I like apocalyptic movies- as I tell friends, I'm good in emergencies. The end of the century has brought us a spate of them: Deep Impact and Armageddon. Of the 2, Deep Impact is far more realistic, but Armageddon is much more fun. In Deep Impact  (Dreamworks-director Mimi Leder who also did Peacemaker) a comet is going to wipe out Earth in an ELE: extinction level event and old salt astronaut Robert Duvall is sent out to blow it up. They keep doing this in these movies: try to blow the objects up. That would, of course, cause a rain of comets or meteors much more devastating than the original.
             I know this because I just read probably the most definitive book ever written about earth impactors: Rain of Iron and Ice, John Lewis, 1997. In loving detail it lists every known damage, injury, and death from meteorites and air bursts (basically when a meteor explodes at altitude like a nuclear bomb, without the radiation) back to 1400 BC. Some highlights: Constantinople-472- hissing humming blinding white fireball, fading to yellow and coppery red; turbulent copper cloud; blinded, burned people cowered in terror; blast wave blew out windows and doors, knocking pedestrians to ground and sailboats flat; hot sulfurous wind; 1 hour of fine black dust <> China- Feb 3,1490   Stones fell like rain- over 10,000 killed <>  7/24/1790  Barbotan, France,  Meteorite crushed cottage- killed farmer and some cattle <> 3/11/1897  Martinsville W.Virginia-  man knocked out, horse killed, walls pierced <> 12/8/1929  Yugoslavia-  Meteor hits bridal party, killed 1  (speak now or forever hold your...) <> 7/19/12  Holbrook,Arizona- Building struck, 14,000 stones fell <> Sept 28, 1934 San Francisco- plowing head on at 7000 ft into great shower of meteors, United airliner rocked by some explosions <> 11/28/54  Alabama- Mrs. Annie Hodges struck by 9lb meteorite that crashed through roof, destroying radio <> 12/5/84 Cuneo Italy,  Strong explosions, blinding flash, windows broken, daytime fireball "bright as sun"  <>  10/9/92  Peekskill, N.Y.  Car trunk, floor, pierced by meteorite.  The big impacts people know about are the 1908 Tunguska Siberian airburst, that blew down trees for 18 miles and was heard in London, and   Arizona's Meteor Crater: about 1 mi. across and 500 ft deep, both explosions about 15 megatons and caused by a rock about 120 ft across.
 In  Armageddon (Jerry Bruckheimer, star. Bruce Willis, Liv Tyler, Ben Afffeck) the world-destroying rock is "size of Texas". Forget there's only one asteroid "the size of Texas": Ceres- and only 4 more over 200 miles diameter and they're not coming calling anytime soon. In Deep Impact  the comet is 6 miles across. Forget that all it would take to destroy human civilization is a rock 2400 ft across (causing a 12 mi. crater, 100,000 megaton explosion, tidal wave over 1000 ft, global winter and occurring every 250,000 years). Since until 2000 years ago, almost all humans lived within 100 miles of the ocean, all it really would have taken was an 800 ft rock hitting the ocean  (1000 megaton explosion, 550 ft. tidal waves moving 450 mph 600 miles away, and happening every 10,000 years). So it's probably not an accident that human civilization is only a paltry 7000 years old out of 3 million years of human habitation and that every religion has legends of brilliant stars, fire, floods, endless dark, and catastrophe. This stuff ain't fiction. It's not if, it's when. A 1 gigaton explosion occurred only 3-5000 years ago in Argentina, leaving a chain of craters 17 miles long. In May '96,  a 1000 ft asteroid passed inside the orbit of the Moon- a 2500 megaton impact if it had visited us. There are estimated to be 565,000 near-Earth asteroids (NEA) this big within 1.3 AU (Earth orbit distance) of the Sun, 2000 bigger than 1/2 mile diameter. There are tidal wave deposits 1000 ft. above sea level on Lanai, far higher than any possible earthquake could cause. In 1178, Monks in Canterbury England, witnessed an incredible sight- "The moon's horn split in two,  a flaming torch sprang up, spewing, hot coals, and sparks. The body of the Moon writhed and throbbed like a wounded snake. This phenomenon was repeated a dozen times. After, the Moon, from horn to horn, took on a blackish appearance " Astronomers think this refers to the creation of 12 mile wide crater Bruno by a 120,000 megaton impact(s), 5 times all the world's nukes put together, from a 2 cu. km rock moving 8 mi/s, but we were lucky: it was 20 times more likely to hit Earth!!

Smaller explosions happen even more frequently (~300 ft rock, 100 megaton, 170 ft tidal wave, size of 1863 Krakatoa explosion, every 1600 years), enough to annihilate entire Eastern seaboard and coastal cities around the world. The waves from Krakatoa (Indonesia) shook boats in London and the sound circled the Earth for 9 days. In fact there's a 100 kiloton airburst about once a year somewhere on Earth, about 12 ten kiloton airbursts/year (size of Hiroshima), but the vast majority are over unpopulated areas (ocean). The most amazing film I've ever seen was taken by a tourist in Grand Teton Park in 1972: over 15 seconds  a brilliant daytime fireball buzzes from one horizon to the other (in a TV documentary). Passing over Zion Nat. Park, Salt Lake City, Pocatello, Idaho Falls, Helena; it slowly faded from view over Alberta. It was an Earth-grazer, moving 8 mi./sec.and never got closer than 31 miles over Montana, but if it had been a shade closer, it could have annihilated one of those cities in a megaton explosion. In 1972, that could have caused a nuclear war.

Moving up towards the title of this opus, a major killer of 1000 gigatons happens about once every 800,000 years, another reason we have only 7000 years of civilization: man was almost wiped out 2-3 times in his evolution.  65 million years ago a 12 mile wide asteroid or comet  slammed into the ocean near the top of the Yucatan and blasted out a 120 mile crater. 100 times the asteroid volume was ripped out of the ground and hurled out at suborbital speeds, to rain down fire and death over the entire globe. The impact punches through the crust, exposing molten lava- vaporizing thousands of cu. miles of ocean water; the pressure waves circling  the globe converge in India, where they rip open the crust and erupt massive fields of lava (Deccan Traps). Within hours or days 2 other large fragments smashed into Manson, Iowa (19  mi. crater) and Popagai, Siberia (60 m