Satellite Falling to Earth? NASA Scientist Puts It Into Perspective

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NASA / Handout / Reuters

The seven-ton Upper Atmosphere Research Satellite (UARS) is deployed by the Space Shuttle Discovery (STS-48) in this NASA handout photo dated September 1991. NASA is expecting the satellite to re-enter Earth's atmosphere.

Every satellite falls back toward earth. But when it’s a giant, 12,500-pound version (about the size of an average elephant), expected to hit somewhere on earth between Thursday and Saturday, people take notice. While the Upper Atmosphere Research Satellite (UARS) rushes at 17,000 mph toward reentry, the NASA satellite could fling as many as 26 pieces of debris in a 500-mile path anywhere from your house to the middle of the open seas.

TIME spoke with Nick Johnson, NASA’s chief scientist for orbital debris, about the upcoming reentry of UARS, the frequency of similar events, any possible danger to people on the ground—there remains a 1-in-3,200 chance of any debris striking a person—and how he predicts reentry locations (not really all that possible, just to tip you off).

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What makes a satellite spin out of control and head toward Earth?

Every satellite is gradually falling back to Earth due to atmospheric drag and solar radiation pressure. Many operational spacecraft, like the International Space Station, must periodically make small maneuvers to return to their original altitudes. UARS was decommissioned in December 2005, and its altitude has been slowly decreasing ever since.

How do you attempt to predict the impact zone?

The U.S. Space Surveillance Network, operated by the Defense Department’s Strategic Command, makes predictions of when the satellite is likely to reenter the dense portion of the atmosphere. However, such predictions cannot be precise. At two hours before reentry, the average uncertainty in reentry time is plus or minus 25 minutes. This is equivalent to plus or minus 7,500 miles.

How often does reentry occur?

On average, one tracked object reenters the atmosphere each day. Most of these are very small. An intact spacecraft or launch vehicle orbital stage typically falls back to Earth once a week.

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What are the risks to everyday life?

The risks are extremely small. To date, no incidents of human injury or significant property damage from reentering debris have been reported throughout the entire Space Age.

How do different sizes of satellites concern you at different levels?

The size of a satellite is not directly related to risk of injury or property damage. Most of all satellites burn up completely. Some components, of either small or large satellites, can survive reentry depending upon their material type and other factors. The larger satellites typically have a greater number of survivable components.

How many satellites or other items are in space right now that could lose control?

All satellites are falling back to Earth, although at different rates. The higher the altitude, the longer it takes to reenter.

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Tim Newcomb is a contributor for TIME. Find him on Twitter at @tdnewcomb. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.