NASA has no active defense against city-killer asteroids, scientists warn

Space officials say tens of thousands of near-Earth asteroids capable of demolishing cities remain untracked — and humanity currently has no way to deflect them.

NASA planetary defense officer Kelly Fast told the American Association for the Advancement of Science in Phoenix, Arizona, that roughly 25,000 asteroids measuring more than 140 meters in diameter orbit near Earth — and the agency has only located about 40% of them.

The alarming admission comes even as NASA has demonstrated it is technically possible to alter an asteroid’s path. In 2022, the agency deliberately slammed a spacecraft called DART (Double Asteroid Redirection Test) into Dimorphos, a small moonlet orbiting a larger asteroid, at 14,000 mph, successfully nudging the rock off course. But scientists caution that a single successful test is a long way from a ready operational 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.”

Ms. Chabot was referring to asteroid 2024 YR4, which last year briefly raised widespread alarm when it was calculated to have a 3.2% chance of striking Earth in 2032. NASA subsequently downgraded that probability to zero — but the episode underscored how quickly a genuine threat could emerge with no defensive infrastructure in place. The James Webb Space Telescope is currently tracking the asteroid, as there remains approximately a 4% chance it could strike the moon, a collision that would produce a flash visible from Earth.

Ms. Fast said her mission was to “find asteroids before they find us,” and noted that tracking is complicated by the fact that near-Earth asteroids travel in the same orbital plane as Earth, making them difficult to spot via conventional telescopes that rely on reflected sunlight. A new space telescope, the Near-Earth Object (NEO) Surveyor, is being developed to detect dark asteroids and comets using thermal signatures — a capability current ground-based systems lack.

However, that tool has faced its own headwinds. A NASA Inspector General report published in 2025 found that funding constraints had already pushed the NEO Surveyor’s projected launch date from 2026 to 2028, with its baseline cost estimate rising from $1 billion to $1.6 billion. Earlier this year, the White House proposed a sweeping 25% cut to NASA’s overall budget — which the Planetary Society called the largest single-year reduction in the agency’s history — raising fresh concerns about the program’s future. Congress ultimately rejected the bulk of those cuts in January 2026, and the NEO Surveyor remains funded and on track for its 2027-2028 launch window.

Despite the gap in defensive readiness, Ms. Fast sought to calibrate the public’s concerns. Small space rocks strike Earth regularly and pose minimal danger, she said. Extremely large asteroids — those capable of mass extinctions, like the one that killed the dinosaurs 66 million years ago — are relatively well-tracked because of their size. The real worry lies in the middle ground.

“We’re not so much worried about the really large ones because we know where those are,” Ms. Fast said. “It’s the ones in between that could pose regional damage.”

Those mid-range rocks, measuring roughly 140 meters in diameter and above, are capable of obliterating a city or triggering regional catastrophe — yet the majority remain undetected.

Ms. Chabot said the capacity to build a real planetary defense system exists.

“We could be prepared for this threat,” she said. “We could be in very good shape. We need to take those steps to do it. If anything keeps me awake, it’s that.”

The international community has begun stepping up. The European Space Agency’s HERA spacecraft, which launched in 2024, is set to arrive at the Dimorphos asteroid system in 2026 to study the aftermath of the DART impact in detail — data that will help scientists understand how to scale up deflection techniques for larger threats. China has separately announced plans to conduct its own kinetic impactor demonstration, with a launch targeted for 2027. Meanwhile, the United Nations has designated 2029 the International Year of Planetary Defense and Asteroid Awareness, timed to coincide with the close flyby of the asteroid Apophis — a 1,100-foot-wide rock that will pass within roughly 20,000 miles of Earth, closer than some satellites.

For now, though, the world’s most proven planetary defense capability — a single DART-class spacecraft — no longer exists in ready form. Scientists say building and maintaining that readiness requires sustained investment and political will, and that the clock may be ticking.