(CNN) -- A satellite whose orbit is degrading is likely to crash back to Earth on Friday, and 26 pieces have a good chance of surviving the heat of re-entry, NASA said.
Despite being pretty sure that the Upper Atmosphere Research Satellite, or UARS, will re-enter the atmosphere sometime Friday afternoon, U.S. time, NASA says there is no way to know where it will fall.
Because the satellite travels thousands of miles in a matter of minutes, even minutes before re-entry, it will be impossible to pinpoint an exact location, Mark Matney of NASA's Orbital Debris team said Wednesday. On top of that, he said, "part of the problem is the spacecraft is tumbling in unpredictable ways and it is very difficult to very precisely pinpoint where it's coming down even right before the re-entry."
NASA says most of the six-ton spacecraft is made of aluminum, which has a relatively low melting temperature and will burn up on re-entry. But about half a ton of material is likely to make it through.
"There are some pieces that are made of stainless steel and titanium and beryllium that have very high melting temperatures, and those pieces will survive," Matney said.
He said NASA has identified 26 pieces, ranging from tens of pounds to a few hundred pounds, that could make it.
Because water covers about 70 percent of the Earth's surface, NASA believes most, if not all the debris that survives will land in an ocean or sea. And even if pieces strike dry land, there's very little risk any will hit people.
Objects this size re-enter the Earth's atmosphere about once a year, according to NASA. This is the largest NASA satellite to re-enter in about 30 years, but at six tons, it's only a fraction of the size of the 75-ton Skylab that fell back to Earth in 1979. Portions of it hit Western Australia.