A satellite's journey through the atmosphere
From the Pacific Ocean, a Navy ship is expected as early as tonight or tomorrow to shoot down, or at least attempt to do so, an out-of-control U.S. spy satellite that is falling toward Earth. Government officials say the satellite, primarily due to its tank containing 1,000 pounds of unused toxic fuel, could pose a safety threat if it is not intercepted.
Believe it or not, approximately 100 to 200 large manmade objects reenter the atmosphere each year, usually with little fanfare. These objects -- mostly satellites and related hardware -- encounter levels of the atmosphere quite different from what we're familiar with closer to the Earth's surface.
Most satellites orbit Earth at altitudes from 100 miles to more than 22,000 miles up, and zoom around the planet at speeds from about 7,000 mph to 17,000 mph.
As a dying satellite loses speed, it also loses altitude and drops through the exosphere, the outer edge of the Earth's atmosphere where the line between space and atmosphere is fuzzy at best. The top of this uppermost layer of the atmosphere is more than 6,000 miles above the ground. The density of molecules, mainly hydrogen and helium gas, is so low that the chance of them bumping into each other is very small. With few collisions to slow them down, some molecules gather enough speed to escape into space. Because of its low density, the exosphere has little effect on a falling satellite, and in fact some satellites orbit Earth within this layer.
Around 430 miles above the Earth's surface, a falling satellite would reach the top of the thermosphere. Temperatures decrease dramatically through the thermosphere, from a searing 3,000°F or so near the top of the layer to a frigid minus 150°F near the bottom. On the other hand, the density of the air increases with decreasing height, causing more and more friction between the atmosphere and satellite as it falls. The heat generated by the friction can begin to breakup, melt or vaporize the satellite, or lat least parts of it, as it reaches the lower end of the thermosphere.
The top of the mesosphere begins about 50 to 55 miles above the ground. After bottoming out at the bottom of the thermosphere, temperatures rise moving downward through the mesosphere, approaching 30°F by the bottom of the layer. But air density continues to increase with decreasing height. Any unprotected components of a falling satellite that make it through the thermosphere are unlikely to survive the increased friction of the mesosphere. It was in this layer that the space shuttle Columbia disintegrated while reentering the atmosphere in February 2003.
About 30 miles from the Earth's surface, temperatures begin to fall again. This marks the top of the stratosphere, which is probably best known for its "good ozone." Ozone gas occurs naturally in the stratosphere and absorbs harmful ultraviolet radiation from the sun, limiting the amount that reaches the ground. But manmade chemicals called chlorofluorocarbons (CFCs) have been conclusively linked to the "ozone hole," a large area of decreased ozone that appears annually over Antarctica from September through December. Although CFCs were banned in the late 1980s, recovery of the ozone hole has lagged due to the long lifetimes (more than 40 years) of CFCs in the atmosphere. Only satellite parts that are protected by specially designed heat shields have a legitimate chance of surviving into the stratosphere.
A final shift in temperature occurs around 5 to 10 miles above the ground. This is the top of the troposphere, the layer that contains the clouds we see, the weather we experience and the air we breathe. Temperatures in the troposphere increase with decreasing height, climbing from around minus 50°F to minus 100°F at the top to the temperatures we feel here at the surface. Ozone found in the troposphere is considered "bad ozone." It forms when pollution from vehicles, factories and other manmade sources interacts with sunlight, and too much of it can result in unhealthy air. Any satellite component that reaches the troposphere is likely to hit the ground at a relatively low speed, slowed by the increasing density of the air, but can still be a threat to people and property.
How much of a threat? According to the Center for Orbital and Reentry Debris Studies, more than 50 objects are believed to have survived reentry and hit the ground over the past 50 years. But in that time, only one person has reported being struck, and she was not injured. Overall, the odds of being hit and hurt by satellite debris is less than one in 1 trillion, far less than the one-in-1.4-million chance of someone in the United States being hit by lightning.