Just over a year ago we found out how much damage a small asteroid could do to a modern city. About 1,500 people were injured seriously enough to go to the hospital, mostly from broken windows, and significant damage was done to the Russian city of Chelyabinsk.
This was a relatively small strike. The asteroid weighed somewhere between 12 and 13 thousand metric tonnes and was about 20 meters in diameter. Much larger objects cross the earth's orbit all the time. (The object that caused the Tunguska event was probably about three times as big but, fortunately, hit an unpopulated area).
And, we now know, an asteroid contributed to the extinction of the dinosaurs. So, one of the things in discussion lately has been asteroid defense. Various methods have been proposed - including steering asteroids into stable orbits so they can be mined, blowing them up with nuclear weapons, deflecting them with lasers...
But a recent study has brought to light another fascinating possibility. We've known for a while that asteroids are covered in dust, and this has always been thought to have been caused by collisions between asteroids breaking off little pieces. (This dust is called regolith).
Wrong. In fact, the dust on an asteroid's surface is caused by thermal fatigue. Asteroids tend to spin and many have irregular orbits. This subjects them to heating and cooling stresses which - and this has been demonstrated by testing on samples - pulverizes the outer surface. Which is why we don't have as many asteroids orbiting really close to the sun - they're baked into pieces.
So, I wonder. Would a pulsed thermal laser be the weapon of choice for asteroid defense? Could we eventually design something that would subject an asteroid to a rapid, intense heat and cold cycling that would break it up into tiny, harmless pieces? I'm not an engineer - maybe it isn't possible - but if it is it would reduce or even eliminate the risk of replacing one huge impactor with three or four smaller - but still highly dangerous - ones.