CAPE CANAVERAL, Fla. -- The space agency unfurled 20 kilometers of cord high above Earth on Thursday in a test of a futuristic new means of space exploration.


The cord is called a space tether. Scientists say tethers could be used someday to change spacecraft altitudes, generate electricity, lower packages to Earth and explore parts of the atmosphere too high for balloons and too low for satellites.


The tether was carried into a 351-kilometer high orbit Wednesday night aboard a Delta rocket that also put the last of a series of 24 military navigation satellites into space.


It's the third time in a year that the National Aeronautics and Space Administration has unwound a tether in orbit. Unlike other tether tests, this one will not attempt to generate electricity as the cord zooms through Earth's magnetic field.


Instead, NASA wants to see how long the thin cord -- it's just eight-tenths of a millimeter (three-hundredths of an inch) thick -- will survive in orbit amid whizzing space junk.


It took nearly two hours for the entire tether to unwind. Sid Saucier, manager of the space systems project office at NASA's Marshall Space Flight Center in Huntsville, Ala., said everything appeared to go as expected.


The tether was wound inside a can aboard the rocket's spent second stage, with one end of the cord hooked to a 26-kilogram aluminum box. The spring-loaded box popped off an hour after liftoff, unwinding the tether as the box dropped.


The polyethylene fiber cord will remain attached to the rocket body until a micrometeorite or a manmade chunk of junk severs it, or until the rocket body falls through the atmosphere and burns up in about a month.


The experiment costs $5 million. In contrast, the failed tethered satellite experiment that flew on space shuttle Atlantis in 1992 cost $379 million. NASA succeeded with its two 1993 tether tests; both were low-priced Delta missions.