Could you build a gravity tractor today?
'With a large asteroid and 20 years' warning time, you would need to use a high-efficiency plasma or ion engine. The craft would have to be about ten times larger than anything that has already flown in space.
'However, the asteroid Apophis could be deflected using existing technology. Because it is going to swing so close to the Earth in 2029, the effect is magnified, so you could use a conventional spacecraft.'
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