How do spacecraft reenter the Earth?

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

Spacecraft reentry 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, 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 down the object to a safer entry speed. Read more about these factors in "What if I threw a penny off the Empire State Building?"

This friction is a mixed blessing, however. Although it causes drag, it also causes intense heat. Specifically, shuttles faced intense temperatures of about 3000 degrees Fahrenheit (about 1649 degrees Celsius) [source: Hammond]. Blunt-body design helped 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 were reusable launch vehicles (RLVs). So instead of merely using ablative material, they incorporated durable insulation. Next, we'll delve more deeply into the modern re-entry process for shuttles.

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:

Leaving orbit: To slow the ship down from its extreme orbit speed, the ship flipped around and actually flew backward for a period. The orbital maneuvering engines (OMS) then thrust the ship out of orbit and toward Earth.
Descent through atmosphere: After it was safely out of orbit, the shuttle turned nose-first again and entered the atmosphere belly-down (like a belly-flop) to take advantage of drag with its blunt bottom. Computers pulled the nose up to an angle of attack (angle of descent) of about 40 degrees.
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. Space shuttles looked and landed much more like airplanes. Once the ship got low enough, the commander took over the computers and guided the shuttle to a landing strip. As it rolled along the strip, it deployed a parachute to slow it down.

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

Space shuttle Columbia tile damage — In this image, NASA workers show where the Columbia suffered tile damage during its maiden flight.
NASA/Space Frontiers/Hulton Archive/Getty Images

Reinforced Carbon Carbon (RCC): This composite material covered 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 reentry, killing all seven crew members.
Fibrous Refractory Composite Insulation (FRCI): These black tiles 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 protected 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 were 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 took 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 that follow to find out more about the challenges posed by space exploration.

Frequently Asked Questions

How does the angle of reentry affect a spacecraft's ability to withstand intense heat?

The angle of reentry is crucial for managing the spacecraft's heat exposure. A steep reentry angle can lead to excessive heating and potential damage, while too shallow an angle might result in the spacecraft bouncing off the atmosphere. The optimal angle ensures the spacecraft can withstand intense heat through controlled deceleration and heat distribution, utilizing thermal protection systems effectively.

What advancements have been made in thermal protection systems since the Space Shuttle?

Since the Space Shuttle era, advancements in thermal protection systems (TPS) have focused on improving heat resistance and durability. New materials and technologies, such as improved ablative coatings, reinforced carbon-carbon, and advanced silica tiles, offer better protection against reentry temperatures.

Lots More Information

Sources

Cuk, Matija, Dave Rothstein, Britt Scharringhausen. "Why do spacecraft need heat shields coming back to Earth but not leaving?" Astronomy Department at Cornell University. Jan. 2003. (May 9, 2008) http://curious.astro.cornell.edu/question.php?number=448
Day, Dwayne A. "Reentry Vehicle Technology." U.S. Centennial of Flight Commission. (May 9, 2008) http://www.centennialofflight.gov/essay/Evolution_of_Technology/ reentry/Tech19.htm
Dumoulin, Jim. "Space Shuttle Orbiter Systems." NASA Kennedy Space Center. (May 9, 2008) http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_sys.html
Hammond, Walter Edward. "Design Methodologies for Space Transportation Systems." AIAA, 2001. (May 9, 2008) http://books.google.com/books?id=uxlKU3E1MUIC&dq=Design+ Methodologies+for+Space+Transportation+Systems&as_brr=3& client=firefox-a&source=gbs_summary_s&cad=0
Jacobson, Nathan S. "As-Fabricatede Reinforced Carbon/Carbon Characterized." NASA. July, 2005. (May 9, 2008) http://www.grc.nasa.gov/WWW/RT/2004/RM/RM01D-jacobson1.html
NASA. "Adventures with Apollo." Ames Research Center. (May 9, 2008) http://www.nasa.gov/centers/ames/news/releases/2004/moon/ adventure_apollo.html
NASA. "HSF - The Shuttle: Entry." NASA. Feb. 13, 2003. (May 9, 2008) http://www.spaceflight.nasa.gov/shuttle/reference/shutref/events/entry/
Pete-Cornell, M. Elisabeth. "Safety of the Thermal Protection System of the Space Shuttle Orbiter: Quantitative Analysis and organizational Factors." Report to the National Aeronautics and Space Administration, Dec, 1990. (May 9, 2008) spaceflight.nasa.gov/shuttle/archives/sts-107/investigation/tps_safety.pdf

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