will be at the same angle each time the satellite passes over a point on Earth.
VII THE FIRST SATELLITES
The first artificial satellite to orbit Earth was Sputnik 1. Built by the Soviet Union and launched on October 4, 1957, Sputnik had an elliptical orbit, ranging in altitude from 225 to 950 km (140 to 590 mi). Sputnik broadcast a steady signal of beeps for 21 days and burned up in Earth’s atmosphere upon reentry on January 4, 1958.
The Soviet Union also launched the first living creature, a dog named Laika, into space on November 3, 1957. Laika flew inside a pressurized chamber aboard the satellite Sputnik 2. She died from overheating and panic after a few hours in orbit. Sputnik 2 reentered Earth’s atmosphere and burned up on April 14, 1958.
The United States launched its first satellite, Explorer 1, on January 31, 1958. Explorer 1 had a highly elliptical orbit, ranging in altitude from 360 to 2,500 km (220 to 1,600 mi). Scientists discovered the Van Allen radiation belts using data transmitted back to Earth from Explorer 1.
On August 10, 1960, the United States launched a surveillance satellite, Discoverer 13, that carried the first artificial object ever retrieved from space. While Discoverer 13 remained in orbit it ejected a capsule earthward, which was then recovered by a team from the U.S. Navy. Later satellites carried cameras that photographed parts of Earth and then ejected recoverable containers of the exposed film toward Earth.

Contributed By:
Leonard R. Kruczynski
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

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Some Facts About Voyager

Posted in Labels: | 12:30 AM



I INTRODUCTION

Voyager, name of two United States spacecraft, launched in 1977, designed to explore the giant outer planets of the solar system and to venture beyond the solar system. The Voyagers took advantage of a rare configuration of the outer planets. This alignment enabled the spacecraft to use the gravitational pull of each planet like a slingshot. As the spacecraft flew by each planet, its gravitational force hurled the spacecraft on to the next planet. As a result the Voyagers never had to rely on large on-board propulsion systems. Voyager 1 visited Jupiter and Saturn, while Voyager 2 flew by Jupiter, Saturn, Uranus, and Neptune, the solar system’s four giant outer planets. Only Pluto, a relatively small outer planet, was not included in the Voyagers’ tour.
Both Voyager spacecraft were launched from the National Aeronautics and Space Administration (NASA) Kennedy Space Center at Cape Canaveral, Florida, by a Titan Centaur rocket. The two-stage rocket boosted the spacecraft into a low orbit around Earth and catapulted the craft toward Jupiter. Voyager 2 was launched first, on August 20, 1977. Voyager 1 was launched on a faster, shorter path on September 5, 1977.
Voyager 1 made its closest approach to Jupiter on March 5, 1979, and Voyager 2 followed on July 9, 1979. Voyager 1 flew past Saturn on November 12, 1980, and Voyager 2 flew past on August 25, 1981. After its Saturn flyby, Voyager 1 continued northward out of the ecliptic plane (the plane in which most of the planets' orbits lie). Voyager 2 passed Uranus on January 24, 1986, and Neptune on August 25, 1989, and then flew southward on its way to interstellar space.
II SPACECRAFT
The Voyager spacecraft were identical. Each weighed 815 kg (1,797 lb), and each was built from 65,000 individual parts. The spacecraft were designed with radiation-hardened parts, and the more sensitive parts were shielded to withstand radiation.
A set of 16 thrusters allowed each spacecraft to control attitude and correct trajectory. Each of these tiny assemblies produced a thrust of only 0.85 newton (3 oz) to allow the craft to perform delicate maneuvers. Because the spacecraft were to travel far from the Sun, they could not use solar panels to collect sunlight for power. Instead they used radioisotope thermoelectric generators, which produce electricity from the heat resulting from the radioactive decay of plutonium. The generators power the instruments, computer, radio, and other systems. The spacecraft are controlled and their data returned to Earth through the Deep Space Network, a global spacecraft-tracking system.
The Voyager spacecraft were equipped with instruments to conduct ten different scientific experiments. The instruments included television cameras, infrared and ultraviolet sensors, magnetometers, plasma detectors, and cosmic-ray and charged-particle sensors. In addition, the spacecraft radios were used to gather information about planetary atmospheres and ring systems.
A message was placed aboard the Voyager spacecraft to communicate with any extraterrestrial beings that might come into contact with the probes. The message was carried on a 30.5-cm (12-in) gold-plated copper phonograph record containing 115 images and a variety of natural sounds selected to portray the diversity of life forms and cultures on Earth. Musical selections from different cultures and eras, spoken greetings in 55 languages, and printed messages from Jimmy Carter, former president of the United States, and from Kurt Waldheim, former United Nations secretary general, were also included.
III SCIENTIFIC HIGHLIGHTS
The Voyager spacecraft provided much new information about the outer planets. The probes revealed that the Great Red Spot of Jupiter is actually a complex swirling storm. They also found an array of smaller storms and eddies throughout the banded clouds of the planet. Nine active volcanoes were discovered on the large Jovian moon Io—the first volcanoes found on another body in the solar system. The spacecraft also found a faint, dusty ring of material around Jupiter beginning at about 99,000 km (about 61,500 mi) and extending outward about 30,000 km (about 18,600 mi).
Data transmitted from the spacecraft as they flew by Saturn revealed atmospheric features similar to those of Jupiter, although smaller. The large Saturnian moon Enceladus was found to be marked by huge faults and valleys. The spacecraft revealed a crater on the moon Mimas so large that the impact that caused it must have nearly broken the moon apart. In Saturn's rings unexpected structures such as kinks and radial spokes were discovered, along with both very thin rings and broad, diffuse rings that had not been observed from Earth.
During its Uranus flyby, Voyager 2 found a previously undetected magnetic field and a high haze layer over the planet's sunlit pole. The craft also found ten moons that had not been detected before. On the innermost of the planet's large moons, Miranda, Voyager 2 discovered fault canyons as deep as 20 km (12 mi), terraced layers, and a mixture of old and new surfaces that indicated great geologic activity. The spacecraft provided information about the planet's rings, which were found to be distinctly younger and made of larger particles than the rings of Jupiter and Saturn.
Voyager 2 revealed Neptune to be an unexpectedly dynamic planet that possesses several large dark spots similar to those on Jupiter. Voyager 2 also found that Neptune had the strongest winds of any planet, reaching speeds of up to 2,000 km/h (1,240 mph). Voyager 2 images of Triton, the largest of Neptune's moons, showed geyserlike eruptions spewing nitrogen gas and dark dust particles far into the moon's tenuous atmosphere. Before continuing on its journey to interstellar space, Voyager 2 transmitted a final image of the entire expanse of the solar system.
To accomplish their original missions, the Voyager spacecraft were initially built to last five years. However, as a result of software improvements, scientists were able to reprogram the Voyagers' on-board computers to greatly extend the usefulness of the two probes. Scientists estimate that both spacecraft have enough remaining power to continue sending data back to Earth until 2020.
In November 2003 two teams of scientists, working independently, debated whether Voyager 1 had become the first spacecraft to reach the outskirts of the heliopause, the region where the solar system ends and interstellar space begins. Scientists believe that the heliopause is where the solar wind, a stream of charged particles from the Sun, encounters the gas and dust that make up interstellar space. They theorize that a spacecraft flying into this region will first encounter a so-called termination shock boundary due to the abrupt slowing of the solar wind as it nears the heliopause.
One team of scientists concluded that Voyager 1 met the termination shock boundary and hence began to approach the heliopause in August 2002. At the time the spacecraft was nearly 13 billion km (8 billion mi) from Earth, or about 86 astronomical units (AUs, one AU is equal to the distance between Earth and the Sun, about 150 million km [93 million mi].) That distance would then indicate approximately how far the solar system extends. The scientists based their findings on instrument readings that indicated the speed of the solar wind declined from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). Scientists believe the heliopause is a dynamic region that expands and contracts based on the constantly changing speed and pressure of the solar wind. By February 2003 data from Voyager 1 showed the solar wind was once again at normal speeds of 1.1 million km/h, indicating that the termination shock boundary had expanded and overtaken the spacecraft.
Another team of scientists, however, who examined the strength of the magnetic field around the spacecraft and the behavior of high-energy particles, concluded that Voyager 1 had not yet reached the termination shock boundary. They reported that the spacecraft was near but not yet there. Both teams agreed, nevertheless, that Voyager 1 had entered unknown territory and had embarked on the final phase of its mission, which includes exploring the termination shock boundary, the heliopause itself, and finally, interstellar space.
Voyager 2 is also due to explore these outer regions. Unlike Voyager 1, a key instrument known as a plasma detector is still functioning on Voyager 2. As a result, scientists expect that Voyager 2 will provide definitive data on the termination shock boundary. Astronomers estimate that it will take the Voyagers 10 to 20 years to cross the heliopause once it is reached and then enter interstellar space.
In their joint mission, Voyager 1 and Voyager 2 explored all the giant outer planets of the solar system, many of the planets' moons, and each of the planets' systems of rings and magnetic fields. The information that the Voyager spacecraft relayed to Earth helped resolve certain key questions in astronomy while raising intriguing new questions about the origin and evolution of the planets in the solar system.

