If an asteroid should be discovered on a catastrophic collision course with our planet, researchers at Glasgow University say mirrors are the best way to save us from annihilation.
Up to 5,000 could be used to focus sunlight on to the asteroid, melting the rock and altering its orbital path away from the Earth.
The doomsday scheme was devised after a team at the university compared nine methods of deflecting near-Earth objects - asteroids and comets. The results were unveiled at the Jodrell Bank observatory in Cheshire as part of celebrations for the 50th anniversary of the launch of the Soviet satellite Sputnik 1, which marked the start of the space age.
The research team compared the mirror technique with eight others, including different types of nuclear explosion and fixing a propulsion system to the asteroid.
The nuclear options and the mirrors would be more effective than the others, but scientists fear the risk of flying debris from a nuclear blast.
The orbiting mirrors would be used to focus sunlight on an area of the asteroid between 0.5 and 1.5 metres wide, heating the rock to around 2,100C - hot enough to melt the surface of the asteroid and create a thrust which would nudge it off course.
The team found that the orbit of an asteroid measuring 150 metres across could be sufficiently modified by a network of 100 mirrors in a few days. For an asteroid the size of the one believed to have wiped out Earth's dinosaurs, a 5,000-strong fleet of spacecraft would need to focus a beam of sunlight on the surface for three or more years.
Dr Massimiliano Vasile, who led the project, said the research was not science fiction but facing up to science fact. He said: "Asteroid impacts are a real threat. The Tunguska explosion in 1908 devastated an area bigger than Greater London. With only ten spacecraft flying in formation, each with a 20 metre mirror, we could deflect a similar-size asteroid into a safe orbit in about six months.
"This technology is genuinely feasible and, unlike methods where an explosion or impacter is used to divert the asteroid, there is no further risk from fragments. We have estimated, for a ten-satellite formation, a launch mass for each individual spacecraft of around 500kg.
"This is a smaller and lighter satellite constellation than, say, the Galileo positioning system, so is well within our launch capabilities."
Pieces of rock and ice enter our atmosphere every day, but most are so small that they go largely unnoticed.
Scientists have calculated that every 26-30 million years, a 10km-sized asteroid (the size of the one believed to have wiped out the dinosaurs) strikes the Earth.
Race against time to save the globe
ALTERNATIVE methods of saving the world from an asteroid were examined but rejected.
• Different types of nuclear explosion were considered but the fragmentation of the resulting debris made this too aggressive a method, not to mention the dangers associated with nuclear power.
• Another option was to fix an electric propulsion system to the asteroid, but this would take too much time to complete before the asteroid impacted with the earth.
• A mass-driver system where material is excavated and catapulted away from the asteroid, would also take too much time.
• A kinetic impactor which would knock the asteroid out of its orbit, was thought to require the launch of too big a space craft as was the option of using a large craft's own gravitational pull to drag the asteroid away from the earth.
The study investigated in each case the mass of spacecraft needed, the warning time required, the orbital deflection achieved and the current readiness of technology. Simulations were run with six different sized asteroids.