The Science Guys
Science Guys > March 2003
What generates all the heat during re-entry when the space shuttle returns to Earth?
The heroic deaths of the space shuttle Columbia’s seven astronauts raised many questions about the safety of the shuttle program. Teams of NASA investigators still are not sure what caused the shuttle to disintegrate in the Earth’s upper atmosphere. Re-entry is a particularly dangerous time for the shuttle, a time during which the shuttle experiences tremendous stress and high temperatures.
There are actually two different phenomena at work to heat the shuttle, compressive heating and friction.
Perhaps you have noticed when you use a bicycle pump that the fitting at the end of the pump gets very hot, very quickly. That heat comes primarily from the action of your muscles pushing on a plunger and compressing the air in the pump. When air (or any gas, for that matter) is compressed it heats up; conversely when it expands it cools.
Now consider the re-entry of the space shuttle or the fall of a meteor through our atmosphere. Initially, the shuttle moves around Earth in the emptiness of space at a tremendous speed. The astronauts slow down by firing some thrusters and gravity begins to pull the shuttle to a lower orbit. As the shuttle gets lower, it eventually begins to plow through the Earth’s atmosphere at initial speed of about 17,000 miles per hour! During re-entry, the shuttle is going so fast, it compresses the air ahead of it. The compression of the air layers near the leading edges of the shuttle is quick, causing the temperature of the air to rise to as high as 3000 degrees Fahrenheit! Being in contact with the shuttle, it heats the shuttle’s surface. Normally, this high temperature will melt almost any material- from the rock of a meteor to the metal skin of a space shuttle. Thus, the shuttle needs a layer of insulation.
The shuttle enters or "attacks" the atmosphere at such an angle that its nose and underside contacts and compresses the air and absorbs most of the heat generated. On the underside are incredibly heat resistant, insulating silica tiles. They conduct heat very poorly and thus keep heat from penetrating to the metal skin of the shuttle. Without the tiles, the 3000 degree air touching the shuttle’s metal skin would melt through it. The shuttle can lose an occasional tile and not incur much heat damage. However, if many tiles are missing the heat can do severe damage. Friction also creates heat as the air molecules rub across the shuttle’s surface. This further contributes to the heat the shuttle must endure.
The space shuttle uses 30 year old technology. Modern rockets have fewer parts and better materials. Using this advanced technology, NASA is now designing a safer, next generation ’space plane’ that may be in use in 10 years.