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"  On Friday the 165-foot-diameter asteroid 2012 DA14 will pass by the planet just 18,000 miles above the atmosphere. The near miss of Earth is a dramatic reminder of the threat posed by its rogue relatives. Planetary scientists estimate there are close to a million near-earth asteroids the size of DA14 or larger. A cousin of DA14 probably strikes Earth every 1000 years or so, and does so with the explosive force of several megatons\u2014enough kinetic energy to destroy a city.  We have the space technology for a mission to deflect a threatening asteroid: We could change its course with the gentle tug of gravity (gravity tractor), slam into it with a hypervelocity bullet (kinetic impact), or vaporize the &amp;quot;topsoil&amp;quot; of an asteroid to divert it from a collision course with us (nuclear explosive).  To execute any of these schemes, though, we need precise knowledge of the orbits and the timing of predicted Earth encounters for the many hazardous asteroids roaming the inner solar system. For 15 years, NASA has been scanning the skies for the largest of these\u2014those larger than 1 km (0.6 miles) in diameter\u2014which could cause global damage. NASA says it has found about 90 percent of these, and none of the known 861 large near-earth asteroids currently threatens Earth. Yet the total of nearly 10,000 known near-earth asteroids, large and small, is scarcely 1 percent of the true population. Because the most likely impact event we could face in coming decades (or possibly tomorrow) will come from an undiscovered asteroid just a few tens of meters across, we humans must set out to find and track the orbits of the 99 percent.  Earth-based telescopes are limited in this search: The atmosphere&amp;#039;s clouds and haze obscure viewing efforts, and these telescopes can observe asteroids only in the night sky. Even new, larger ground-based observatories will take decades to find the majority of small objects.  A more efficient solution is space-based telescopes. They can see small asteroids, warmed by the sun, glowing brightly in the infrared (heat) part of the spectrum. Space telescopes can operate around the clock. Placed far from Earth, they are much better at seeing asteroids that are circling inside Earth&amp;#039;s path around the sun.  Canada is about to launch the suitcase-size, Earth-orbiting NEOSSat search telescope to prove these advantages. NASA is considering mounting asteroid sensors on a geosynchronous communications satellite, and its Jet Propulsion Lab is proposing an infrared telescope that would hover at the L1 Lagrange point, 1 million miles sunward of Earth.  The private B612 Foundation has set its sights even higher. Its Sentinel telescope would circle the sun on the inside track, similar to the planet Venus&amp;#039;s orbit, to look outward past Earth at the asteroid swarm. Sentinel&amp;#039;s perch is especially good for finding objects that spend most of their time in Earth&amp;#039;s daytime sky. In just six and one-half years, its half-meter-diameter infrared telescope will locate and follow the trajectories of more than 90 percent of asteroids larger than 140 meters, a population that numbers in the hundreds of thousands. Private asteroid mining companies like Planetary Resources and Deep Space Industries will use this new asteroid catalog to find prospecting targets. Finding new asteroids is also critical to NASA&amp;#039;s plans to send astronauts to one or more asteroids in the late 2020s. NASA is also looking at a robotic mission to capture a small, 500-ton asteroid and return it to an orbit around the Moon for astronaut exploration and resource prospecting. Finding small asteroids in the right orbits is necessary to identify &amp;quot;grabbable&amp;quot; targets for the mission.  In his new book, Near-Earth Objects: Finding Them Before They Find Us, JPL astronomer Don Yeomans says there are three essential steps to defending Earth from rogue asteroids. First, find them early. Second, find them early. Third, find them early. All of our plans for deflecting an asteroid depend on accurate knowledge of orbits and close-earth encounters. A space-based search would cost an estimated $500 million over 10 years, much less than 1 percent of NASA&amp;#039;s budget. For us on Earth, gliding with our fingers crossed through a cosmic swarm of asteroids like 2012 DA14, that&amp;#039;s cheap insurance. Tom Jones is a veteran astronaut, planetary scientist, and co-author of Planetology: Unlocking the Secrets of the Solar System. "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/space/telescopes/a8719/to-save-earth-from-asteroids-we-must-find-them-early-15102080/>