Heads up for meteorites.
The paradox of meteorites is they're extremely rare, but they're falling all the time.
That meteorite that streaked across the Siberian sky last Friday and blew to pieces, shattering glass and injuring more than 1,000 people, was the biggest event of its kind in a century. In 1954, a woman in Alabama became the only person known to have been hit by a meteorite. The risk of a meteorite ruining anyone's day is vanishingly small.
Yet between 37,000 and 78,000 tons of material reach the Earth's surface from space annually. Most of that is dust, but an estimated 18,000 to 84,000 meteorites bigger than 10 grams - the weight of four pennies - hit the Earth's surface every year.
The Siberian meteorite weighed an estimated 7,000 tons, which makes it a big one. The Willamette Meteorite, found near the Portland suburb of West Linn, weighed only 15 tons, and it's the largest meteorite ever found in North America and the sixth-largest in the world (the University of Oregon has a replica).
On the other hand, the Siberian meteorite wasn't really all that big. On the very same day as the Siberian light-and-sound show, an asteroid (or big meteor) more than 25 times as large, 150 feet in diameter and weighing 190,000 tons, passed within 17,000 miles of Earth, a near miss by astronomical standards. The asteroid that is believed to have wiped out the dinosaurs 66 million years ago was six miles in diameter.
Even the Siberian meteorite could have caused much more damage if it had fallen on a more densely populated area. If the asteroid that passed on the same day had a slightly different orbit, the effects of its impact could have been catastrophic. And if another six-mile-wide asteroid came on a collision course - well, friends, it's been good to know you.
Meteorites have been venerated and feared by cultures throughout history; a rock that falls from the sky is an intrinsically impressive object. The possibility of large impacts has been imagined since the moon's craters were identified as the product of collisions. Now it's known that the power of an impact can cause mass extinctions. And for the first time, the ability to predict, and perhaps avert, a killer impact is close at hand.
The National Aeronautics and Space Administration has been making a serious effort to catalogue asteroids for 20 years. And now a private group, the B612 Foundation, is raising money for a space telescope that would conduct a thorough inventory of asteroids if their orbits intercept the Earth's. Compiling a reliable list of near-Earth asteroids is difficult and expensive; deflecting any deadly threat that might be identified would be harder and would cost even more. Humans tend not to spend much time or money preparing for catastrophic events where the chances of it happening anytime soon are virtually nil.
The possibility of seeing asteroids as a source of profit as well as threat might alter the calculation. Private companies are forming to explore the possibility of mining asteroids - a six-mile-wide missile is a killer, but a six-mile-wide nugget is the mother lode. A good inventory of asteroids that pass nearby would be needed both by space miners and Armageddon avoiders. Perhaps a combination of fear and greed will motivate humans to do what they can about the danger of asteroids.