What goes up must come down.NASA's redundant Upper Atmosphere Research Satellite is no exception to the rule.It's expected to plunge back towards Earth on Friday.Most of its bus-sized bulk is expected to burn up in the atmosphere it spent 14 years studying.But about 26 pieces weighing a total of around half a tonne will come crashing down to Earth somewhere.NASA says the craft won't be meeting its maker, in a manner of speaking, because the debris will miss North America.But they say the chances of anyone being hurt by the falling debris are very slim.Dr. William Ailor, Director, Center for Orbital and Re-entry Debris Studies, Aerospace Corporation, said, "We've had over the last 40-some years or 50 years , whatever it is, of putting things into space. We've had a lot of debris come down, sometimes a hundred objects a year or more. We've had only one person brushed on the shoulder by a very light piece of debris in all that time and so we all think that the probability of getting hit by something is very low."NASA reckons the odds of anyone being hit by a piece of plunging satellite are 1-in-3, 200.But the scientists say anyone who gets to see the satellite's swansong is in for a spectacular treat.