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Earth, like all the planets and moons in our solar system, has been playing in traffic for a long time. Torturously scarred bodies like Mercury and our moon are testimony to how dangerous near-Earth space can be, especially going back to the solar system's earlier days, during what's known as the heavy-bombardment period, when planets and moons were still accreting and there was plenty of leftover material flying free. Since then, things have quieted down, but only in relative terms.
An asteroid as big as 2012 DA14 is estimated to pass close to us every 40 years or so and enters the atmosphere every 1,200 years. Bigger rocks, those measuring up to a third of a mile, are less common but would be far more devastating, causing a 5,000-megaton blast and a 7.1-magnitude shock and spreading pain on a continent-wide scale. "We're talking serious trouble here," says Yeomans. Once you get up to 1.3 miles, all of Earth would suffer, with growing seasons disrupted and weather patterns altered. "There are probably no objects left as big as the one that killed the dinosaurs," says Yeomans. "But these smaller ones would still be horrendous, both physically and economically."
Since the 1990s, astronomers were requesting funds to track near-Earth objects, and in 1995 Congress agreed, authorizing NASA to establish a full-time monitoring program. The space agency mostly turned the job over to three observatories--on Maui; in Tucson, Ariz.; and in Socorro, N.M. Those facilities have detected about 98% of all the known NEOs, though 2012 DA14 was spotted by amateur astronomers in Spain and the WISE infrared spacecraft has found more than 150 near-Earth objects. There is, says Yeomans, a "small army" of professionals and amateurs worldwide contributing to the work, but it is very much a NASA-dominated field.
Just how you define the near in near-Earth is a very precise thing. In order to qualify for the cosmic watch list, an object must come within 1.3 astronomical units (AUs) of the sun. A single AU is the Earth-sun distance, or 93 million miles. To earn the sobriquet potentially hazardous asteroid, a rock has to come a whole lot closer--within 0.05 AU, or 4.65 million miles of us--and measure at least 330 ft. "The asteroid that just passed is at the threshold of what would cause us worry if it were going to hit us," says Lindley Johnson, director of the Near Earth Object Program Office in NASA's Washington headquarters.
The composition of an asteroid plays an important role too--but only in a pick-your-poison kind of way. The overwhelming share of space rocks are made of some kind of silicate, a relatively porous material that doesn't survive the plunge through Earth's atmosphere intact. That leads to the kind of airburst that wiped out Tunguska. Metallic asteroids are far less common but tend to survive re-entry. This is what happened when a 165-ft. nickel-iron object struck the ground near Winslow, Ariz., 50,000 years ago, leaving a crater 4,000 ft. across and 570 ft. deep and doing massive regional damage. The site is a tourist attraction now, but long ago, it was witness to a very bad day.