The interceptor that the Navy used had never been tested against anything nearly as fast-moving as the spy satellite destined to crash to Earth March 6.
"From that point of view, it was pretty remarkable they were able to hit it," said Geoffrey Forden, a physicist at MIT who calculated the potential dangers of the plummeting satellite. But when it comes to why the Pentagon did it, he said, "They have been very opaque about their decision-making process."
Said John Pike, a space policy analyst and director of GlobalSecurity.org: "The claim there was danger from the fuel is not the most preposterous thing the Pentagon has ever said. But it seemed to be a bit of a stretch."
The Navy shot the satellite down on Wednesday using an Aegis missile designed as part of the "Star Wars" program to intercept enemy missiles.
Tracking the satellite with radar, a ship in the Pacific shot a three-stage missile called an SM-3. An off-board tracking system would have gotten it close to the path of the 17,000-m.p.h. satellite, Pike said, "sort of like your GPS navigator." Then, the missile's own heat-seeking guidance would have allowed it to home in exactly.
The Aegis system had been tested 13 times against dummy missiles near Hawaii, Forden said, and it succeeded in 11 of those.
The satellite was moving two to three times faster than the test targets, he said, and would have been brighter from reflected sunlight. To deal with that, engineers made some last-minute adjustments in the software, said Lt. Col. Karen Finn, a Pentagon spokeswoman.
But did they have to do it?
Forden and other scientists question the danger posed by the satellite, which had been out of control since its radio broke after its 2006 launch.
The Pentagon said 1,000 pounds of unspent hydrazine fuel was frozen, said Jonathan McDowell, a Harvard astrophysicist who has worked on NASA research satellites.
If that were the case, it would pose more of a danger than liquid fuel, which would probably burn up on contact with the atmosphere. Hydrazine, which is toxic, can cause nerve damage and breathing problems if inhaled, and is listed as a possible carcinogen.
"If you imagine this big iceberg of frozen hydrazine coming through the atmosphere," he said, some of it could potentially remain solid before it hit the Earth.
But, he said, in his experience working with NASA astronomy satellites, hydrazine fuel would freeze only if it were consistently shielded from the sun. You would expect this errant satellite to tumble, he said, keeping the fuel liquid.
"Why was this satellite's hydrazine frozen when others aren't frozen?" McDowell asked. "I'd like someone to press them on this."
Attempts to reach a Pentagon scientist were unsuccessful.
Frozen or not, the fuel might not have survived reentry into Earth's atmosphere, scientists said.
McDowell questioned why this satellite was shot down when so many other satellites and spent rockets fall without anyone worrying about them. "Big chunks of metal fall out of the sky all the time," he said.
The 75-ton Skylab fell in 1979, and while it didn't have a big chunk of fuel, "overall the risk from that was much higher," McDowell said.
McDowell said some hydrazine reportedly survived the crash of the space shuttle Columbia in 2003, but the difference was that the shuttle was an immense vehicle designed to withstand the heat of reentry.
Some of the scientists said they worried that the missile shot would encourage the Chinese to continue developing technology to shoot down our satellites.
"You know it's going to make it more difficult for the U.S. to tell anyone else not to develop antisatellite technology," said Laura Grego, an astrophysicist at the Union of Concerned Scientists.
The Chinese used a similar missile system last year to shoot down their own satellite in a test. "We made a big to-do about it and rightly condemned them," MIT's Forden said.
Harvard's McDowell said the United States started experimenting with antisatellite missiles in the 1960s, in some cases shooting nuclear warheads at them. One test, he said, killed about half the satellites in orbit at the time.
In the 1980s, the United States shot down several more satellites in further testing, he said, in one case sending an antisatellite missile from under an F-15 fighter plane. Then, for 20 years, there were no satellite-destroying tests in space, he said. "Last year, the Chinese test reopened the door."
Pike said he wasn't sure last week's show would make a difference to the Chinese. "They want to be able to shoot down our spy satellites . . . they needed no encouragement," he said. "It would be stupid not to and they're not stupid."
Contact staff writer Faye Flam at 215-854-4977 or firstname.lastname@example.org.
Staff writer Tom Avril contributed to this article.
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