When it comes to comet impacts, the denizens of Earth may be living on borrowed time. Of course, comets are only about half the problem — there are plenty of asteroids whizzing around the inner solar system too — so we decided to have a look and see just how close modern society has come to destruction since 1900, and how close we're going to come over the next 100 years. The answers, provided in our nifty infographic, aren't reassuring.
NASA's list of potentially hazardous asteroids (PHAs) currently numbers 959. That's 1,000 asteroids that astronomers pretty much know are going to get closer than 7.5 million kilomters to Earth, about 20 times the distance from here to the Moon. Five of those are expected to come between Earth and the Moon over the next century.
So we'll have a few close shaves but nothing to worry about, right? Not so fast. The total number of PHAs and comets astronomers think are out there is probably more like 20,000. That means we've mapped about 5% of the objects that stand a good chance of hitting us. So take the future part of this chart as a best-case scenario. The past five close encounters, however, show just how vulnerable we are:
1) The Comet of 1491. This one must've scared the hell out of some folks. At a little less than four times the distance to the moon, this was the closest pass ever recorded at the time, and no one knows for sure how big it was. Little did our ancestors know how much more interesting things would get.
2) Tunguska, 1908. One of the most famous Earth lcose calls of all time, it was also a pipsqueak. For a long time scientists believed a comet perhaps 60 meters in diameter exploded over Siberia with a force of as much at 30 megatons, or about 2,000 times more powerful than the atomic bomb dropped on Hiroshima, though nothing solid ever hit the planet. All those pictures of flattened forest certianly look impressive, but last year, scientists re-crunched the numbers and found that the comet oculd've been as small as 30 meters, and the blast just 5 megatons. In other words, much smaller objects can do way more damage than we ever thought before. Gulp.
3) The Great Daylight Fireball of 1972. The name says it all — it doesn't get much closer than this. Size estimates range from 3 to 14 meters in diameter, depending on whether it was ice or rock. Whatever it was, the object called US19720810 burned through the atmosphere from Utah to Canda for about a minute and a half. Luckily, the space rock struck a glancing blow — had it hit Earth directly, it could've blasted us with 1/2 a Hiroshima worth of energy.
4) 2004 FH and 2004 FU162. At 30 meters in diameter and made of solid rock, 2004 FH would be a thumper of Tunguska proportions if it ever hit home. In the right (or wrong) place, it could detroy a city. As it was, it passed 43,000 kilometers above Earth on on March 18, 2004.
Three weeks later, FU 162 came whizzing along. Astronomers basically discovered it at bascially the same time as the 6-meter in diameter rock soared just 6,400 kilometers above Earth's surface.
5) Comet Hyakutake. Now we're getting into civilization-threatening territory. At 2 kilometers in diameters, this comet only got within about 40 lunar distances to Earth in 1996. Compared ot our other close calls, that's pretty comfortable, but considr this: it was discovered less than two months before its closest pass. Had it been on a collision course with Earth there's almost nothing we could've done other than brace for the millions dead, massive climate disruption, crop failure, 500-foot high tsunami...you get the idea.
6) 1999 AN10. In a little less than 20 years, our usually quiet Earth-Moon system is going to have a lot of visitors. In August 2027, AN 10 is going to get about one lunar distance from Earth, and we'll get a chance to see just how big this bad boy is. Estimates range from 1/2 to 2 kilometers in diameter, plenty large to leave a dent in humanity if it ever gets closer.
7) 2001 WN5. Just six months after AN10 comes a callin' WN5 will get even closer, just about splitting the difference between Earth and the Moon. At 700 meters in diamters, this asteroid has a got potential for major dmaage, but current odds of impact are rated a negligible.
8) 99942 Apophis. By far the most famous of the end-bringing objects we know about in our solar system, astronomers thought for a while that this 270 meter-wide rock had an almost 3% chance of hitting us. Since then, odds have been lowered to 1 in 43,000 that it could slam into Earth in 2029. But if it passes through a gravitaitonal keyhole — a tiny region in space that could tweak its orbit ever so slightly — an impact could still happen on April 13, 2036.
9) 2005 WY55. Just 200 meters wide, astronomers think this asteroid could still pack a wallop. Right now it's scheduled to get within about 75,000 km of Earth, but impact odds are big enough to kep in mind — currently they're rated at around 1 in 70,000. If our number comes up on that faeful dayin May 2065, look out — blast yield estimates from this rock range to 1100 megatons.
10) 2000 WO107. Depending on how well humanity holds up under climate change, bird flu, and all the other things that could potentially kill us off, we might be able to look up and see WO107 zoom by in December 2140. The 400 meter-wide rock isn't scheduled to hit us — it should get about half way between Earth and the Moon — but if calculations are off by even a little bit (and all of the future examples here have some uncertainty) we could care a lot.
Sources: NASA's Near Earth Object Program, Harvard List of PHAs
Additional research by Nivair H. Gabriel. Image by Stephanie Fox.