Space Exploration

I

Introduction

Space Exploration, science and engineering of manned and unmanned space travel. Space exploration, or astronautics, is interdisciplinary in that it draws upon the findings of such fields as physics, astronomy, mathematics, chemistry, biology, medicine, electronics, and meteorology.

Manned and unmanned space probes have provided a vast new source of scientific data on the nature and origin of the solar system and the universe (see Cosmology); Earth-orbiting satellites have improved global communications, weather forecasting, navigational aids, and reconnaissance of the Earth’s surface for the location of mineral resources and for military purposes.

The space age and practical astronautics commenced with the launching of Sputnik 1 by the Union of Soviet Socialist Republics (USSR) in October 1957 and of Explorer 1 by the United States in January 1958. In October 1958 the National Aeronautics and Space Administration (NASA) was created in the United States. Since then, there have been over 4,000 launches of spacecraft of all varieties, mostly into Earth orbit. Twelve men have walked on the Moon’s surface and returned to Earth. Several thousand objects—mostly spent, upper stages of space-launch vehicles and inert spacecraft—are circling the Earth.

II

The Physics of Space

The boundary between the atmosphere of the Earth and space is diffuse rather than sharp. Because the density of air diminishes gradually with increasing altitude, the air in the upper atmosphere is so thin that it merges almost imperceptibly with space. At 30 km (19 mi) above sea level, the barometric pressure is one-eightieth of that at sea level; at 60 km (37 mi), it is 1/3,600; at 90 km (56 mi) it is 1/400,000. Even at an altitude of 200 km (124 mi), sufficient residual atmosphere remains to slow down artificial satellites by aerodynamic drag; thus long-duration satellites must have a higher orbital altitude.

A

Matter and Radiation in Space

By ordinary standards, space is a vacuum. Space, however, does contain very minute quantities of gases such as hydrogen and small quantities of meteoroids and meteoric dust (see Meteor; Meteorite). X-rays, ultraviolet radiation, visible light, and infrared radiation from the Sun and stars all traverse space. Cosmic rays, consisting mainly of protons, alpha particles, and heavy nuclei, are also present. See also Astronomy.

B

Gravitation

The law of universal gravitation states that every particle of matter in the universe attracts every other particle with a force directly proportional to the products of their masses and inversely proportional to the square of the distance between them. Consequently, the gravitational pull exerted by the Earth upon all other bodies (including spacecraft) diminishes with distance from the Earth. The gravitational field, however, extends to an infinite distance; gravity does not cease to act at any altitude. Objects in a spacecraft are said to be weightless when it is in orbit around the Earth (or around any other celestial body) because they do not experience the normal effects of weight, since all are moving in the same way under the influence of gravity. Under these conditions, the objects float freely in the craft.

Aerodynamic forces on the lifting surfaces (for example, the wings) of an aircraft keep it up against the force of gravity, but a space vehicle cannot stay aloft in this way because of the absence of air in space. The spacecraft, therefore, must orbit if it is to remain in space. Aircraft flying in the Earth’s atmosphere can use propellers and wings for propulsion and manoeuvring, but spacecraft cannot do so because of the lack of air. A space vehicle must rely on the reaction of rockets for propulsion and manoeuvres, using Newton’s laws of motion. When a spacecraft fires a rocket blast in one direction, the reaction imparts momentum to the spacecraft in the opposite direction.