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Tens of thousands of near-Earth asteroids capable of destroying entire cities remain untracked, and humanity currently has no active system ready to deflect one if it were found to be on a collision course with Earth, space officials have warned.

Speaking at a meeting of the American Association for the Advancement of Science in Phoenix, Arizona, NASA planetary defense officer Kelly Fast said that roughly 25,000 asteroids measuring more than 140 meters in diameter orbit near Earth. Of those, only about 40% have been identified and cataloged.

The sobering assessment comes even as NASA has demonstrated that altering an asteroid’s trajectory is technically possible. In 2022, the agency carried out the Double Asteroid Redirection Test (DART), deliberately slamming a spacecraft into Dimorphos — a small moonlet orbiting a larger asteroid — at approximately 14,000 miles per hour. The impact successfully changed Dimorphos’ orbit, marking the first time humanity intentionally shifted the path of a celestial body.

But scientists stress that a single experimental success does not equate to an operational planetary defense system.

“We don’t have [another] Dart just lying around,” said Nancy Chabot, a planetary scientist at Johns Hopkins University who led the DART mission. “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 was referring to asteroid 2024 YR4, which briefly triggered global concern last year when initial calculations showed a 3.2% chance of impacting Earth in 2032. NASA later downgraded that probability to zero after additional observations refined its trajectory. However, the episode served as a stark reminder of how rapidly a potentially serious threat can emerge — and how limited current defensive readiness remains.

Although the risk of an Earth strike has been eliminated, asteroid 2024 YR4 is still being closely monitored. The James Webb Space Telescope is tracking the object, as there remains roughly a 4% chance it could strike the moon. Such an impact would produce a flash visible from Earth but would pose no direct danger to the planet.

Fast summarized the mission of NASA’s planetary defense program succinctly: “find asteroids before they find us.” She noted that detecting these objects is inherently challenging. Near-Earth asteroids tend to travel in the same orbital plane as Earth, meaning they are often positioned in areas of the sky where conventional ground-based telescopes — which depend on reflected sunlight — struggle to detect them, especially if they are dark and absorb most incoming light.