Robert Gates said the operation "speaks for itself", adding the US was prepared to share some technology with China.
The comments came after China said the missile strike could harm security in outer space.
US officials are confident that the satellite's potentially toxic fuel tank was destroyed by the missile.
Marine Gen James Cartwright said there was a 80-90% chance the tank had been hit.
A fire ball, vapour cloud and spectral analysis indicating the presence of hydrazine all indicated this, he told reporters.
It would take 24-48 hours for officials to confirm whether the operations had been completely successful, he said.
The satellite, USA 193, was struck 153 nautical miles (283 km) above earth by an SM-3 missile fired from a warship in waters west of Hawaii.
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Owner: National Reconnaissance Office
Launched: 14 Dec 2006
Weight: 2,300 kg (5,000lbs)
1,134kg (2,500lbs) could survive re-entry
Carrying hydrazine thruster fuel
Mr Gates said the issue of whether the technology would work was already decided.
"I think the question over whether this capability works has been settled," he said, quoted by AFP news agency.
"The question is what kind of threat, how large a threat, how sophisticated a threat [the US faces]."
The US approach was one of "complete transparency", he said.
"We provided a lot of information... before it took place," he said, adding: "We are prepared to share whatever appropriately we can."
China called on the US on Thursday to provide more information about the mission.
Russia suspects the operation was a cover to test anti-satellite technology under the US missile defence programme.
Operatives had only a 10-second window to hit the satellite, which went out of control shortly after it was launched in December 2006.
The missile needed to pierce the bus-sized satellite's fuel tank, containing more than 450kg (1,000lbs) of toxic hydrazine, which was otherwise expected to survive re-entry.
The US denies the operation was a response to an anti-satellite test carried out by China last year, which prompted fears of a space arms race.
US officials had said that without an attempt to destroy the fuel tank, and with the satellite's thermal control system gone, the fuel would have been frozen solid, allowing the tank to resist the heat of re-entry.
If the tank were to have landed intact, it could have leaked toxic gas over a wide area - harming or killing humans if inhaled, officials had warned.
The US has denied that it shot down the satellite to prevent parts of it from falling into the hands of foreign powers.
Gen Cartwright said most of the satellite's intelligence value was likely to have been destroyed.
SATELLITE DESTRUCTION1 SM-3 missile launched from a US Navy cruiser in Pacific Ocean
2 The three-stage missile headed for collision location, where the relative "closing" speed was expected to be 10km/s (22,000mph)
3 Satellite came in range at altitude of 247km (133 nautical miles), close to edge of Earth's atmosphere
4 Missile made contact with satellite with objective of breaking fuel tank, freeing hydrazine into space
5 Much of the debris will burn up but an as yet unknown amount is expected to be scattered over hundreds of kilometres
Published: 2008/02/21 20:52:58 GMT
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