As highlighted by the Chelyabinsk meteor impact of 2013 and the frequent fly-bys of asteroids past our planet, we need to keep watching the skies for any space rocks that may pose a threat. The international teams of astronomers doing just that have now hit a milestone: 15,000 near-Earth objects (NEOs) have been discovered, and there's plenty more still to find.

Any comet or asteroid whose orbit brings it close to that of Earth's is classified as a NEO, and coordinated efforts from NASA and the ESA, among others, are keeping tabs on them to make sure we don't have any unexpected run-ins. The tally of identified objects is growing faster and faster: 5,000 have been discovered in the last three years alone, a figure which previously would've taken decades to reach. That's thanks to increasingly powerful telescopes like the Pan-STARRS in Hawaii and the Catalina Sky Survey in Arizona, which together have discovered around 90 percent of the known NEOs today.

"The rate of discovery has been high in the past few years, and teams worldwide have been discovering on average 30 new ones per week," says Ettore Perozzi, manager of the ESA's NEO Coordination Centre in Italy. "A few decades back, 30 were found in a typical year, so international efforts are starting to pay off. We believe that 90% of objects larger than 1,000 m (3,281 ft) have been discovered, but – even with the recent milestone – we've only found just 10% of the 100 m (328 ft) NEOs and less than 1% of the 40 m (131 ft) ones."

An impact by an object over 1,000 meters wide would cause widespread destruction, but even a 40 m one could have serious effects. The Chelyabinsk meteor, which injured 1,500 people and damaged several thousand buildings, is estimated to have had an initial diameter of just 20 m (66 ft). Identifying these threats, which are smaller on a cosmic scale but still pack a punch, is a priority.

To that end, space agencies are developing new instruments to help spot these rogue rocks before they strike: the ESA's "fly eye" telescopes, designed to keep watch over a large patch of sky, are due to come online in 2018, and the Large Synoptic Survey Telescope in Chile is due to begin sweeping the skies in 2022.

Source: ESA