Launch and Reentry

Spaceflight became possible with the development of rockets that had suffi­cient power to break free of Earth’s gravity. To break free, a rocket must reach escape velocity, which is just over

25,0 miles per hour (40,200 kilome­ters per hour). Takeoff and reentry are the two most dangerous times in a spaceflight. Before the 1950s, some sci­entists argued that the human body could not survive the stresses of a space launch. The earliest flights by astro­nauts proved such views wrong. Astronauts have flown faster than any humans have before and have returned back to Earth unharmed.

Conventional rocket motors work both in air (for takeoff) and in space. Most rockets used to launch spacecraft are multistage vehicles propelled by chemical fuel burned in liquid or solid

form. The propellants must include oxy­gen, or the fuel will not burn, because there is no air in space.

Booster rockets provide extra thrust during takeoff. In a multistage rocket, boosters and lower stages separate and fall away as soon as their fuel is burned up. Only the topmost stage reaches space. The load a rocket lifts into space is called its payload-this could be a satellite, a manned spacecraft, or a robot space probe.

A rocket is streamlined for efficient, controlled, high-speed flight through the air. A spacecraft designed to return to Earth, like the Space Shuttle, also is streamlined, but it has wings. The wings help the Space Shuttle land like a con­ventional airplane after it has reentered Earth’s atmosphere.

Returning to Earth from space is potentially as dangerous as leaving it. A spacecraft must decelerate (slow down), using braking rockets, and approach at a precise angle so it does not hit the Earth’s atmosphere too fast. Reentry is accompanied by a rapid rise in tempera­ture. Air gets trapped in front of the spacecraft, which is moving so fast the air cannot escape. Compression (squeez­ing) of the air raises the temperature to more than 10,000°F (5,540°C). Spacecraft would burn up unless protected by a heat shield of tough, insulating material.