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The Ride Down
When the space shuttles returned to Earth, they hit speeds of 7,700 meters a second (more than 17,200 mph). The friction between fast-moving spacecraft and the atmosphere's molecules generates intense heat of up to 1,477 degrees Celsius (2,691 degrees Fahrenheit). The energy generated during re-entry is intensive enough to heat a home for more than 40 years. For unmanned objects, such as space junk or satellites falling back to Earth, the stress and the high temperatures can cause these objects to break apart. Unmanned spacecraft that re-enter the atmosphere typically break up at altitudes between 84 kilometers and 72 kilometers (52 and 45 miles) above Earth, according to NASA. Items that do survive the fall hit the Earth at very low velocities.
What's Up There?
On average, one piece of man-made space debris has fallen back to Earth every day for more than 40 years, according to NASA̵7;s Orbital Debris Program Office. The heat of re-entry vaporizes most of these objects. But some objects survive, typically ending up in the oceans or in sparsely populated areas. More than 2 million kilograms (4.4 million pounds) of space junk -- the equivalent mass of 25 space shuttles -- orbits around the Earth, according to the NASA Goddard Space Flight Center. There are at least 19,000 pieces of space junk that are the size of a softball or bigger orbiting Earth.
NASA's Rules for Safety
NASA has rules for space junk aimed at reducing the odds of damage or harm from debris re-entering Earth̵7;s atmosphere. No NASA spacecraft may remain in low Earth orbit for more than 25 years after the mission ends or 30 years after launch. NASA requires that the risk of significant injury from a piece of debris re-entering Earth̵7;s atmosphere must be no greater than 1 in 10,000. ̶0;Significant injury̶1; is defined as a blow that delivers 15 joules of energy to the unprotected human body -- the equivalent of a 2-pound frozen chicken falling out of your freezer on your foot. The risk of an individual being hit and injured by a piece of debris is estimated to be less than 1 in 1 trillion, according to The Aerospace Corp. An individual has a much greater chance of being hit by lightning (1 in 1.4 million) or killed in a hurricane (1 in 6 million).
A Tap on the Shoulder
The only person known to have been struck by space debris is Lottie Williams of Tulsa, Oklahoma. Williams was out walking when a Delta II rocket body disintegrated as it re-entered the atmosphere. About a half hour later, Williams was struck on the shoulder by a piece of the falling debris. In a Feb. 16, 2012, article in Texas Christian University̵7;s "TCU Magazine," NASA astronomer-physicist Mark Matney said Williams thought she was being tapped on the shoulder by someone. The debris that struck Williams was described as a clothlike, 5-inch-long section of fiberglass. Matney said the odds of being struck by any particular piece of space debris are trillions to one. There̵7;s no need for hard hats, Matney told the magazine.