The Upper Atmosphere Research Satellite prior to deployment from the orbiter Discovery, STS-48.

(NASA)

Originally published September 23, 2011

There has been a lot of hype about the Upper Atmosphere Research Satellite (UARS) that will be re-entering Earth's atmosphere, with pieces of the satellite expected to crash land anywhere from South America to Canada. Many are worried whether or not they will be safe outside, or if a giant chunk of space metal will fall from the sky and do worse than ruin their day.

Our expert Andy Cox answers some of those questions to try to calm some of those fears.

UARS was decommissioned in 2005, ending its 14-year science mission

The Upper Atmosphere Research Satellite, or UARS, was launched in 1991 aboard space shuttle Discovery. UARS measured the chemical composition of the upper atmosphere, leading to a better understanding of Earth’s ozone layer and the role of the upper atmosphere in the Earth’s climate.

The target date for UARS to re-enter the atmosphere is Friday, September 23

As of Friday morning, NASA expects UARS to begin breaking up while entering the Earth’s atmosphere either late Friday, Sept. 23 or early Saturday, Sept. 24, United States time. NASA says, "There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent."

The risk of you - or anyone else in the world - being struck by debris is extremely remote.

The risk of any human casualties - that is, injuries or deaths - caused by UARS debris is 1 in 3,200. Put another way, the chance that nobody in the world will be injured by debris is about 99.97 percent.

The chance that you will be struck by debris is about 1 in 21 trillion. What does that mean? The National Weather Service says that your chance of being injured or killed by lightning in any given year are about 1 in 775,000.

The orbit of UARS can be tracked, but it cannot be controlled.

When a satellite has enough propellant on board (like Skylab in 1979), it can be moved to an orbit designed to bring it down to Earth in an uninhabited area. UARS used all its propellant when it was decommissioned and moved to a disposal orbit in 2005. That means its orbit cannot be changed now to control where it lands. This is known as an uncontrolled entry.

UARS will break up into smaller components during re-entry.

NASA performed a detailed analysis in 2002 of what would happen to the six-ton UARS during re-entry. As it travels through the Earth's atmosphere, UARS will begin to break up into 100 to 200 pieces, 26 of which are expected to make it to the Earth's surface.

The pieces that are expected to survive should vary between 1 pound and 350 pounds when they reach the surface. The differences in mass, shape, and size among these pieces means they will fall at different rates and land in different locations along the path of UARS re-entry. This range, which is called the debris footprint, is expected to be about 500 miles long.

The 1 in 3,200 risk estimate doesn't take into account a specific re-entry location.

The location of UARS at any one time can be tracked very accurately. However, the factors that affect how quickly it slows down are not as predictable. The density of the atmosphere at that level, which directly affects the amount of drag, cannot be measured accurately. The density can also vary based on solar activity.

Even small differences in the amount of drag can have a significant impact. This makes it very difficult to pinpoint the re-entry time and location of an uncontrolled entry like UARS until very close to re-entry time.

NASA's simplified public risk calculation is based on the expected debris field area compared to the population in the area that could be affected.

The orbit of UARS is tilted about 57 degrees with respect to the equator. This means the debris footprint - the predicted area where debris will strike the Earth - can only fall between 57 degrees north latitude and 57 degrees south latitude.The population density on land areas in that range is about 36 people per square mile.The 2002 NASA analysis of UARS estimates that the total surface area of the Earth that could be affected by the 26 pieces of debris is about 240 square feet - roughly the size of a one-car garage - spread across the debris footprint.

When those factors are combined together, the result is a 1 in 3,200 chance of at least one human casualty.

Even the final prediction will not be 100 percent certain.

The U.S. Strategic Command's Joint Space Operations Center is responsible for issuing tracking and impact prediction updates during the last few days before UARS re-entry. Their final prediction should be issued about two hours before entry.

At that point, the ground track - that is, the path along the Earth's surface - of the final UARS orbit will be known. However, the unpredictable atmospheric drag could mean a +/- 25 minute difference in when along that orbit UARS will begin to break up, which translates to about +/- 7,500 miles along the ground track.

The uncertainty to either side of the ground track will be much less. If the final UARS orbit does not travel directly overhead, your risk will basically drop to zero.

The bottom line: pay attention, but don't worry.