Contributed By:
Dennis L. Mammana

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NASA or National Aeronautics and Space Administration

Posted in Labels: , | 12:28 AM


NASA  Satellite 

I
INTRODUCTION

NASA or National Aeronautics and Space Administration, agency of the United States government responsible for developing space exploration. NASA conducts both human and unpiloted spaceflights, and also trains astronauts and carries out scientific experiments in space. NASA’s areas of space research include planetary science, astrophysics, astronomy, Earth science, astrobiology, and aerospace medicine. The agency studies the performance and design of airplanes (aeronautics) and rockets as well.
NASA has a wide range of educational outreach programs in the form of public lectures, Websites, and television (NASA TV) for classrooms and the general public. In addition, NASA works to increase international alliances for space activities, and to establish partnerships between private companies and government agencies in the aerospace industry. NASA is a civilian agency, separate from the military and intelligence-gathering space programs conducted by the United States Department of Defense (DOD).
II ORGANIZATION OF NASA
NASA is based in Washington, D.C., and is under the guidance of the president of the United States. The president is mainly responsible for determining NASA’s budget, which directly affects the mission and goals of the agency.
NASA is organized into four main mission directorates: the Aeronautics Research Mission Directorate, the Exploration Systems Mission Directorate, the Science Mission Directorate, and the Space Operations Mission Directorate. NASA employs thousands of scientists, engineers, and technicians at its major installations across the country. These facilities include the John F. Kennedy Space Center in Merritt Island, Florida; the Lyndon B. Johnson Space Center in Houston, Texas; the Goddard Space Flight Center in Greenbelt, Maryland; the George C. Marshall Space Flight Center in Huntsville, Alabama; and the Jet Propulsion Laboratory in Pasadena, California.
Scientists from institutions outside the U.S. government and from other countries commonly work with NASA on particular projects and space missions. Satellites launched by NASA are used by the National Oceanic and Atmospheric Administration (NOAA) and other government agencies. Although NASA is a civilian agency, it sometimes shares launch facilities with the U.S. Air Force and has launched satellites and space probes from the Cape Canaveral Air Force Station (CCAFS), next to NASA’s Kennedy Space Center in Florida, and from Vandenberg Air Base in California. Many members of NASA’s astronaut corps are also active members of the military. NASA has used research conducted by the military such as the X-15 piloted rocket program in the 1960s and the Clementine Moon orbiter.
Throughout its history, NASA has faced budget and funding challenges, and has had to prioritize a mix of human space missions and unpiloted satellite and robotic missions, along with aeronautical research. Developing and operating human space programs have been more expensive than unpiloted programs. Debate has centered on whether robotic space exploration using orbiters, landers, and rovers is more economical and scientifically productive than human missions. In turn, decisions must be made about the size and scale of resources devoted to different areas of research such as Earth science, astrophysics, extrasolar planets, or planetary science within our solar system.
III FOUNDING AND EARLY HISTORY OF NASA
The National Aeronautics and Space Administration was established in 1958 under the National Aeronautics and Space Act. The United States Congress passed the “NASA Act” in the wake of the Soviet Union’s successful launch of the first space satellite (Sputnik) in 1957. The act established NASA as a civilian agency that would use space for peaceful purposes. President Dwight Eisenhower wanted to separate civilian space activities from the military space program conducted by the Department of Defense.
NASA replaced the National Advisory Committee for Aeronautics (NACA) that was created by the United States Congress in 1915. NASA absorbed various aviation and space organizations that were already established, enabling a single agency to coordinate efforts effectively among all interested parties: scientists, engineers, universities, and politicians.
From the beginning NASA was a major player in what became known as the “space race” between the United States and its Cold War rival, the former Union of Soviet Socialist Republics (USSR). Both nations worked to develop rockets that could carry satellites and piloted missions into space. Soviet scientists scored the first victory on October 4, 1957, when they launched the beach-ball-sized artificial satellite, Sputnik (later referred to as Sputnik 1), into orbit around Earth. The USSR went on to record several more historic firsts, including a series of dramatic achievements that culminated on April 12, 1961, when cosmonaut Yuri Gagarin became the first human to orbit the planet in a spacecraft. Just a few weeks later, on May 5, the United States launched its first piloted spacecraft in which astronaut Alan B. Shepard, Jr., made a suborbital flight (a complete orbit of Earth was not made). This American space success prompted President John F. Kennedy to propose a program to send humans to the Moon before the end of the decade of the 1960s.
IV NASA’S HUMAN SPACE PROGRAMS
