Air Force Space Commander Gen. Kevin Chilton said it was often difficult to know exactly what happened to satellites orbiting from 125 to 22,400 miles above the earth.

"We're at a point where the technology's out there and the capability for people to do things to our satellites is there. I'm focused on it beyond any single event," Chilton said.

Satellites are also vulnerable to man-made and natural events affecting their ground stations and the links between the station and the satellite, he told reporters last week.

Theresa Hitchens of the Center for Defense Information cautioned against jumping to conclusions about the Chinese incident.

Beijing may have been testing its capability to track satellites, not damage them, Hitchens said. "We don't know their intent, and we don't have the capability to know."

Hitchens also noted current technology made it difficult to identify anything smaller than a baseball in the orbits where spy satellites fly, a capability that needed to be improved.

Hitchens also emphasized that it would be extremely difficult to disable a satellite with a laser -- and even U.S. scientists had not developed a system to do that.

But there is growing concern among lawmakers about U.S. efforts to develop such anti-satellite weapons.

House of Representatives lawmakers tried to block a planned test of Starfire, a satellite and star tracking program, for fiscal 2007 after learning it could also be used as an anti-satellite weapon. The funds were reinstated only after the Air Force assured lawmakers it would be used only for tracking.

The Chinese incident also underscored the need to develop an international code of conduct for space. Currently, there are no specific rules or treaties governing behavior of the 40 countries that operate satellites, and about a dozen countries that have launch capability, Hitchens said.