15,000 'city-killer' asteroids still undetected in space: NASA

Scientists have warned that thousands of 'city-killer' asteroids remain untracked, raising concerns about planetary defence. Dr Kelly Fast of NASA said around 15,000 near-Earth objects, at least 140 metres across, are still undiscovered and could cause severe regional destruction. Detection is difficult due to challenging orbits, and only 40% of such objects are currently catalogued.