The United States finally caught up with the USSR on February 20, 1962, when a Mercury rocket carried John Glenn around Earth three times (see Mercury program). In 1963 Mercury gave way to the Gemini program, which would send two astronauts into space in a bigger and better version of the Mercury spacecraft.
The Gemini program was phased out in 1967 by the Apollo program, a $25-billion project devoted to fulfilling President Kennedy’s goal of landing a U.S. astronaut on the Moon by the end of the decade. In May 1969 Apollo 10 came within 15,240 m (50,000 ft) of the Moon’s surface. Two months later, on July 20, 1969, Apollo 11 astronaut Neil Armstrong became the first human being to walk on the Moon. His words at that moment are part of history: “That’s one small step for a man, one giant leap for mankind.”
Five more Apollo missions reached the Moon. The only failed mission occurred in 1970 when technical problems forced Apollo 13 to return to Earth before landing on the Moon. Apollo’s final Moon mission was completed in December 1972.
Apollo technology was used in 1973 to launch NASA’s first space station, Skylab, where astronauts could work and live in space for several months. Apollo-Soyuz in 1975 brought the United States and the USSR together for a joint mission in Earth orbit.
NASA’s next major human space program was the space shuttle, which had its first flight in 1981. The shuttle was designed to be a reusable space vehicle that could be launched on a regular schedule to carry humans, scientific equipment, satellites, space probes, and cargo into space. A fleet of five shuttles was built. The shuttle was launched with a combination of liquid fuel and solid fuel rockets and glided back from space to land like an aircraft.
The original ambitions of the shuttle program were only partly realized. Fewer shuttle launches were made than planned. Catastrophic accidents destroyed the Challenger shuttle after launch in 1986 and the Columbia shuttle during reentry in 2003, killing both crews. Investigations traced the accidents to design flaws and to management problems. Both accidents led to design and procedure changes, and to a temporary stoppage of shuttle flights. Following the Columbia disaster, NASA also announced plans to retire the shuttle in 2010, after completion of the International Space Station (ISS), a human orbital research facility.
The Constellation program is NASA’s successor to the space shuttle. The program uses a human-piloted capsule design similar to Apollo technology, and is intended to be safer than the space shuttle. Plans call for the Constellation program to take astronauts back to the Moon and on to Mars, and possibly to nearby asteroids. The first flight of the Orion piloted capsule is scheduled for 2015. The long gap between the retirement of the space shuttle and the beginning of Orion piloted flights concerns some NASA officials, however. The training of astronauts and the staff expertise for operating human-piloted missions may suffer. Russian space capsules will take American astronauts to the International Space Station between the end of the shuttle program and the beginning of the Constellation program.
NASA is also an active participant in the construction and staffing of the International Space Station. The United States, Canada, Japan, Russia, and nine members of the European Space Agency (ESA) collaborate on this project. At least one NASA astronaut is included in each of the ISS crews that have continuously resided on the station since 2000.
V NASA’S UNPILOTED SPACE MISSIONS
Beginning in the 1960s, NASA launched a series of spacecraft that flew by all the major planets of the solar system from Mercury to Neptune, as well as by asteroids and comets. Notable missions include the Mariner series to Mercury, Venus, and Mars; the Pioneer craft to Jupiter and Saturn; and the Voyager probes to Jupiter, Saturn, Uranus, and Neptune. A flyby of the dwarf planet Pluto is scheduled for 2015. NASA has also put space probes into orbit around the Sun, the Moon, Venus, Mars, Jupiter, and Saturn, with a Mercury orbiter set for 2011, and an asteroid belt orbiter for 2015. These orbiting space probes include Ulysses around the Sun; Lunar Orbiter around the Moon; Magellan around Venus; Viking, Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter around Mars; Galileo to Jupiter and its moons; and Cassini to Saturn and its moons and rings.
NASA robot probes have landed on the Moon with Surveyor and on Mars with Viking, Mars Pathfinder, and Mars Exploration Rovers. More Mars landers are scheduled, with a possible return of samples in the future. NASA also collaborated on the Huygens probe that was attached to Cassini and landed on Saturn’s moon Titan. Other NASA missions such as Genesis and Stardust have returned samples to Earth of the particles emitted by the Sun and by comets in space.
NASA has also helped design, launch, and operate a fleet of space telescopes, including the Hubble Space Telescope (HST), the Infrared Space Observatory, the Spitzer Space Telescope, the Chandra X-Ray Observatory, the Compton Gamma Ray Observatory, and the planned James Webb Space Telescope. As part of research into Earth science, NASA launches satellites that monitor Earth from space to observe weather, climate, oceans, land, and resources, as well as auroras and the magnetosphere. In addition, sophisticated scientific NASA satellites have studied astronomical and astrophysical phenomena, including black holes and radiation from the big bang that started the universe. Other NASA space observatories look for evidence of extrasolar planets around other stars. NASA also helps support ground-based telescopes that study objects in space or search for asteroids and comets that come near Earth’s orbit.

