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The effort to find objects that might threaten Earth is far from complete, and NASA admits it won’t meet a 2020 congressional deadline to find the bigger ones.

Two years ago, without warning, a house-sized meteor exploded 30 km above Chelyabinsk, Russia; it produced a shock wave that damaged thousands of buildings and injured around 1500 people (see the article by David Kring and Mark Boslough, , September 2014, page 32). It was a startling reminder of the sea of asteroids and other near-Earth objects (NEOs) through which our planet moves.

Of the nearly 13 000 NEOs that have been cataloged, 1600 are considered potentially hazardous in that their paths might cross Earth’s orbit. Scientists have estimated that around 22 500 NEOs are larger than 100 m in diameter. But the Minor Planet Center, the central registry of NEOs, located at the Smithsonian Astrophysical Observatory, has cataloged just 7840 of them, says the center’s acting deputy director José Luis Galache. The good news is that astronomers have located more than 90% of the objects larger than 1 km, including 200 potentially hazardous ones. None of those have been determined to pose a threat to Earth for at least the next 100 years or so, says Jim Green, director of planetary science at NASA.

Of greater concern, perhaps, is that fewer than 1% of an estimated 1 million NEOs larger than 20 m have been spotted, says Gerhard Drolshagen, comanager of the European Space Agency’s (ESA’s) near-Earth objects division. Relatively small objects can still wreak havoc: The impact from an 80-m asteroid would produce a crater the size of Washington, DC, says Green. The 1908 meteor that exploded in the Tunguska event over Siberia, flattening 80 million trees in a 2000-km2 area, was an estimated 60 m in diameter. And the Chelyabinsk meteor was calculated to be 17–20 m.

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