When it comes to asteroids, you want to be like the Rolling Stones and put time on your side (yes, you do). Luckily, steps are being taken to survey and detect near-Earth objects, or NEOs.
NASA addresses NEO detection through two surveys mandated by U.S. Congress. The first, known as the Spaceguard Survey, seeks to detect 90 percent of NEOs 1 kilometer (0.621 miles) in diameter. Congress had set the original deadline as 2008, but the work continues as astronomers keep discovering and learning more about these enigmatic rocks. The second survey, the George E. Brown Jr., Near-Earth Object Survey, seeks to detect 90 percent of near-Earth objects 459 feet (140 meters) in diameter or greater by 2020. Both surveys rely on powerful telescopes to repeatedly scan large areas of the sky.
As of March 2012, those telescopes had discovered 8,818 near-Earth objects. Almost 850 of those NEOs were asteroids with a diameter of approximately 1 kilometer or larger. Nearly 1,300 were labeled as potentially hazardous asteroids, or PHAs. PHAs must be at least 492 feet (150 meters) wide and must come within 4.65 million miles (7.48 million kilometers) of Earth [source: NASA]
Now, if you're prone to panic, remember that the key word is "potentially." Not every space rock that makes a close approach to Earth will make an impact. Still, it's a sobering number, especially when you realize that the solar system likely contains hundreds of thousands, or even millions, of asteroids. How many have we just not seen? And how many will go unnoticed until it's too late?
As we grapple with that final question, we must face a harsh reality: Despite our best efforts, a catastrophic impact could be in Earth's future. Next, we'll consider a few civil defense strategies that might be necessary if an asteroid comes knocking.