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Jupiter`s Moon :- Callisto (astronomy)

Posted in Labels: , , | 12:19 AM


Callisto (astronomy), large satellite of the planet Jupiter. The moon orbits Jupiter at an average distance of 1,883,000 km (1,170,000 mi). Callisto completes an orbit and rotates in the same amount of time, once every 16.69 Earth days. Its nearly circular orbit parallels Jupiter’s equator.
Callisto is spherical and is the third-largest moon in the solar system. The moon has a radius of 2,403 km (1,493 mi), making it nearly the same size as the planet Mercury. Since Callisto consists mostly of low-density water ice, however, the moon is only one-third as massive as rocky, metallic Mercury. Callisto’s interior is probably not differentiated into a rocky core surrounded by lighter icy material, like that of the other three large moons of Jupiter—Io, Europa, and Ganymede. Instead, scientists believe that the entire moon is a mixture of rock and ice, with the percentage of rock in the mixture increasing toward the moon’s center.
Callisto is the most heavily cratered world known. This cratering suggests that the surface has not changed in billions of years and that there is little internal activity affecting it. If Callisto’s internal activity was strong enough, it would melt or disturb the ice near the moon’s surface, and many of the craters would be erased. Instead, Callisto seems to be the most geologically quiet of the four large moons of Jupiter. This is probably because Callisto is the outermost of the large moons. No large outer satellite of Jupiter subjects it to the gravitational tug-of-war with Jupiter that can produce internal heating. The only sign of change on the surface is the slumping of older crater walls that occurs as the ice that forms them flows downward over extremely long periods of time. Despite Callisto’s apparent stillness, however, in 1998 the Galileo spacecraft revealed signs that the moon may have a liquid or slushy ocean beneath its surface. Callisto may also have a tenuous atmosphere composed mostly of carbon dioxide.
The largest crater on Callisto is Valhalla, a 300-km (190-mi) basin surrounded by a system of concentric rings 1,500 km (930 mi) wide. In addition, Callisto has 12 known crater chains. Astronomers believe that these chains were caused by comets or asteroids that broke up when they passed too near Jupiter and crashed into Callisto. The longest chain, Gipul Catena, is about 640 km (about 400 mi) long.
Callisto was discovered independently by Italian astronomer Galileo and German astronomer Simon Marius. Callisto and the three other large moons of Jupiter—all of which were found in 1610—are collectively known as the Galilean moons. Marius named Callisto and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Callisto was a nymph who was changed into a bear by Zeus’s jealous wife Hera. In Greek mythology, Callisto is the basis of the constellation Ursa Major. Craters on Callisto are generally named for heroes and heroines of northern mythologies. The United States Voyager space probes and the United States Galileo orbiter have provided a wealth of information about Callisto. When the Voyager probes passed Jupiter in 1979 they provided some detailed photographs. Galileo made its first flyby of Callisto in 1996 and examined the moon several times before its mission ended in 2003.

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Summary Of The Iron Ring

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Book Summaries help you understand books studied in schools and give you insights that make for great book reports. Gain a new perspective by reading about the author, and learn how settings, characters, and themes help make these books acclaimed works of literature.
The Iron Ring
Published 1997

I ABOUT THE AUTHOR
Lloyd Alexander is a towering figure figure in young adult literature with his fiction earning awards, critical praise, and a large audience. He did not come by his fame and popularity easily; he labored for many years and endured frequent rejections before achieving renown. The tremendous success of his second novel for young adults, The Book of Three (1964), made him almost overnight one of the foremost writers for young people. The book's lyrical prose, complex characters, and well-structured plot justifiably garnered critical acclaim and great acceptance from the book-buying public.

He was born in Philadelphia on January 30, 1924 to Edna Chudley Alexander and Alan Audley Alexander, a stockbroker. As a youngster, he was an avid reader of mythology and folk tales. These early readings may have inspired The Arkadians (1995) and his other recent writings for young adults and younger children that focus on African and Asian cultures as well as that of Ancient Greece.

Alexander worked as a teenager to earn money for college but only attended a semester at West Chester State Teacher's College before joining the army in 1942, where he worked as an intelligence agent. While stationed for a time in Wales, he developed a passion for Celtic folklore and culture that inspired his Prydain Chronicles. He was later stationed in Paris as a counterintelligence agent. After being discharged from the army, he attended Sorbonne University in Paris, where he not only received a college degree but met and married his wife, Janine Denni.

