Shoemaker-levy-9 (1994: probably large enough to wipe out human life on earth. 2000 square km of forest (80m trees) leveled. Asteroid/meteor exploded 5-10 km above the ea(cid:396)th"s su(cid:396)fa(cid:272)e. Possi(cid:271)ly a (cid:272)o(cid:373)et still up fo(cid:396) de(cid:271)ate. Lots of close calls: happen all the time; asteroids that could seriously damage earth frequently pass by. Protecting against asteroid impacts: there are millions of dangerous asteroids left in the solar system, sophistication of protection measures depends on how early warning arrives, steps for protection: Shoot lots of bombs at the comet; a fraction of a millimeter per second is enough to deflect it past the earth. Launch a massive spacecraft to gravitationally pull it into a safe orbit. Inflate a huge balloon that is pushed with a rocket thereby moving the comet gently (to cm/s) without breaking it. Do(cid:374)"t ha(cid:448)e (cid:373)a(cid:374)y optio(cid:374)s; (cid:373)ay(cid:271)e t(cid:396)y (cid:374)uki(cid:374)g the (cid:373)eteo(cid:396) a(cid:374)d hope it works. It took a long time to reach complexity.