| body of scientific knowledge as it relates to space exploration; it is sometimes also called astronautics. Space science draws on the conventional sciences of physics, chemistry, biology, and engineering, as well as requiring specific research of its own. The particular disciplines that are relevant depend on the type of mission being planned. There are four basic categories of space mission. The sounding rocket is restricted to suborbital flights with maximum altitude between 35 and 1,300 mi (55–2,100 km). Artificial satellites orbit the earth at altitudes between one hundred and several thousand miles. Space probes travel to the moon and planets. The final and most complex category is human spaceflight, of which the Apollo moon landings, the space shuttle, and the Skylab, Mir, and International space stations are the outstanding examples. The problems that space science must deal with include prediction and control of trajectories and orbits, telecommunications between spacecraft and earth, spacecraft design and fabrication, and life-support systems for human spaceflight. Trajectories and Orbits The key contribution of physics is celestial mechanics, the laws that govern the motions of bodies moving under the influence of gravitation. By combining Newton's law of universal gravitation and his laws of motion, the path of a rocket in the earth's vicinity can be calculated. This path, known as the trajectory, is strictly determined by the initial thrust imparted to the rocket, the gravitational field of the earth, and the atmospheric drag encountered. Although the manner in which these factors interact is highly complex, it is possible to determine accurately in advance the trajectory of any rocket and even to alter its course by remote control. If a satellite or unpowered spacecraft is close to the earth, the effects of other heavenly bodies can be ignored and its orbit will be a conic section: circular or elliptical for a satellite that remains in a closed orbit around the earth, and parabolic or hyperbolic for a spacecraft or space probe that escapes the earth's gravitational field into an open orbit. The criterion that separates the closed and open orbits is the escape velocity, which for the earth is 7 mi (11.3 km) per sec. If the initial thrust provided by a rocket gives the object a speed greater than the escape velocity, it will move away from the earth in an open orbit; if the final velocity is smaller than the escape velocity, it will remain at finite distance from the earth in a closed orbit; if the final velocity is less than 5 mi (8 km) per sec, the flight will be suborbital and the object will follow an arc that returns it to earth. A satellite in orbit around the earth typically travels at a height of several hundred miles with a velocity of about 5 mi (8 km) per sec and a period of revolution of 90 min. For certain satellites, however—such as communications satellites—synchronous orbits are desirable; at a distance of 22,300 mi (35,900 km), a satellite's period is exactly 24 hours, so it appears to hover over the same point on the earth's surface. Circular orbits are usually the most desirable but are the hardest to achieve. If a satellite is launched eastward near the equator, it receives a boost from the earth's rotation, but the resulting orbit necessarily lies in the earth's equatorial plane. For some applications, polar orbits, which pass near both of the earth's poles, are preferred. In a polar orbit, a satellite will periodically pass directly over every point on the earth's surface. Translunar and interplanetary trajectories are highly complex, because no simplifying assumptions can be made; the gravitational influences of the sun, moon, and other planets must be considered. Such gravitational forces can be exploited advantageously; for example, in the slingshot effect, a space probe is accelerated as it swings past a planet on the correct trajectory. Telecommunications Control over unmanned space probes and artificial satellites is maintained from the ground at control centers, where huge electronic computers analyze data and determine the exact moment when a change should be made. These instructions are relayed to the spacecraft by signals carried on certain radio frequencies. Instruments inside the craft also use radio signals to send data back to earth. Radio contact with spacecraft divides naturally into three categories: tracking, telemetry, and control. Tracking is the continuous reporting of a satellite's or space probe's position in space. Telemetry is the transmission of data back to earth by an on-board instrument (e.g., camera, Geiger counter, or magnetometer). Control includes the overall direction of a spacecraft to achieve the intended trajectory. Commands are specific control signals that order execution of a specific maneuver, such as turning on a camera or firing a retro-rocket Spacecraft Design and Fabrication Spacecraft employ booster rockets for propulsion and small adjustable retro-rockets for changing the orientation of the craft. Rocket propulsion systems vary from the tiny Aerobee sounding rocket to the giant Saturn V used in the Apollo project. For interplanetary flights, propulsion by nuclear or solar energy may be possible. Also being considered are ion and photon engines, which very efficiently provide low thrust that can build up very high velocity during a long flight. Landing on the earth or any planet with a significant atmosphere raises the problem of atmospheric friction, which can instantly burn up any spacecraft. In the manned space program, shielding that comes apart is used to absorb the frictional energy as the material of the shielding vaporizes. Also, a spacecraft enters the atmosphere at a shallow angle to avoid the friction produced by excessively high velocities. Without the development of modern electronics based on miniaturized transistor circuitry, space exploration would have been practically impossible. Unmanned space probes and satellites carry on-board computers of varying degrees of sophistication, and even on manned missions, maneuvering the spacecraft requires the rapid calculation and response available only through computerized devices. The instruments carried on spacecraft measure almost every conceivable physical parameter. Devices for measuring micrometeorite density, cosmic rays, magnetic fields, and solar wind were aboard even the early artificial satellites. Television cameras for both visible and infrared light are carried by most space probes. In addition, many spacecraft carry telescopes for different wavelengths of the spectrum, ranging from infrared to X rays and gamma rays. An important technique in space science is called multispectral scanning. Images are formed using only certain selected wavelengths; the data can be used to compile a single, detailed color photograph, or can be studied separately. Certain space probes carry more specialized devices, such as ultraviolet spectrographs for studying stars, and coronographs and spectroheliographs for studying the sun. Life Support for Human Spaceflight Long-range life support must be provided in manned spaceflight. This includes oxygen, food, and recycling of waste material. Shielding is also provided against encounters with micrometeorites and cosmic radiation that could damage the spacecraft or be a health hazard for its occupants. The spacesuit is a miniature life-support system for the individual astronaut; it provides sufficient oxygen at the correct pressure to sustain normal body functioning. In more advanced projects like Apollo, the space shuttle, Skylab, Mir, and the International Space Station, a "shirt-sleeve" environment, in which the astronauts do not have to wear any life-support equipment, is provided in a large capsule. Space biology (or exobiology) and space medicine study the reactions of human, animal, and plant life to the physical stresses encountered in space, such as weightlessness and radiation exposure. Attention is also given to the psychological effects on a group of people working together in confined quarters under demanding conditions. Bibliography See S. E. Zabusky, Launching Europe: An Ethnography of European Cooperation in Space Science (1995); P. S. Harderson, The Case for Space: Who Benefits from Explorations of the Last Frontier (1997); L. P. Sarsfield, The Cosmos on a Shoestring: Small Spacecraft for Space and Earth Science (1998); S. A. Stern, ed., Our Worlds: The Magnetism and Thrill of Planetary Exploration as Described by Leading Planetary Scientists (1999). ____________________The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved. -44788- | 