Alexander bounced from one job to another for years, working as a cartoonist, artist, advertising writer, and editor, while writing novels in his spare time. Unable to find a publisher for his first three novels, all for adults, Alexander struggled to support his family. He turned his frustration into humor and wrote a book And Let the Credit Go (1955), about the travails of writing for publication. In the early 1960s, he turned his attention to young audiences and wrote Time Cat, which was published in 1963 (republished in 1996). While writing this book he came across Welsh folklore, which rekindled his youthful interest in Celtic mythology and culture, thus inspiring The Book of Three, the first of the Prydain Chronicles, a series of daring yet humorous adventures set in a land of mysterious magic.

Since then, Alexander's reputation has climbed not only among critics, but among a large audience that includes adult readers as well as young ones. C. S. Lewis once wrote that a way to tell whether a book for young readers is good is to read and enjoy it as a youngster and then read it again years later—if one still liked the book as an adult, then it is probably good literature. Nearly all of Alexander's books meet this criterion, with graceful prose, interesting characters, sharp wit, and complex plots, all appealing to young and old readers. He has proven himself to be a master craftsman in several types of novels: sword-and-sorcery fantasies like the Prydain Chronicles, adventures set in ancient cultures and mythological worlds like The Arkadians, and a series of fine melodramatic mysteries featuring the courageous and versatile Vesper Holly. He has also written a more realistic group of novels about war and its effects—the Westmark Trilogy. Alexander's recent writing seems to chiefly focus on the mythologies, folklore, and cultures of the world. In addition to The Arkadians, The Remarkable Journey of Prince Jen (1991) tells of adventure and magic in a land much like ancient China, and a book for younger readers, The Fortune Tellers (1992), focuses on Cameroon in west central Africa.




II OVERVIEW
The Iron Ring has superbly developed characters who are fun to spend time with, wondrous events, fearful dangers, and a complex exploration of what one of the characters calls the 'twilight' between right and wrong. The premise on which the story is built seems simple but becomes ever more challenging as the tale advances. King Tamar of Sundari is visited by Jaya, ruler of Mahapura, a land in a valley in the Snow Mountains of the far north. In a game of chance, Jaya wins Tamar's life and commands Tamar to come to him in Mahapura. This would be a dull quest if there were not complications, and there are many, involving cantankerous birds, a kingdom of snakes, the king of the monkeys, and many other wonderful beings and places. The novel has romance, drama, action, and suspense, all set in a richly detailed context that exemplifies the essence of the ancient Indian culture, with the cultural standards and expectations presented fluidly as coherent parts of the plot.

III SETTING
The events take place in an imaginary land based on the societies of ancient India. This fictional world features some of the topography of the real India such as rivers, forests, grasslands, and a northern mountain range. The Danda-Vana forest is especially important because it still has remnants of the early days of the world when animals could talk. It is in and near this forest that Tamar meets the woman he shall love, the king of the monkeys, the Naga-loka, and a talking bird. Talking animals would seem to be very strange, but Tamar takes the wonders he experiences in stride.

IV THEMES AND CHARACTERS
The underlying idea for the novel is to use its plot and characters to illustrate fundamental characteristics of Indian culture. A good example is the caste system which is an essential aspect of Indian society; in The Iron Ring, the characters represent different castes and how they respond to each other shows how the castes are related. Tamar is a kshatriya and is supposed to marry only within his warrior caste or with a brahmana—a member of the other elite caste. His love of Mirri, a cow-tender and thus a member of the shudra caste, is forbidden by caste rules because her caste is far below his. Their relationship epitomizes some of the unfairness of a system in which one's birth into a caste is supposed to determine what one does with one's life. To emphasize this point, Alexander has Tamar's mentor Rajaswami explain that the chandalas are the lowest of the low. If a chandala's shadow falls on what one is eating then the food is polluted and must be thrown out. If a member of any other caste is touched by a chandala then that person loses his caste status and instantly becomes a chandala. It is characteristic of Alexander's sympathy with ordinary people that he would have Tamar be forced to work for a chandala and in so doing learn the injustice of how chandalas are treated and understand the nobility of the work that they do. It is at the chandalas' hands that society's poorest and most miserable people at least receive reverential treatment after death, as respect is at last shown for their spirits.

Namaste, the showing of respect for the spirit in someone, is very important to the characters in the novel. It is symbolized by pressing one's hands together toward someone; it is more than a greeting, it is symbolic of one of the most important sources of motivation for the characters. A good person respects the spirit of others, and the characters live in persistent awareness that their physical lives are only part of their total lives—all creatures are spiritual beings. This helps explain one of the mysteries of Mahapura, where people can be heard but not seen. This symbolizes the spirituality of people; their physical lives are heard, but their spiritual lives remain unseen until a person can fully perceive the truth behind the truth.

For Tamar and most of the members of his warrior caste—the kshatriya—dharma is of paramount importance. A kshatriya is supposed to live by a warrior's code and to follow the rules of war. Alexander describes dharma as 'goodness, virtue, righteousness, conscience; a code of proper conduct, a deep and driving sense of obligation to do what is right.' It may be the most remarkable aspect of this remarkable novel that the concept of dharma is well worked out in the story and displayed from many angles. Two aspects of dharma are vital to the plot. One is that someone's dharma varies according to caste and animal species, and the other is that the people of India are human beings and not archetypes and that some do not follow their dharma as well as they should.

In The Iron Ring those who defy their dharma are usually evil. This amount of complexity for the novel's presentation of dharma would be exceptionable in itself, but Alexander has a fine understanding of human nature, and his characters in The Iron Ring are too multifaceted to be easily separated into groups of good dharma followers and bad dharma followers. Fallibility is a trait in even the best of people, and Tamar and his friends sometimes fail to live up to their dharma. Even more important, fanatical following of dharma can be foolish and even lead to evil, thus good sense should rule. Tamar spends much of the novel nearly getting his self and others killed by obeying the warrior caste's dharma rigidly; Mirri has to work hard to teach him to use common sense. Tamar eventually learns that there are higher ethics than dharma, although dharma is very important. Compassion, for instance, is a high virtue that transcends everyday dharma. This does not mean that dharma is arbitrary; the rules of dharma emphasize virtue, and the practice of dharma encourages one to understand and appreciate the ways of others. Tamar understands that monkey dharma allows for monkeys to take objects that do not belong to them; it is part of monkey nature. This sort of understanding helps him keep his equilibrium when meeting with the snake people, scholarly bears, and other unusual folk: they have their dharma as he has his and he respects them for it.

