NASA has sounded the alarm about a cosmic threat that, until recently, many might have dismissed as the stuff of science fiction. According to recent briefings delivered at the American Association for the Advancement of Science in Phoenix, Arizona, the agency estimates that there are roughly 25,000 medium-sized asteroids—each at least 140 meters in diameter—circling near Earth’s orbit. These so-called "city killer" asteroids are large enough to cause regional devastation if they were to strike a populated area, yet NASA admits it only knows the whereabouts of about 40% of them. The rest, for now, remain invisible to even the most advanced telescopes.
"If any of these 'city-killer' asteroids hits a populated place on Earth, it can cause regional damage," warned Kelly Fast, acting planetary defense officer for NASA, as reported by multiple outlets including The Times and The Telegraph. The threat is not merely theoretical. In 2025, a space rock dubbed YR4, about the size of a football pitch, set off alarm bells among astronomers when it appeared to have a small but real chance—initially estimated at roughly 3.2%—of colliding with Earth in 2032. Further analysis later downgraded that risk to zero, but YR4’s orbit still brings it close enough that it could pass through the Earth and Moon system, with a 4% chance of striking the Moon. If it does, the resulting flash would be visible from Earth with the naked eye, according to NASA scientists.
What’s perhaps most unsettling is how little we know about these cosmic neighbors. "What keeps me up at night is the asteroids we don’t know about," Fast admitted, echoing a sentiment that is gaining traction among planetary defense experts. Small space rocks hit our planet regularly, but they typically burn up in the atmosphere, posing little threat. The real worry is the mid-sized objects—too big to ignore, too small to be reliably tracked with current technology. "And we’re not so worried about the large ones from the movies because we know where they are. It’s the ones in between, about 140 meters and larger, that could really do regional rather than global damage and we don’t know where they are," Fast explained, as quoted by LADbible and The Sun.
NASA’s current approach relies heavily on ground-based telescopes that spot asteroids by detecting sunlight reflected off their surfaces. But many asteroids travel alongside Earth in its orbit around the Sun, making them especially hard to spot. "It takes time to find them, even with the best telescopes," Fast said. The agency has set an ambitious goal: to identify more than 90% of near-Earth objects larger than 140 meters. So far, only about 40% of this target has been achieved, leaving approximately 15,000 potentially dangerous objects unaccounted for.
To close this gap, NASA is pinning its hopes on the upcoming Near-Earth Object Surveyor mission, a space telescope set to launch next year. Unlike its predecessors, this new observatory will use thermal imaging to detect both bright and dark asteroids, including those invisible to traditional optical telescopes. "We’re searching the skies to find asteroids before they find us, and get them before they get us," Fast told The Telegraph. The mission, if successful, could be a game-changer for planetary defense, but for now, the world remains vulnerable.
Despite these efforts, the reality is sobering: humanity currently has no way to deflect a medium-sized asteroid on short notice if one were detected on a collision course with Earth. In 2022, NASA’s Double Asteroid Redirection Test (DART) mission proved it was possible to alter the path of a space rock by deliberately crashing a spacecraft into Dimorphos, a small moon orbiting an asteroid, at 14,000 mph. The experiment was hailed as a milestone, but it was just that—an experiment. "We don’t have [another] DART just lying around. If something like YR4 had been headed towards the Earth, we would not have any way to go and deflect it actively right now," said Nancy Chabot, planetary scientist at Johns Hopkins University and lead on the DART mission, according to The Times and LADbible.
Chabot’s concerns go deeper than just technical readiness. "We could be prepared for this threat. We could be in very good shape. We need to take those steps to do it. If anything keeps me awake, it’s that," she said. The lack of investment in planetary defense has left the world exposed. "We worry about these city killer asteroids," Chabot reiterated. "We could be prepared but I don’t see that investment being made." Her remarks highlight a growing consensus among experts: while the science to protect Earth exists, the political and financial will is lagging far behind.
It’s not just about money, though. The challenge is also technological. As Fast pointed out, "Telescopes are straining to spot them because they accompany the Earth in orbit around the Sun." Many asteroids are dark, reflecting little sunlight, and can only be detected by their heat signatures—a capability the Near-Earth Object Surveyor will provide, but which is not yet operational.
Meanwhile, the James Webb Space Telescope is tracking YR4, the asteroid that caused so much concern last year. Although the risk of an Earth impact in 2032 has been ruled out, the episode served as a wake-up call. If YR4 were heading straight for us, there would be little the world could do. "If something like YR4 had been headed towards the Earth, we would not have any way to go and deflect it actively right now," Chabot emphasized, underscoring the urgency of the situation.
For now, the best defense is vigilance. Small space rocks will continue to pepper Earth’s atmosphere, and the largest "planet killers"—like the one that wiped out the dinosaurs—are already being tracked. But the mid-sized, city-destroying asteroids remain an elusive threat. As Fast put it, "We’re not so much worried about the really large ones because we know where those are. It’s the ones in between that could pose regional damage."
With new technology on the horizon and a growing awareness of the risks, the hope is that humanity can get ahead of the next big one. But until the world matches its scientific know-how with real investment and preparation, the night sky will continue to hold secrets that keep even NASA’s top experts awake.