Satellite chunks earthbound

A six-ton Nasa satellite that is on course to crash back to Earth is not expected to hit North America, the US space agency said in its latest update.

Re-entry is expected some time between 7pm tonight and 7pm tomorrow night, South African time.

"It is still too early to predict the time and location of re-entry with more certainty, but predictions will become more refined," Nasa said.

All but 26 pieces of the Upper Atmosphere Research Satellite are expected to burn up on re-entry. The surviving chunks will include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims. The parts may weigh as little as 1kg or as much as 158kg.

Orbital debris scientists said the debris will fall over an area of about 800km. The risk to human life and property from Uars was "extremely small", with a one in 3200 chance that someone, somewhere in the world would be hit, Nasa said.