The growth in Tamar is important. Most of the aspects of Indian culture that Alexander explores are embodied in him, but they have their importance primarily because he is a very human character. A young man who has been strictly educated in the dharma of his caste, he is too rigid in his following of his dharma and in his expectations of how others should follow their dharma. This is a common characteristic of young people who have not had the rules they have learned, tested by the realities of life. His rigidity makes him somewhat annoying; it also helps to round out his characterization and make him a fully human figure. That strong and clever Mirri sees something in him to love is a good sign; that she does so before she learns his caste and that he is a king suggests that he can be related to on a human level, in spite of his bombastic declarations of dharma.

The Iron Ring has an epic structure and, as in most epics, the main character must endure much disappointment and suffering before he becomes the man he ought to be. Tamar's recognition of the validity of the dharma of others encourages him to be patient with them, to understand them, and to be merciful to them. For instance, he does not punish Hashkat, king of the monkeys, for taking something that does not belong to him. He is merciful because he recognizes the monkey's dharma.

Tamar's maturation occurs at a credible pace, with no one event transforming him into a great man. Through love of Mirri he learns to appreciate a member of a low caste. Through shared hardships he learns to recognize that people should not be judged only by their membership in a particular caste or animal species and stereotypes do not do justice to individual spirits. The demands of namaste require that he learn to see the value of each individual spirit. This may explain why the young man who was willing to fight to the death at any provocation eventually spares an old man's life during a battle and why he wants to spare the life of Nahusha, a very evil man who has caused great suffering. His desire to capture Nahusha and turn him over to the chandalas as Nahusha had done to him is in no way a desire for revenge. He has learned that revenge would make him no better than Nahusha. Instead he is motivated by what he learned while serving a chandala, humility and a reverence for the spirits of others, and he hopes to give Nahusha the same enlightening, ennobling experience.

The other characters of the novel are developed to a lesser degree than Tamar, but are nonetheless finely drawn. Hashkat, the man who was changed into a monkey by a wandering rishi, shows the most growth of the remaining characters. Attracted by bright objects, he is first seen being squeezed to death by a snake angry that Hashkat was trying to take a gem from the snake's forehead. He is a lesson in defying stereotypes; although true to the monkey dharma that if something is not nailed down then it must be free to be taken, he shows courage, intelligence, and fortitude as he fulfills his obligations to Tamar, who saved his life. That he prefers to remain a monkey rather than be changed back to a man is consistent with his characterization; even though a courageous human warrior—he was born a kshatriya—and a trustworthy friend, he still prefers the chaotic dharma of the monkeys.

Rajaswami is a gentle man who has nonetheless taught Tamar to follow dharma rigidly and believe that the caste system is an unalterable absolute. His affection for Tamar eventually causes him to grow enough to recognize that his rigid teachings are contrary to namaste; after Tamar has become a chandala, Rajaswami embraces him without concern for his own caste. He has learned of higher virtues than dharma, and to his credit he chooses to follow them.

Mirri is the catalyst for the growth of the other characters. She presents a problem in terms of character development since she does not seem to change much during the narrative. Once she commits herself to Tamar, she sticks with him—even when he tells her to go away. She is from the start intelligent, courageous, clever, and understanding. Even though she seems to exhibit little growth she is a very important figure. Not only is she Tamar's lady love, but her insistence on common sense behavior affects all of her companions. Even old Garuda, a battered old eagle who looks like a worn-out buzzard, perceives Mirri's important ability to help others grow. He takes to her quickly, and with her recovers some of his lost courage and much of his health. Without her influence he would not have been able to transcend his querulousness and fear and achieve those acts of courage that are essential to saving Tamar and defeating Nahusha. Her active nobility helps Rajaswami grow, not only by showing him the merits of someone of a low caste, but by impelling him to acts of courage and cleverness, as when he helps her fake her death. She is for Tamar the battering ram that knocks down his barriers to other people; she helps him change from a stubborn young man to a compassionate, loving, and conscientious leader.

V LITERARY QUALITIES
The Iron Ring follows an epic structure. Epics were originally poems that told stories; they would follow at length the fortunes of war or the fabulous adventures of people caught in events much larger than themselves. The central characters nearly always grow in some significant way. In Homer's Iliad, Achilles grows from a pouting, spoiled, and childish man into a warrior with the stature to make the moral choice between a lifetime of honor and a premature death that will save the lives of many Greek warriors; in Homer's Odyssey, Odysseus endures many travails, as Tamar does, before arriving at his goal. The Iron Ring features an epic journey, many obstacles that must be overcome, a goal to be reached, and like the Iliad and Odyssey, a concern with the issues of leadership and moral behavior.

VI SOCIAL SENSITIVITY
The Iron Ring is a triumph of exploration of the customs and values of an ancient culture that is likely to be remote to most of its western readers. It explores the complexity of dharma, a code of honorable conduct that varies for each caste and for each kind of animal. For instance, a member of the kshatriya—the warrior caste—is expected to conduct himself according to the virtues of forthrightness and courage, whereas a member of the brahmana—the scholarly, priestly caste—is expected to conduct himself according to the virtues of peace and learned study; on the other hand, the monkeys have their own dharma, which is basically that if something is not nailed down then it is free for the taking. What all dharma has in common is, as Alexander puts it, 'a deep and driving sense of obligation to do what is right.'

