No way to stop 15,000 ‘city-killer’ asteroids yet: NASA planetary defence chief flags detection gaps

Earth currently has no reliable mechanism to stop thousands of potentially devastating “city-killer” asteroids that remain undetected, NASA’s acting Planetary Defence Officer Dr Kelly Fast said at the American Association for the Advancement of Science (AAAS) conference in Phoenix, Arizona, TOI reported.

Fast said scientists estimate there are about 25,000 near-Earth asteroids measuring roughly 140 metres or larger — large enough to cause regional devastation in the event of an impact. However, only around 40% of these objects have been identified so far, leaving nearly 15,000 untracked.

“What keeps me up at night is the asteroids we don’t know about,” Fast said, noting that while very small meteoroids typically burn up in Earth’s atmosphere and the largest “planet killers” are largely catalogued, mid-sized objects remain a major blind spot in planetary defence efforts


Why mid-sized asteroids pose concern?

According to NASA, asteroids measuring 140 metres (about 460 feet) or more can cause severe regional damage, including destruction of metropolitan areas, fires and infrastructure collapse. These objects are harder to detect through traditional visible-light telescopes because many are dark and reflect little sunlight. Some also follow Earth-like orbits, making early detection more difficult.

The potential impact of such objects is illustrated by the 1908 Tunguska event in Siberia, when an object estimated to be under 100 metres exploded in the atmosphere, flattening roughly 2,000 square kilometres of forest.

Limited defence capability?

NASA and its international partners have conducted asteroid defence tests, including the 2022 Double Asteroid Redirection Test (DART) mission, which successfully altered the orbit of a small asteroid’s moonlet by crashing a spacecraft into it. However, experts have stated that such deflection methods require years of advance warning and suitable target conditions.

Currently, there is no dedicated deflection spacecraft on standby, and planetary defence systems do not have the infrastructure for rapid-response missions if a dangerous asteroid is detected with limited lead time.

Detection efforts underway

To address detection gaps, NASA is developing the Near-Earth Object (NEO) Surveyor, an infrared space telescope designed to identify dark asteroids by detecting their heat signatures. The mission aims to significantly improve identification rates and help meet the goal of cataloguing 90% of near-Earth asteroids larger than 140 metres within the next decade.

Ground-based surveys, including those by the Vera C Rubin Observatory, and global collaborations such as the International Asteroid Warning Network (IAWN), are also expected to strengthen early warning systems.

While the probability of a large asteroid impact in the near term remains low, NASA officials have indicated that improved detection and tracking are critical to enabling any future deflection efforts.

With inputs from TOI