Do you ever wonder what will bring on the apocalypse? Maybe you’ve heard that the Sun will one day blow up the Earth, but I wouldn’t think twice about it. Sure, there are rogue stars eating their planets in the night sky, but Earth has another 5 billion years before the Sun loses it and swallows the inner planets (including us). The odds are better—which is to say much worse—that when death comes from above, it’ll be the classic dinosaur-slayer: an asteroid.But hey, don't worry. NASA is on it! Following the adages “an ounce of prevention is worth a pound of the cure” and “the first step to recovery is acknowledging you have a problem,” the space agency continues to map our local solar system, looking for asteroids headed for Earth.Using their "Wide-field Infrared Survey Explorer" (WISE), NASA has produced its most accurate count yet of the number of "potentially hazardous asteroids." These are asteroids whose orbits bring them within five million miles of Earth and which are large enough to pass through Earth's atmosphere. As it turns out, there are only 4,700 of them! And we already know where 20 to 30 percent of them are. That’s comforting, right?This is fairly close to what previous estimations had found, but the infrared telescope provides a much more credible estimate on the total number and size of these potentially hazardous asteroids. Conventional telescopes give a dot that varies in degrees of brightness—meaning a small shiny object and big dull object look roughly the same. By looking at their heat a more accurate estimation of their size—and potential for destruction—can be made.Scientists at NASA are playing up the less-apocalyptic aspects of their findings. Amy Mainzer, principal investigator on the survey at NASA’s Jet Propulsion Laboratory, explained that "because they will tend to make more close approaches to Earth, these targets can provide the best opportunities for the next generation of human and robotic exploration."
NASA puts the size of a potentially hazardous asteroid at about 100 meters, the size necessary for the asteroid to reach the Earth’s surface. But that doesn’t mean they need to reach the surface to make an impact. In 1908 an asteroid or meter that is estimated to be around 50 meters across exploded above Tunguska, Siberia in remote Russia. It exploded with the strength of a 15 megaton nuclear bomb (the light from the explosion was seen in London) and flattened trees over 830 square miles.Of course, as recently as 2004, NASA researchers pointed out that a global-ecological-catastrophe asteroid—about 2 kilometers in diameter, equal to a million megatons of power—would either come with years of warning, as it would likely make orbiting several passes before smashing into us and turning our atmosphere into flames. The other option? We’’ll first discover a deadly asteroid as it comes screaming across the sky. But as long as NASA keeps scanning the sky, that’s not going to happen.
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“In this simulated view of the near-Earth asteroid population, potentially hazardous asteroids (PHAs) are denoted in orange. Less dangerous near-Earth asteroids are blue. Earth’s orbit is green.” Credit: NASA
Asteroid 243 Ida. Credit: NASA, August 28, 1993

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