The caste structure of society is also presented in the novel. At the apex of the caste system are brahmanas, followed by the kshatriyas. Next comes the merchant and farm owner caste, the vaishyas, then the 'peasants and unskilled laborers,' the shudras. Beneath these are those who are to be shunned by the higher castes, the panchamas, who do most of the most unpleasant work, and at the very bottom are the chandalas, who are virtual nonpeople whose very touch is supposed to degrade any member of any other caste to the level of chandala. One's caste dictates whom one should marry, whom one should socialize with, and what job one should have. Of the castes, the brahmans, kshatriyas, and chandalas receive the most coverage, as kshatriya Tamar learns lessons not only about honor and dishonor, but of the merits of others different from himself.

VII TOPICS FOR DISCUSSION
1. Why is Tamar surprised at the lies and deceit of other members of his caste?

2. What is the significance of personal obligation in The Iron Ring? What different kinds of obligation are to be found in the novel?

3. Is the list that identifies the different characters in the tale necessary?

4. Is a social system based on dharma a good one? What are its strengths and weaknesses? Would American society benefit from dharma? What social problems might it help to fix?

5. What does the iron ring symbolize?

6. Why does Mirri want Tamar to go to Mahapura? Does her explanation make sense?

7. Why is Rajaswami horrified by the sight of chandalas? Why does he later change his mind?

8. What makes Nahusha a bad man?

9. Will Tamar be a good king?

10. Can Tamar, after the defeat of Nahusha, trust the kings who promise to have their kingdoms live peacefully with each other and his?

11. Is Ashwara a fool for not fighting to defend himself? Is his refusal a betrayal of his followers?

12. Why cannot Tamar and his companions see the people they hear in Mahapura?

13. In what ways does Tamar grow and mature during the novel?

14. Which of the characters other than Tamar is best drawn? Do any of them grow and become more than what they were when they were first introduced?

15. What are Jaya's limitations?

VIII IDEAS FOR REPORTS AND PAPERS
1. What are the origins of dharma? Who practiced it in the past? Who practices it now?

2. Characters often bow to the spirit within other characters. What is the origin of this custom? What is its religious and philosophical foundation?

3. The society of The Iron Ring has elaborate obligations incurred by characters for kindnesses and services performed for them by other characters. How important a concern was this system of obligations in ancient India? How important is it in modern Indian culture?

4. What is the origin of the Indian caste system? What are the castes and what are the duties of each? How strong is the caste system in present-day India?

5. What role have elephants played in Indian culture? Do they have any religious standing? What tasks have they been set to? How have they been captured? How have they been trained?

6. Monkeys have a long and complex history in Indian culture. What roles have they played in the past? What roles do they play now?

7. The Naga-loka are a mysterious folk. Who are they in Indian folklore? What are Indian attitudes toward snakes.

8. Why are people very afraid of Indian tigers? How have tigers played an important role in any aspect of Indian society? What is their status in India today?

9. What is a durbar? Does Alexander portray it accurately in The Iron Ring?

10. What are ashramas? What is life like in one?

11. What are rakshasas? Where do they appear in Indian folklore or literature? What do they represent?

12. What are the powers of rishis? Do they play a distinctive role in Indian literature?

13. Why would a king have a suta? How important was a suta's job?

14. Were there women kshatriyas? If so, what was their dharma?

15. What different castes were allowed to intermarry?

16. What are the limits of dharma in The Iron Ring? Are these limits reflected in real-life Indian society?

IX RELATED TITLES
Alexander's focus presently seems to be on exploring cultures ancient and modern in different parts of the world. The Iron Ring focuses on ancient India, The Arkadians focuses on the roots of Ancient Greek culture, and The Remarkable Journey of Prince Jen focuses on ancient China. This novel shares with The Iron Ring a quest structure and features a Prince (instead of a king) journeying north to a fabled kingdom. It also features a low-born young woman who becomes the bride of the prince, eccentric companions including wise men, and fabulous animals. Although this may make The Iron Ring seem to be a repeat of The Remarkable Journey of Prince Jen in a different setting, the two books are otherwise distinct: the characterizations are different, the interactions with animals are much more extensive in The Iron Ring, and the cultures portrayed are markedly dissimilar. The Fortune Tellers, a book for younger readers, takes place in Cameroon and tells of how a carpenter consults a disreputable fortune teller and how the carpenter then seems to prosper.

Contributed by: Kirk H. Beetz

Source: Beacham's Guide to Literature for Young Adults. Copyright by Gale Group, Inc. Reprinted by permission.


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Black Hole, an extremely dense celestial body

