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. The barometric pressure, which is a measure of atmospheric density, is 760 torrs at sea level. (One torr is defined as the pressure caused by the weight of a column of mercury 1 mm/about 0.04 in high at sea level.) At 30 km (19 mi) above sea level, the barometric pressure is 9.5 torrs; at 60 km (37 mi), 0.21 torr; at 90 km (56 mi), 0.0019 torr. 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.
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 meteorites and meteoric dust. X rays, ultraviolet radiation, visible light, and infrared radiation from the sun all traverse space. Cosmic rays, consisting mainly of protons, alpha particles, and heavy nuclei, are also present.
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. A spacecraft is said to be weightless when it is in orbit around the earth (or around any other celestial body) because the centrifugal effect (which acts away from the center) is then equal and opposite to the force of gravity. Under these conditions, objects in a spacecraft seem to float in space. In the same way, the moon does not fall toward the earth because of the centrifugal effect that balances the force of gravity.
Aerodynamic forces on the lifting surfaces (for example, the wings) of an aircraft keep it aloft 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 winged surfaces for propulsion and maneuvering, but spacecraft cannot do so because of the lack of air. A space vehicle must rely on the reaction of rockets for propulsion and maneuvers, based on Newton's laws of motion. When a spacecraft fires a rocket blast in one direction, reaction against the rocket exhaust imparts momentum to the spacecraft in the opposite direction.
Humans in Space
Space is a hostile environment for humans in a number of ways. It contains neither air nor oxygen, so human beings are unable to breathe. The vacuum of space can destroy an unprotected human body in a few seconds by explosive decompression. Temperatures in space in the shadow of a planet approach absolute zero; on the other hand, temperatures can become fatally high under direct solar radiation. Energetic solar and cosmic radiations in space may also be fatal to an unshielded person who is not protected by the atmosphere of the earth. These environmental conditions can also affect the instruments and devices used in spacecraft, so the design and construction of these materials are dictated by the space environment. Experiments in weightlessness for long periods of time have been studied intensively to discover what adverse effects this condition will have on humans in space .Mission Pilot Performs Free-floating Camera Work
Astronaut Robert Cabana, pilot of Shuttle Mission STS-41, prepares to use a specialized camera on the mid-deck of shuttle Discovery. The camera is just one of many pieces of equipment carried on every shuttle flight to record all aspects of the mission.

Spacewalker and Manned Maneuvering Unit
Astronaut Bruce McCandless floats free above the earth in a manned maneuvering unit (MMU) during a shuttle mission. McCandless helped design and was the first to fly the MMU, which is propelled by small nitrogen thrusters controlled by the astronaut's hands. Because no umbilical cord attaches the astronaut to the spacecraft, it gives much greater mobility than was available to earlier spacewalkers.
Humans can be protected against the space environment in several ways. At present, they are enclosed inside a hermetically sealed cabin or space suit, with a supply of pressurized air or oxygen to approximate conditions on earth. Air conditioning controls the temperature and humidity inside the cabin or space suit. Absorbing and reflecting surfaces on the outside of the spacecraft regulate the amount of heat radiation affecting the craft. Furthermore, space journeys are carefully planned to avoid the intense radiation belts around the earth. On long interplanetary voyages of the future, heavy shielding might be necessary to protect against solar radiation storms; or crews might be sheltered in a central position within the spacecraft with supplies and equipment to surround and shield them. For lengthy space journeys, or for prolonged stays in an earth-orbiting satellite, the effects of weightlessness might be reduced by spinning the craft so that the centrifugal effect provides artificial gravity. For this purpose, the spacecraft might be shaped like a large wheel that spins slowly around its own axis, or it might be built like a dumbbell, both ends of which rotate around the center of gravity of the dumbbell.