The reason why they need to use a solar sail is because they want the very small probe (10kg?) to hit the asteroid in the opposite direction, a retrograde orbit which would otherwise require an insane amount of fuel (after being put on an escape trajectory it would need to first cancel out the earth's orbital momentum and then basically speed up to a likewise high velocity in the OPPOSITE direction). They are doing this to hit the asteroid at a very high impact speed.
While Apophis may not literally be capable of wiping us out (it "only" weighs 46 million kilograms) it might be able to wreck our civilization. So rather than putting the fate of our species into the hands of an untried technology (no solar sail has yet imparted substantial delta-V to its spacecraft) may I suggest an alternative? By using Jupiter as a gravity assist we could send a much heavier probe to hit it at comparable speeds. For example the Juno spacecraft, recently launched to the gas giant weighs almost 8000kgs.
Jupiter could sling a spacecraft around so as to completely cancel its orbital momentum (with no fuel expenditure!). Then it will fall directly towards the sun and, if guided correctly, could hit Apophis broadside. Considering it will be falling from a height of several hundred million miles it would pack quite a blow. Admittedly, the impact will be on the side rather than head on but that should be okay since all we have to do is assure that Apophis doesn't pass through the keyhole which is only 600m wide.
Don't get my wrong, I hope that solar sails become widely used for the (slow, cheap) transport of cargos in the solar system. It's just that I wouldn't base the defense of earth on them. Then again, if you were able to very accurately control the asteroid impactor, not only could you control IF the asteroid was going to go through the keyhole but WHERE it was going to go through. Then you could determine where, on earth, the asteroid was going to eventually going to hit.
Say on an unfriendly nation (that was preferably on another continent)."
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