Estimates of where debris might fall will be narrowed hours before impact.
And a UK team studying the trajectory says the most likely time for re-entry could be after 23:00 GMT Friday, and as late as 03:00 GMT on Saturday.
Most of the decommissioned and now unpowered spacecraft should simply burn up, but modelling work suggests perhaps 500kg could survive to the surface.
UARS is the largest American space agency satellite to return uncontrolled into the atmosphere in about 30 years.