The $750 million (£468 million) Upper Atmosphere Research Satellite (UARS) satellite, launched 20 years ago to study climate change, is set to breach the atmosphere within weeks.
In a new alert issued this week, officials warned pieces could land in densely populated areas on six continents including parts of Britain, Europe, North and South America and Asia.
Nasa claimed the risk to public safety from the “dead” satellite – which is orbiting just over 155 miles above the earth with an inclination of 57 degrees – was “extremely small”.
But senior space agency officials admitted they were “concerned” about the risk to billions of people when it starts falling uncontrolled out of orbit at any stage from later this month.
Nasa admitted more than half a tonne of metal from the satellite, which ran out of fuel in 2005, will survive as the majority of it will burn up after entering Earth's atmosphere.