Interesting article, so I have quoted several long sections below:
Various plans have been put forward to deflect incoming asteroids. For example, lasers or giant space mirrors could evaporate ices on their surfaces, creating jets that propel them away from Earth. And half-painting an asteroid could make it radiate heat differently on each side, slowly nudging the object off course.
But many of these plans require several years of advance warning in order to push the asteroids into safe orbits. If an asteroid or comet is found barrelling towards the planet with a year or less to impact, "that's a case where perhaps our only option is to attempt a big kinetic kill", says Durda.
Now, Didier Massonnet and Benoît Meyssignac of France's National Centre for Space Studies have come up with a new projectile to fire at the asteroid in such a "kill". They advocate capturing a small, 40-metre asteroid and "parking" it a stable Lagrange point 1.5 million kilometres from Earth, where the gravity of the Earth and the Sun balance.
If a larger asteroid were then found to be on a collision course with Earth, the small rock could be moved into its path within eight months, says the team. This "David's stone" would be too puny to cause any damage to Earth if things went awry, says the team. "Such an asteroid capture would be one of the most remarkable achievements of mankind," they write in Acta Astronautica.
But other experts say the plan is not realistic. It relies on using a small hopping robot to excavate rock at tens of metres per second from the little asteroid in order to provide the force to capture it and send it towards the larger rock. The capture would take a year of digging and would require the robot to remove 66% of the small rock's mass.
"To have a mechanical device work all on its own - without a person to kick it - in an essentially unknown surface environment full of dust and debris, is a very difficult thing to do technically," Durda told New Scientist.
Gerhard Hahn and Ekkehard Kührt of the German Aerospace Centre in Berlin agree. "It sounds rather like science-fiction," they told New Scientist in an email.
But Dario Izzo, an aerospace engineer at the European Space Agency's Advanced Concepts Team in The Netherlands, says the capture is technically feasible. "We can do it, but it would be really expensive," he told New Scientist.
Izzo is now working on a strategy based on ESA's plans for its Don Quijote mission. That mission is designed to put one spacecraft in orbit around an asteroid to watch as another is sent crashing into it. Don Quijote will be a technology demonstration mission, but Izzo's team has been working on ways to use just an impactor spacecraft to deflect a dangerous asteroid.
As a test case, the team used the orbital parameters of Apophis, a 400-metre-wide asteroid that will pass by Earth in 2029. During that pass, it may change course enough to hit Earth when it returns again in 2036 - a possibility that now has a one in 5000 chance of happening.
The team developed formulae to find out how much Apophis could be deflected by a 700 kilogram (1540 pound) spacecraft. "We found there are loads of trajectories, of launch windows, that would allow us to obtain a deflection," Izzo says. If a spacecraft were to launch by 2026, it could hit Apophis and change its speed by 0.01 millimetres per second – a tiny change, but enough to prevent it from colliding with Earth a decade later, he says.
"Taking out a killer asteroid – with a tame one"
Maggie McKee
NewScientist
26 April 2006
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