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.

To close that detection gap, NASA is developing the Near-Earth Object (NEO) Surveyor, a dedicated space telescope designed to identify asteroids and comets by detecting their thermal signatures rather than relying solely on visible light. By observing infrared emissions, the spacecraft will be able to locate darker objects that current ground-based systems frequently miss.

However, the NEO Surveyor program has faced financial and scheduling challenges. A NASA Inspector General report published in 2025 found that funding constraints had pushed the mission’s projected launch date from 2026 to 2028. The baseline cost estimate has also risen significantly, increasing from approximately $1 billion to $1.6 billion.

Compounding concerns, the White House earlier this year proposed a sweeping 25% reduction to NASA’s overall budget — a move the Planetary Society described as the largest single-year cut in the agency’s history. The proposal raised fresh uncertainty about the future of planetary defense initiatives, including NEO Surveyor. Congress ultimately rejected most of the proposed reductions in January 2026, allowing the telescope project to remain funded and on track for a 2027–2028 launch window.

Despite the current limitations in defensive readiness, Fast sought to provide context and avoid unnecessary alarm. Small space rocks routinely enter Earth’s atmosphere and typically burn up harmlessly. Extremely large asteroids — those capable of triggering global mass extinctions like the object that wiped out the dinosaurs 66 million years ago — are generally well tracked because their size makes them easier to detect.

The greatest vulnerability, she said, lies in the intermediate category.

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

Asteroids measuring approximately 140 meters in diameter and larger fall into that middle range. An impact from such an object could obliterate a major metropolitan area or cause widespread regional devastation. Yet the majority of these potentially hazardous objects remain undiscovered.

Chabot emphasized that the technical capacity to build a robust planetary defense system already exists. What is required, she said, is sustained political will, funding, and long-term strategic planning.

“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.”

International efforts to strengthen planetary defense are also underway. The European Space Agency’s Hera spacecraft, launched in 2024, is scheduled to arrive at the Dimorphos asteroid system in 2026. Hera will conduct a detailed assessment of the aftermath of the DART impact, collecting critical data to help scientists better understand the physics of kinetic impact deflection and how such techniques might be scaled for larger threats.

China has separately announced plans to conduct its own kinetic impactor demonstration mission, with a launch targeted for 2027.

Meanwhile, the United Nations has designated 2029 as the International Year of Planetary Defense and Asteroid Awareness. The timing coincides with the close flyby of asteroid Apophis, an approximately 1,100-foot-wide object that will pass within about 20,000 miles of Earth — closer than some orbiting satellites. Although Apophis poses no impact risk during that encounter, the event is expected to serve as a high-profile reminder of the importance of preparedness.

For now, scientists say humanity has proven it can nudge an asteroid off course under controlled conditions. But the world does not yet maintain a ready-to-launch deflection mission or a fully mature planetary defense infrastructure. Building and sustaining that capability, they warn, requires long-term investment and coordinated international commitment — because the next serious threat may not provide decades of advance notice.s