WASHINGTON — Debris from an obliterated U.S. spy satellite is being tracked over the Pacific and Atlantic oceans but appears to be too small to cause damage on Earth, a senior military officer said Thursday, just hours after a Navy missile scored a direct hit on the failing spacecraft.

Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff and an expert on military space technologies, told a Pentagon news conference that officials have a ”high degree of confidence” that the missile launched from a Navy cruiser Wednesday night hit exactly where intended.

It was an unprecedented mission for the Navy, so extraordinary that the final go-ahead to launch the missile Wednesday was reserved for Defense Secretary Robert Gates rather than a military commander.

Cartwright estimated there was an 80 percent to 90 percent chance that the missile struck the most important target on the satellite — its fuel tank, containing 1,000 pounds of hydrazine, which Pentagon officials say could have posed a health hazard to humans if it had landed in a populated area.

Alluding to a video clip of the missile smashing into the satellite, which he showed at the news conference, Cartwright said, ”We have a fireball, and given that there’s no fuel (on the tip of the missile), that would indicate that that’s a hydrazine fire.”

The video showed the three-stage SM-3 missile launching from the USS Lake Erie at 10:26 p.m. EST, northwest of Hawaii, and of the missile’s small ”kill vehicle” — a non-explosive device at the tip — maneuvering into the path of the satellite and colliding spectacularly.

He said the satellite and the kill vehicle collided at a combined speed of 22,000 mph about 130 miles above Earth’s surface, and that the collision was confirmed at a space operations center at 10:50 p.m. EST.

Asked about the satisfaction felt among those in the military who had organized the shootdown on short notice by modifying missile software and other components, Cartwright smiled widely.

”Yes, this was uncharted territory. The technical degree of difficulty was significant here,” Cartwright said. ”You can imagine that at the point of intercept there were a few cheers that went up.”

He cautioned, however, that more technical analysis was required to determine for certain what debris was created and where it might go. The satellite was described as the size of a school bus and weighed about 5,000 pounds.

Unlike most spacecraft that fall out of orbit and re-enter the atmosphere, this satellite had an almost full fuel tank because it lost power and became uncontrollable shortly after it reached its initial orbit in December 2006. Cartwright said the hydrazine alone was justification for undertaking the unprecedented effort to use a Navy missile interceptor to attempt to destroy the satellite in orbit.

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 — and 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.

And Cartwright cited two other sources of information that 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 re-entry 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 shootdown 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 Hawaii 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.

The elaborate intercept may trigger worries from some international leaders, who could see it 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 shootdown 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,” Foreign Ministry spokesman 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.”