By Paul Reynolds
World affairs correspondent, BBC News website
From ship to space: SM3 missile being fired at satellite
The anti-satellite operation carried out by the US Navy is seen by experts as a useful test of the anti-ballistic missile system being developed by the Bush administration.
A spy satellite, which has not functioned since being launched, was hit in low orbit by a missile fired from a ship in the Pacific.
The Pentagon said the satellite had to be brought down because the toxic fuel it was carrying had frozen and so could not be used to guide the satellite into the earth's atmosphere to burn up.
The fuel would be a risk to humans if it remained frozen after re-entry, it said. A similar fuel tank on the crashed shuttle Columbia had survived to fall into a wood in Texas.
"There was a genuine reason to bring this down, but this was also an opportunity to test the anti-ballistic missile system," said Andrew Brookes, aerospace analyst at the International Institute for Strategic Studies in London and a former RAF pilot.
"The SM-3 missile they used forms the terminal phase of the anti-missile system, and they were grateful for a chance to test it. President Bush said he would have an anti-missile system up and working before he left office and although he has not got the whole thing, he can say he delivered on this. The rest of the system is still very much to be done.
"It is also good for the Japanese, who use this missile. It is the proof of the pudding. It is wonderful kit and it was a wonderful opportunity."
Russia and China
There have been suggestions that the US did this as a reply to the Chinese who brought down a satellite at a much higher altitude in January 2007.
The US and others protested at that time, on the grounds that China had not consulted under the 1967 Space Treaty as it was supposed to and that it had created debris which would migrate downwards through the paths of other craft in space, including the international space station.
"I don't think it was done to teach the Chinese or Russians a lesson," said Andrew Brookes. "The Americans already know how to shoot down a satellite. They did so from an aircraft in 1985 and higher up than this time, but it created debris, the last of which fell into the sea only this year."
However, opinion is divided on whether this operation was also designed to send a message to Russia and China. They have both submitted to the UN a draft treaty which would prohibit the deployment of weapons in space and the use or threat of force against satellites or other craft.
The US has rejected any such treaty, stating in 2006: "The United States will oppose the development of new legal regimes or other restrictions that seek to prohibit or limit US access to or use of space."
The Union of Concerned Scientists in the US believes that the satellite strike will lead to new dangers in space.
"The potential political cost of shooting down this satellite is high," said Laura Grego, an astrophysicist with the UCS's Global Security Program. "Whatever the motivation for it, demonstrating an anti-satellite weapon is counterproductive to US long-term interests, given that the United States has the most to gain from an international space weapons ban.
"If the Pentagon demonstrates that its missile defence systems can destroy satellites, it will be very difficult to convince other countries that they shouldn't develop a similar anti-satellite capability."
The US military said the motivation for the strike was as stated - the risk from the fuel.
The Vice-Chairman of the Joint Chiefs of Staff, General James Cartwright, said: "This is all about trying to reduce the danger to human beings. That was a decision that was taken by the president after listening to all the technical arguments. That was the calculation: hydrazine equals hazard to human beings, and we tried to do what we could to mitigate it. "
So he rejected the idea that it was simply to send a message.
"Remember that we did that 20 years ago. There's really no need to go back to that data point."