So how often do meteorites hit the Earth? In fact a startling 20,000 meteorites are estimated to rain down on our planet every year. The vast majority land far from human habitation, or in the ocean, and are never found. Moreover most measure only a few centimeters in diameter, with almost none of sufficient size to form a crater. A meteorite over 10km in diameter like that which made the dinosaurs extinct comes only once in tens of millions of years, so the odds of a meteorite colliding with the Earth and wiping out all life are extremely slim. However it is said that even the impact of a meteorite measuring 10m across would have the power of the A-bomb that destroyed Hiroshima, packing enough energy to destroy a city, so the threat is certainly not one to be taken lightly.
© NASA
Where do meteorites come from? In the Solar System between Mars and Jupiter lies the asteroid belt, a densely packed band of small celestial bodies. Over 330,000 asteroids have been discovered, and the orbits of just under 140,000 tracked. Of these, as many as 4600 have been identified as having the potential to pass close to Earth (as of 2007). See the NASA NEO (Near Earth Object) program Website for the latest updates.
If an asteroid 10km in diameter were to hit our planet, it would wipe out all of human civilization. Even a direct hit by a 10m meteorite on a densely populated region would trigger a disaster of cataclysmic proportions. So starting in the 1990s, astronomers from across the globe began working on a monitoring system able to project the risk of such a collision, for example by discovering asteroids in the vicinity of Earth and calculating their orbital paths. Aided by the emergence of sophisticated observation apparatus, they have succeeded in discovering many small celestial bodies. The observation network now extends around the world, with Japan launching its own Spaceguard Association in 1996, and establishing the Spaceguard Center at the Bisei Observatory in Okayama in 2000 to play its part in the monitoring operation. So have astronomers discovered any objects that could actually hit the Earth? Observations to date have found no astronomical bodies posing an immediate danger. However, hundreds of thousands of undiscovered celestial objects are thought to exist, so something threatening the world could be identified any day. If detected early enough, such a threat could be countered, for example by firing a missile to knock the object off course. Discovering that a giant meteorite was heading our way might provide the perfect opportunity for the world's nations to stop making war and unite in defense of the planet we all call home.