No debris remains in space from the U.S. destruction a year ago of an errant spy satellite loaded with toxic hydrazine fuel, the head of the Pentagon's Strategic Command said.

By contrast, some of the debris caused when China used a ground-based ballistic missile to destroy one of its defunct weather satellites will stay in orbit for another 80 or 90 years, said Air Force Gen. Kevin Chilton, the command's chief.

"Every bit of debris created by that (U.S.) intercept has de-orbited," Chilton told a symposium on air warfare hosted by the U.S. Air Force Association in Orlando, Florida, on Thursday.

The U.S. military used a ship-launched Raytheon Co Standard Missile-3 missile to destroy a crippled National Reconnaissance Office satellite on Feb. 20, 2008. It was shot apart at an altitude of about 130 miles.

The intercept was interpreted by many analysts as a demonstration of U.S. capabilities in response to a Chinese anti-satellite test a year earlier. The Chinese satellite had been in polar orbit at an altitude of about 537 miles. Since it was higher up, it will take longer for the debris to re-enter the earth's atmosphere.

Space junk is a threat to the 800 or so commercial and military satellites estimated to be operating in space as well as to the International Space Station.

The Strategic Command, which coordinates U.S. military operations in space, said it is now tracking about 2,200 pieces of orbiting junk created by the Chinese anti-satellite demonstration in January 2007

Chilton, in a followup session with reporters, said the last bits of debris from the U.S. intercept, which he said had been codenamed Burnt Frost, de-orbited as early as last July or August.

On Feb. 25, 2008, the U.S. Defense Department said a space operations center at Vandenberg Air Force Base, California, had been tracking fewer than 3,000 pieces of debris from the U.S. intercept, all smaller than a football.

After the U.S. operation, the Pentagon said there had been no reports of debris landing on Earth and it was unlikely any would remain intact after re-entering the atmosphere.

Chilton said the orbiting debris problem had worsened, with some 18,000 bits now being tracked by the United States. On Feb. 10, a dead Russian military communications satellite collided with a commercial U.S. satellite that was part of the Iridium global communications network.

This collision highlighted a need for more investment in sensors that could help keep track of debris and improve U.S. "space situational awareness," he said.