Posted in Labels: | 11:07 AM


Black Hole
I INTRODUCTION
Black Hole, an extremely dense celestial body that has been theorized to exist in the universe. The gravitational field of a black hole is so strong that, if the body is large enough, nothing, including electromagnetic radiation, can escape from its vicinity. The body is surrounded by a spherical boundary, called a horizon, through which light can enter but not escape; it therefore appears totally black.
II PROPERTIES
The black-hole concept was developed by the German astronomer Karl Schwarzschild in 1916 on the basis of physicist Albert Einstein’s general theory of relativity. The radius of the horizon of a Schwarzschild black hole depends only on the mass of the body, being 2.95 km (1.83 mi) times the mass of the body in solar units (the mass of the body divided by the mass of the Sun). If a body is electrically charged or rotating, Schwarzschild’s results are modified. An “ergosphere” forms outside the horizon, within which matter is forced to rotate with the black hole; in principle, energy can be emitted from the ergosphere.
According to general relativity, gravitation severely modifies space and time near a black hole. As the horizon is approached from outside, time slows down relative to that of distant observers, stopping completely on the horizon. Once a body has contracted within its Schwarzschild radius, it would theoretically collapse to a singularity—that is, a dimensionless object of infinite density.
III FORMATION
Black holes are thought to form during the course of stellar evolution. As nuclear fuels are exhausted in the core of a star, the pressure associated with their energy production is no longer available to resist contraction of the core to ever-higher densities. Two new types of pressure, electron and neutron pressure, arise at densities a million and a million billion times that of water, respectively, and a compact white dwarf or a neutron star may form. If the star is more than about five times as massive as the Sun, however, neither electron nor neutron pressure is sufficient to prevent collapse to a black hole.
In 1994 astronomers used the Hubble Space Telescope (HST) to uncover the first convincing evidence that a black hole exists. They detected an accretion disk (disk of hot, gaseous material) circling the center of the galaxy M87 with an acceleration that indicated the presence of an object 2.5 to 3.5 billion times the mass of the Sun. By 2000, astronomers had detected supermassive black holes in the centers of dozens of galaxies and had found that the masses of the black holes were correlated with the masses of the parent galaxies. More massive galaxies tend to have more massive black holes at their centers. Learning more about galactic black holes will help astronomers learn about the evolution of galaxies and the relationship between galaxies, black holes, and quasars.
The English physicist Stephen Hawking has suggested that many black holes may have formed in the early universe. If this were so, many of these black holes could be too far from other matter to form detectable accretion disks, and they could even compose a significant fraction of the total mass of the universe. For black holes of sufficiently small mass it is possible for only one member of an electron-positron pair near the horizon to fall into the black hole, the other escaping (see X Ray: Pair Production). The resulting radiation carries off energy, in a sense evaporating the black hole. Any primordial black holes weighing less than a few thousand million metric tons would have already evaporated, but heavier ones may remain.
The American astronomer Kip Thorne of California Institute of Technology in Pasadena, California, has evaluated the chance that black holes can collapse to form 'wormholes,' connections between otherwise distant parts of the universe. He concludes that an unknown form of 'exotic matter' would be necessary for such wormholes to survive.


Reviewed By:
Jay M. Pasachoff

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Great Andromeda Spiral Galaxy

Posted in | 11:00 AM

Great Andromeda Spiral Galaxy
I INTRODUCTION



Andromeda Galaxy
The Andromeda Galaxy, a spiral galaxy similar to our own Milky Way Galaxy, is the farthest object from Earth visible to the naked eye. Its whirlpool of stars can be seen from the Northern Hemisphere in the constellation Andromeda. The Milky Way and Andromeda galaxies are part of a group of galaxies called the Local Group, which in turn is part of larger group called the Virgo Cluster.
Robert Gendler/NASA
Great Andromeda Spiral Galaxy, also known as M31 and as the Andromeda Galaxy, large spiral galaxy in the constellation Andromeda, about 2.5 million light-years from Earth.
The Great Andromeda Spiral Galaxy is the largest nearby galactic neighbor to the Milky Way Galaxy, Earth’s home galaxy, and is part of the Local Group. Because it is so near, it appears very bright, with a total magnitude (a measure of its brightness as seen from Earth) of 3.4, and is easily visible to the naked eye on a clear, dark night. Its immense diameter of about 200,000 light-years makes it appear five times larger than the full Moon in our sky.
II STUDYING THE ANDROMEDA GALAXY
The Great Andromeda Spiral Galaxy is the most studied of external galaxies because astronomers can view features in it that they believe are also present in the Milky Way, but are made invisible by the Milky Way’s thick intervening clouds of dust. Astronomers study the Andromeda Galaxy’s spiral arms; the birth of stars, dense spherical groups of stars called globular clusters, as well as looser star groupings called open clusters; interstellar matter; and supernova remnants. The mass of the Andromeda Galaxy is believed to be equivalent to between 300 billion and 400 billion Suns.
From the 1970s to the 1990s, observations made with ground-based telescopes and with the Hubble Space Telescope showed that the galaxy appears to have a double nucleus—two bright areas near its center, instead of the usual single bright spot. In 1999 astronomers mapping the motion of stars near the galaxy’s center discovered that the stars orbit the center in an elliptical path instead of in the more usual circular path found in most other galaxies. The stars move much more slowly at one end of the ellipse than at the other. Astronomers believe that stars bunch up at the slow point in the orbit, like cars in a traffic jam, creating one of the bright areas. The other bright area, at the other end of the ellipse, occurs because stars pass close to the black hole (a region of space so dense that not even light can escape its gravitational pull) at the center of the galaxy. Astronomers believe that all galaxies have a similar, giant black hole at their center. Matter from the stars is more likely to fall into the black hole at this close point in the stars’ orbit, and matter accelerating toward the black hole glows to produce the second bright area.
The Andromeda Galaxy and our Milky Way Galaxy are moving toward each other at about 120 km (75 mi) per second. Calculations indicate that the two spiral galaxies will eventually collide to form a single, giant elliptical galaxy. A series of close passes that may begin in about 2 billion years will first pull their spiral arms into long streams of stars, with complete merger occurring by about 5 billion years from now. The black holes at the heart of each galaxy will combine as a single supermassive black hole, possibly resulting in a powerful quasar. Recent computer models suggest our Sun and its solar system will survive but likely will be thrown into the distant outer halo of the new combined galaxy.
III HISTORY OF OBSERVATION
The first recorded observation of this galaxy was in ad 905 by Persian astronomer al-Sufi, who described it as the “little cloud” in his Book of Fixed Stars (964). The telescopic discovery of this object is often attributed to German astronomer Simon Marius who described the soft glow of the object in 1611 or 1612.
French astronomer Charles Messier entered the object as the 31st item in his Catalogue of Nebulae and Star Clusters (1771-1784) on August 3, 1764. The name M31 is referred to as the object’s Messier number, in which the M is followed by the number in Messier’s catalog.
Messier studied the galaxy with several instruments, including an excellent telescope, but could see no individual stars within, so he theorized that the object was a nebula, or cloud of gas and dust. It was not until 1923 that the American astronomer Edwin Hubble measured the distance to M31 and showed that it was not a nearby nebula but actually a distant galaxy like the Milky Way.

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