Asteroids typically top the list of extraterrestrial objects that could hit Earth. A 9-mile wide asteroid that crashed into what is now Mexico’s Yucatan Peninsula was partly responsible for the dinosaurs’ extinction about 65 million years ago.
The 2004 announcement that 900-foot long Apophis had more than a 2 percent chance of colliding with Earth in 2029 revved up research on asteroid detection and defense, when scientists recalculated the odds down to 1 in 250,000.
Luckily, nothing of that size is in Earth’s path currently, so “we may be safe for at least a few million years,” said planetary scientist Jay Melosh of Purdue University.
But smaller threats may be looming.
NASA expects that roughly every 100 years, an asteroid larger than 55 yards wide will strike. The impact could cause local catastrophes like massive floods, destruction of entire cities and agricultural collapse. Around once every few 100,000 years, chunks of rock more than three-fifths of a mile wide — the equivalent of about 12 New York City blocks — could come tumbling through the atmosphere causing much more serious problems, on a global scale. Acid rain would kill crops, debris would shield Earth from sunlight, and firestorms would ensue, according to NASA’s Near Earth Object Program.