How do spacecraft re-enter the Earth's atmosphere?

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­Launching a spacecraft into space is one thing. B­ringing it back is another.

Spacecraft re-entry is tricky business for several reasons. When an object enters the Earth's atmosphere, it experiences a few forces, including gravity and drag. Gravity will naturally pull an object back to earth. But gravity alone would cause the object to fall dangerously fast. Luckily, the Earth's atmosphere contains particles of air. As the object falls, it hits and rubs against these particles, creating friction. This friction causes the object to experience drag, or air resistance, which slows the object down to a safer entry speed. Read more about these factors in "What if I threw a penny off the Empire State Building?"

Space Shuttle Image Gallery 

meteor entering eath's atmosphere
Pete Turner/Stone Collection/Getty Images
Objects that enter the Earth's atmosphere face a rough trip. See more spacecraft and space shuttle pictures.

This friction is a mixed blessing, however. Although it causes drag, it also causes intense heat. Specifically, shuttles face intense temperatures of about 3000 degrees Fahrenheit (about 1649 degrees Celsius) [source: Hammond]. Blunt-body design helps alleviate the heat problem. When an object -- with blunt-shaped surface facing down -- comes back to Earth, the blunt shape creates a shock wave in front of the vehicle. That shock wave keeps the heat at a distance from the object. At the same time, the blunt shape also slows the object's fall [source: NASA]. The Apollo program, which moved several manned ships back and forth from space during the 1960s and 1970s, coated the command module with special ablative material that burned up upon re-entry, absorbing heat.

Unlike the Apollo vehicles, which were built for one-time use, space shuttles are reusable launch vehicles (RLVs). So instead of merely using ablative material, they must incorporate durable insulation. On the next page, we'll delve more deeply into the modern re-entry process for shuttles.

The Demise of the Satellite
Satellites don't have to stay up in Earth's orbit forever. Old satellites sometimes fall back to Earth. Because of the harsh conditions of re-entry, they can severely burn up on their way down. However, some of them can survive the fall and hit the Earth's surface. In controlled falls, engineers manipulate the propulsion systems on a satellite to make it fall in a safe place, like the ocean.

The Descent of a Space Shuttle

Re-entering Earth is all about attitude control. And, no, this doesn't mean astronauts need to keep a positive attitude (although that's always helpful). Rather, it refers to the angle at which the spacecraft flies. Here's an overview of a shuttle descent:

  1. Leaving orbit: To slow the ship down from its extreme orbit speed, the ship flips around and actually flies backwards for a period of time. The orbital maneuvering engines (OMS) then thrust the ship out of orbit and toward Earth.

  1. Descent through atmosphere: After it's safely out of orbit, the shuttle turns nose-first again and enters the atmosphere belly-down (like a belly-flop) to take advantage of drag with its blunt bottom. Computers pull the nose up to an angle of attack (angle of descent) of about 40 degrees.

  1. Landing: If you've seen the movie "Apollo 13," you might remember that the astronauts return to Earth in their command module and land in the ocean where rescue workers pick them up. Today's space shuttles look and land much more like airplanes. Once the ship gets low enough, the commander takes over the computers and glides the shuttle to a landing strip. As it's rolling along the strip, it deploys a parachute to slow it down.

space shuttles' leading wing edge and nose
NASA
The leading edges and nose of the shuttle use RCC material.

The trip back to Earth is a hot one. Instead of the ablative materials found on the Apollo spacecraft, today's space shuttles have special heat-resistant materials and insulating tiles that can sustain re-entry heat.

  • Reinforced Carbon Carbon (RCC): This composite material covers the nose and edges of the wing, where temperatures get the hottest. In 2003, Columbia's RCC was damaged during liftoff, causing its burn-up on re-entry, killing all seven crew members.

  • broken insulation tiles
    NASA/Space Frontiers/Hulton Archive/Getty Images

    In this image, NASA workers show where the Columbia suffered tile damage during its maiden flight.

    High-temperature reusable surface insulation (HRSI)­: These black silica tiles are on the bottom of the shuttle and various other places that can reach up to 2,300 degrees Fahrenheit (1,260 degrees Celsius).

  • Fibrous Refractory Composite Insulation (FRCI): These black tiles have replaced HRSI tiles in many places because they are stronger, lighter and more heat resistant.

  • Low-temperature reusable surface insulation (LRSI): These white silica tiles are thinner than HRSI tiles and protect various areas from temperatures up to 1,200 degrees F (649 degrees C).

  • Advanced Flexible Reusable Surface Insulation (AFRSI): Made of silica glass fabric, these exterior blankets are installed on the forward upper section of a shuttle and withstand temperatures of up to 1,500 degrees F (816 degrees C). Over the years, these have taken over for much of the LRSI material on a shuttle.

  • Felt reusable surface insulation (FRSI): This material sustains temperatures of up to 700 degrees F (371 degrees C) and is made of heat-treated white Nomex felt (a material used in firefighters' protective clothing).

Take a look at the links on the next page to find out more about the challenges posed by space exploration.

Bitter Reminders
Just as the Challenger disaster in 1986 reminded us how risky shuttle launches are, the Columbia disaster reminded us just how dangerous atmospheric re-entry is. In 2003, the space shuttle Columbia and its seven crew members burned up as they were returning to Earth. After investigation, NASA discovered that damage to the left wing (that actually occurred during liftoff), let hot air in upon re-entry and caused the shuttle to lose control and burn up.

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Sources

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    http://curious.astro.cornell.edu/question.php?number=448
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