In September 1991, NASA deployed the Upper Atmosphere Research Satellite (UARS) from the space shuttle Discovery. Its purpose was to measure chemical compounds in the ozone layer, atmospheric winds and temperatures, and energy produced by the Sun. These measurements helped scientists determine the role of the upper atmosphere in climate regulation. After 14 years of gathering data, the UARS was decommissioned on December 14, 2005. Scientists left it floating around in space with the knowledge that it would eventually return to Earth. The "eventually" is now close at hand. This Friday, to be specific.
On Friday, chunks of the UARS that do not burn up during re-entry will come crashing down to the Earth's surface. NASA analysts estimate a total of 532 kilograms of satellite components will be landing somewhere between 57 degrees North latitude and 57 degrees South latitude, a gap from Alaska to the southern tip of South America. Do not worry about being hit by a piece of satellite though. NASA has calculated the chance of 1 in 3200 that a person would get hit, and 1 in 22 trillion that that person will be you.
How can they know that? By using the physics of falling objects, universal gravitation, and knowing the masses and velocities of the pieces that will soon be hurtling towards Earth. It's still kind of a long shot, but considering how much of the Earth is covered in water, plus adding in the uninhabitable areas like deserts and high mountain ranges, the regions where a satellite fragment might actually hit a person are drastically reduced. NASA will have to be tracking the satellite during its re-entry in order to really know exactly where the pieces are going to land. Hopefully we will get to see some part of the process, for it will be - as the Science author put it - like "a $750 million fireworks display."
Science article about re-entry: http://news.sciencemag.org/sciencenow/2011/09/scienceshot-place-your-bets-on.html
Science article about the "dangers" of the falling UARS: http://news.sciencemag.org/sciencenow/2011/09/the-sky-is-falling-but-no-big-de.html?ref=hp
NASA site tracking the UARS: http://www.nasa.gov/mission_pages/uars/index.html