An out of control satellite, the size of a bus, is due to hit the earth this evening, it is still not known where.
The chances of it landing on a person are around one in 3,000.
A remote UK air force base is a the forefront of international efforts to track the dead six-ton satellite.
RAF Fylingdales, which is high on the North York Moors, near Whitby, is using its giant radar to track the Upper Atmosphere Research Satellite (UARS).
The Fylingdales base forms part of a worldwide network of powerful radars and tracks all objects in orbit bigger than 10cm.
The station was originally built at the height of the Cold War to track any incoming ballistic missile attack – a role it still performs.
An RAF spokeswoman said: “The Space Operations Room at Royal Air Force Fylingdales is manned 24 hours a day by specialist Royal Air Force and civilian personnel, and its operators will be working to track the UARS object as it returns to the atmosphere.
“The Solid State Phased Array Radar (SSPAR) is being tasked by the United States Air Force and the Royal Air Force to concentrate its radar energy towards the object in order to track its final orbit.
“This information will then be used by various different agencies to predict the path of re-entry into the Earth’s atmosphere.”
The spokeswoman said: “With the majority of the Earth being covered by water, the chances of an impact on solid ground are slim to negligible.
“Whilst the object may survive the harsh conditions of its journey back to Earth, it is important to remember that parts of the satellite – if not all of it – will burn up on re-entry.”
The old Nasa research satellite is expected to come crashing down through the atmosphere this evening, Irish time.
An estimated 26 pieces – representing 1,200lb – are expected to survive.
Yesterday, the Aerospace Corporation in California, predicted that re-entry will occur over the Pacific Ocean.
The 20-year-old Upper Atmosphere Research Satellite will be the biggest Nasa spacecraft to fall uncontrolled from the sky in 32 years.
It is expected to break into more than 100 pieces as it enters the atmosphere, most of it burning up.
The heaviest metal parts are expected to reach Earth, the biggest chunk weighing about 300lb. The debris could be scattered over an area about 500 miles long.
The 35ft long UARS was launched in 1991 by the Space Shuttle Discovery to study the ozone layer. It was decommissioned in 2005.