An old satellite set to fall to Earth in pieces will do so on Friday afternoon ET, but won't be over North America when it comes down, NASA says.

As of 7 a.m. ET Thursday, the U.S. space agency was predicting that the bus-sized Upper Atmosphere Research Satellite (UARS) would re-enter Earth's atmosphere on Friday afternoon, but could not yet predict the time and location of re-entry precisely.

"The satellite will not be passing over North America during that time period," it added.

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The Upper Atmosphere Research Satellite, shown here being launched using a robotic arm from the space shuttle Discovery in 1991, has a mass of nearly six tonnes. ((NASA))

NASA said it will be able to provide more precise estimates of the time and location of re-entry within the next 24 to 36 hours.

The UARS satellite, launched in 1991 to take measurements of the ozone layer, has a mass of almost six tonnes and is a little over 10 metres long with a diameter of about 4.5 metres.

It was decommissioned in 2005 and other satellites have taken over the types of measurements it used to make.

Much of the satellite is expected to burn up during re-entry, but about 450 kilograms of debris may survive and fall to the ground, including a couple of dozen potentially hazardous pieces.

NASA expects any surviving components to land between 57 north degrees latitude — about 300 kilometres south of the boundary between the territories and the provinces — and 57 degrees south latitude.

The debris is expected to be scattered across an area about 800 kilometres long.