Cartwright said experts were still watching the debris fields and he could not yet rule out that hazardous material would fall to Earth.
But he said that as of Thursday morning, debris had only been seen in the atmosphere—none had been detected surviving re-entry. He indicated that debris appeared unlikely to pose a problem.
"Thus far we've seen nothing larger than a football," he said, referring to debris in the atmosphere spotted by radars and other sensors.
The military concluded that the missile had successfully shattered the satellite because trackers detected a fireball. Cartwright said it was unlikely that the fireball could have been caused by anything other than the hydrazine in the tank.
Cartwright said two other sources of information also indicate the fuel tank was hit: the appearance of a vapor cloud and the results of spectral analysis, or the study of light emissions, from devices aboard two aircraft that operate from the Pacific test range associated with the Pentagon's missile defense testing.
Debris from the satellite has started reentry and will continue through Thursday and into Friday, Cartwright said.
The size of the debris is smaller than the Pentagon had forecast and most of the satellite's intelligence value was likely destroyed, Cartwright said.
Analysts had said one of the reasons for the shoot-down was that officials worried that without it, larger chunks of the satellite could fall and be recovered, opening the possibility of secret technology falling into the hands of the Chinese or others.
Gates arrived in Hawai'i less than two hours before the missile was launched.
His press secretary, Geoff Morrell, said Gates had a conference call during his flight with Cartwright and Air Force Gen. Kevin Chilton, head of Strategic Command. They told him that "the conditions were ripe for an attempt, and that is when the secretary gave the go-ahead to take the shot and wished them good luck," Morrell said.
At 10:35 p.m. EST, Gates spoke to both generals again and "was informed that the mission was a success, that the missile had intercepted the decaying satellite, and the secretary was obviously very pleased to learn that," said Morrell.
China and Russia have expressed concern over the shoot-down, saying it could harm security in outer space. The elaborate intercept could be seen see as a thinly disguised attempt to test an anti-satellite weapon—one that could take out other nations' orbiting communications and spy spacecraft.
Within hours of the reported success, China said it was on the alert for possible harmful fallout from the shoot-down and urged Washington to promptly release data on the action.
"China is continuously following closely the possible harm caused by the U.S. action to outer space security and relevant countries," China's Foreign Ministry spokesperson Liu Jianchao said at news conference in Beijing. "China requests the U.S. to fulfill its international obligations in real earnest and provide to the international community necessary information and relevant data in a timely and prompt way so that relevant countries can take precautions."
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AP writer Pauline Jelinek contributed to this report from Washington.
Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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