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Space exploration Returning to Earth


flying in the space

Returning to Earth involves the problem of decreasing the spacecraft's great speed. To do this, an orbiting spacecraft uses small rockets to redirect its flight path into the upper atmosphere. This action is called de-orbit. A spacecraft returning to Earth from the moon or from another planet also aims its path to skim the upper atmosphere. Air resistance then provides the rest of the necessary deceleration (speed reduction).

At the high speeds associated with reentering the atmosphere from space, air cannot flow out of the way of the onrushing spacecraft fast enough. Instead, molecules of air pile up in front of it and become tightly compressed. This squeezing heats the air to a temperature of more than 10,000 °F (5,500 °C), hotter than the surface of the sun. The resulting heat that bathes the spacecraft would burn up an unprotected vehicle in seconds. Insulating plates of quartz fiber glued to the skin of some spacecraft create a heat shield that protects against the fierce heat. Refrigeration may also be used. Early spacecraft had ablative shields that absorbed heat by burning off, layer by layer, and vaporizing.

Many people mistakenly believe that the spacecraft skin is heated through friction with the air. Technically, this belief is not accurate. The air is too thin and its speed across the spacecraft's surface is too low to cause much friction.

For unpiloted space probes, deceleration forces can be as great as 60 to 90 g's, or 60 to 90 times the acceleration due to gravity, lasting about 10 to 20 seconds. Space shuttles use their wings to skim the atmosphere and stretch the slowdown period to more than 15 minutes, thereby reducing the deceleration force to about 11/2 g's.

When the spacecraft has lost much of its speed, it falls freely through the air. Parachutes slow it further, and a small rocket may be fired in the final seconds of descent to soften the impact of landing. The space shuttle uses its wings to glide to a runway and land like an airplane. The early U.S. space capsules used the cushioning of water and "splashed down" into the ocean.

Contributor: James Oberg, M.S., Spaceflight Engineer; author, UFOs and Outer Space Mysteries.
Source : World Book 2005

Space exploration
What is space?
Getting into space and back
Living in space
Meeting basic needs in space
Communicating with Earth
The dawn of the space age
Space probes
Probes to Venus
Probes to Jupiter and beyond
Probes to comets
Human beings enter space
Apollo: Mission to the moon
Exploring the moon
Returning to Earth
The International Space Station
Space shuttles
Types of shuttle missions