A five tonne, 20-year-old satellite has fallen out of orbit and is expected to crash somewhere on Earth on or around 24 September, according to Nasa.
Nasa says the risk to life from the UARS - Upper Atmosphere Research Satellite - is just 1 in 3,200.
It could land anywhere between 57 degrees north and 57 degrees south of the equator - most of the populated world.
However, most of the satellite will break or burn up before reaching Earth.
Scientists have identified 26 separate pieces that could survive the fall through the earth's atmosphere, and debris could rain across an area 400-500km (250-310 miles) wide.
Nasa said scientists would only be able to make more accurate predictions about where the satellite might land two hours before it enters the Earth's atmosphere.
The 1 in 3,200 risk to public safety is higher than the 1 in 10,000 limit that Nasa aims for.
However, Nasa told reporters that nobody had ever been hurt by objects re-entering from space.
Members of the public are not allowed to keep pieces of the satellite that may fall to Earth, or sell them on eBay, as they remain the property of the US government.
The UARS was launched in 1991 by the Discovery space shuttle, and was decommissioned in 2005.
The latest satellite re-entry is much smaller than Skylab, which re-entered the earth's atmosphere in 1979.
It was some 15 times heavier than the UARS, and when it crashed in Western Australia the US government had to pay clean-up costs to the Australian government.
Sputnik 2 crashed on Earth in 1958, travelling from over New York to the Amazon in 10 minutes. It was viewed by many people and left a trail of brightly coloured sparks behind it.