A missile launched from a Navy ship struck a dying U.S. spy satellite passing 130 miles over the Pacific on Wednesday, the Pentagon said.
It was not clear whether the operation succeeded in its main goal of destroying a tank that carried a toxic fuel called hydrazine. U.S. officials said the fuel could pose a hazard to humans if it landed in a populated area.
Some scientists, including Massachusetts Institute of Technology research professor Geoffrey Forden, said their calculations showed the chances of actual harm to be exceedingly low.
Forden found a 3 percent chance that an individual somewhere in the world would be injured by the hydrazine if any fell to Earth. For a resident around Los Angeles, for example, the risk of being harmed is one in a billion, Forden said. The government estimates the risk of being struck by lightning at one in 700,000.
Forden and his colleagues concluded that the possibility of the toxic gas making it to land is very close to zero. "The amount of pressure on that tank will be enormous - about 50 times the gravitational force on Earth," he said. "We can't see how it would possibly make it through the atmosphere."
Harvard astrophysicist Jonathan McDowell separately concluded the chance that a piece of the satellite might strike someone directly is about one in a million. The chances the hydrazine will land within 100 yards of someone if the tank makes it through Earth's atmosphere, he said, are higher: about two in 100. But "if people just walk away from it, they won't be harmed," he said.
McDowell and Forden said that NASA and the Defense Department should release the data used by the government to conclude the health risks were so great that the satellite should be destroyed by a missile - at an estimated cost of more than $30-million - before it could enter Earth's atmosphere and break apart.
Skeptics in the arms control community have speculated that the administration chose to undertake the mission partly so it can test potential antisatellite weapons and missile defense technology. Some have also speculated that Washington may want to send a message to China, which conducted an antisatellite test last year, or to keep potentially valuable technology from falling into the wrong hands.
Pentagon officials have denied the claim and said they take the risks seriously, noting that a similar tank on the space shuttle Columbia fell unbroken to Earth during a 2003 disaster.
The USS Lake Erie, armed with an SM-3 missile designed to knock down incoming missiles - not orbiting satellites - launched the attack at 10:26 p.m. EST, according to the Pentagon. It hit the satellite as the spacecraft traveled at more than 17,000 mph.
Information from the Associated Press and Washington Post was used in this report.