Could a spy satellite identify any of us from our shadow?
Spy satellites could soon be able to identify someone from space by looking at their shadow.
They would use a computer program that searches for the movement of shadows on the ground, and then identifies their owners from the way they walk.
The technique - called gait analysis - relies on the fact that someone's walking style is very difficult to disguise.
It could be used to monitor known criminals and suspected terrorists using satellites or spy planes. It could even be used in the hunt for Osama Bin Laden.
There is however a significant catch. The system, being developed by Nasa, is useless once the sun goes in.
And, although there has been an explosion in satellite imagery and technology in recent years, it is still impossible to recognise someone with confidence using pictures taken in orbit.
Images from high-altitude aircraft and spacecraft show only the tops of their heads.
Experts say aerial shots are no good for monitoring someone's stride length and walking rhythm.
However, that is not true of shadows. According to Dr Adrian Stoica of Nasa's Jet Propulsion Laboratory in California, video from space could provide enough data to confirm a suspect's identity - as long as details of the person's walking pattern were on file.
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