Dr Kelly Fast, the head of planetary defence at the US<\/a> agency overseeing efforts to track nearby asteroids<\/a> and comets, said roughly 15,000 mid-sized objects remain unaccounted for, and scientists also warn Earth, which is now about halfway through its lifespan<\/a>, currently lacks the means to stop one if it were heading our way.<\/p>\n Speaking at the American<\/a> Association for the Advancement of Science<\/a> (AAAS) conference in Arizona last week, Dr Fast said medium-sized asteroids pose the biggest worry, via The Sun<\/a>.<\/p>\n “What keeps me up at night is the asteroids we don’t know about,” she said. “Small stuff is hitting us all the time, so we’re not so much worried about that. And we’re not so worried about the large ones from the movies because we know where they are.”<\/p>\n Dr Fast continued: “It’s the ones in between, about 140 metres and larger, that could really do regional rather than global damage, and we don’t know where they are. It’s estimated there are about 25,000 of those, and we\u2019re only about 40 per cent of the way through.<\/p>\n “It takes time to find them, even with the best telescopes.”<\/p>\n![\"Scientists](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/02\/aHR0cHM6Ly9ldS1pbWFnZXMuY29udGVudHN0YWNrLmNvbS92My9hc3NldHMvYmx0OGJiZjE2YzJkN2EyMDllNS9ibHRhMjc1MGQ2.webp\"\/)
Scientists have warned over ‘city-killer’ asteroids which could hit Earth (Getty Stock Images)<\/p>\n
![\"NASA's](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2026\/02\/aHR0cHM6Ly9ldS1pbWFnZXMuY29udGVudHN0YWNrLmNvbS92My9hc3NldHMvYmx0OGJiZjE2YzJkN2EyMDllNS9ibHQ1MDhhYzYx.webp\"\/)
NASA’s Dr Kelly Fast said medium-sized asteroids pose the biggest worry (Kevin Carter \/ Contributor \/ Getty Images)<\/p>\n