Perhaps the most well known of all ELE's are massive comets crashing into Earth, like in Armageddon or Deep Impact. The most popular theory explaining dinosaur extinction is a 6 mile long (10 kilometer) comet smashing into the tip of the Yucatan peninsula, 580 miles due south of present-day New Orleans. Its impact was 2 million times larger than the most powerful man-made explosion ever created and left a 112 mile crater (180k).
But where do these big rocks come from? On researching ELE comets, I learned about a spherical group of ice chunks far far outside our solar system called the Oort cloud:
Just as our planet has an orbit around our sun, our entire solar system (including our Oort Cloud) has an orbit within our Milky Way galaxy. More accurately, our solar system has two orbits: every 225 to 250 million years, it rotates around a semi-flat pancake of star systems we call the Milky Way galaxy. Our solar system goes round and round the Milky Way like a slow second hand on a clock, but every 28 to 30 million years it also orbits up and down, i.e., out of and into the thicker part of the Milky Way. In tandem, these two orbits make our solar system resemble a horse on a carousel oscillating up and down and round and round.
During one of these downward orbits into the dense thicket of the Milky Way's stars, our solar system increases its chances of coming close to stars, red dwarfs, black holes or anything else with significant gravitational pull. That could be a severe threat to us because any nearby gravity can distend the Oort Cloud and then—as the approaching star moves off again—its pull diminishes and launches comets out of our solar system's spherical Oort cloud shell towards the strongest source of our solar system's gravity—our sun... and thus to Earth.
Fortunately for us, Jupiter's massive gravity has repeatedly absorbed those comets; in fact, we even saw a set of these comets smash into Jupiter as recently as 1994: the Shoemaker-Levy comets.
Had Jupiter not been good enough to swallow these rocks for us, here's what the impact site might have looked like had any of them hit Earth:
In this image, the impact would have vaporized America's entire Northeastern seaboard and killed countless North Americans for thousands of miles surrounding the impact site. The explosion would have hurdled so much debris into our atmosphere that it would have likely triggered another ice age or or at least destroyed so many crops that it would have sparked worldwide hunger... and panic. And, shockingly, that's the best case scenario.
For a time, scientists thought comet 99942 Apophis would hit earth in 2029, but its trajectory now looks like it will miss Earth by 25,000 kilometers... and then return to hit Earth in 2035. While the chances are slim—1 in 45,000 that this comet will hit Earth sometime in the 21st century—this scenario is still terrifyingly possible. A comet the size of a house can cause a Hiroshima-sized explosion; 99942 Apophis is 320 meters (1/5 mile) wide and would release the equivalent of 880 megatons of TNT, at least 44 times the size of the 1908 Tunguska event that levelled 80 million trees over 2,150 square kilometers (830 square miles).
The good news is that our solar system is on an upward orbit out of the Milky Way's dangerous dense star clusters, meaning our civilization is looking forward to another 30 million years of relative freedom to evolve. The bad news is that this upward orbit is only about 1 million years out of its 30 million year orbit... and it takes about 1 million years for comets from the Oort Cloud to reach Earth.
Making matters worse, there might be many comets out there we don't know about—NASA has actually said it knows how to find these comets, but it just doesn't have enough funding to do the job properly. Ironically, that might be a blessing in disguise—NASA needs about 20 years' lead time to successfully alter a comet's trajectory, so if we all knew a comet were only a year away, we might all be better off not knowing the end is nigh.
Tomorrow's monster in the closet: Outbreak
A hat tip to Chris Walker for pointing me to Lucifer's Hammer by Larry Niven and Jerry Pournelle, which recounts the global struggles of recovering from a massive comet blast; recommended reading if this is your particular brand of obsession.