A senior scientist working with the United States’ space agency has delivered a chilling assessment of a threat lurking in our cosmic neighbourhood, saying that Earth currently has no effective defence against a vast number of potentially devastating asteroids and that this stark reality “keeps me up at night.” Dr. Kelly Fast, NASA’s Planetary Defense Officer, made the comments at a scientific conference, where she highlighted a worrying gap in humanity’s understanding and preparedness for near-Earth objects that could inflict catastrophic regional damage.
According to Dr. Fast, scientists estimate that there are approximately 25,000 asteroids larger than about 140 metres — roughly half the size of a football field — orbiting in or near Earth’s path. These mid-sized rocks are too small to be classified with the largest “planet killers” that can cause global extinction events, yet they are far larger than the tiny fragments that harmlessly burn up in Earth’s atmosphere on a regular basis. The real concern, she said, is the roughly 15,000 of these objects that remain undetected and untracked by telescopes and space-based surveys, creating a blind spot in planetary defence efforts.
“What keeps me up at night is the asteroids we don’t know about,” Dr. Fast said, emphasising that current detection systems, even the most advanced telescopes, struggle to spot these bodies because of their size, surface properties and the way they can follow orbits that position them close to Earth without reflecting much sunlight. This means many of these “city killers” could be lurking in space without astronomers’ knowledge, and a strike from such an object could obliterate a large metropolitan area if it were to collide with Earth.
The threat poses a dilemma for planetary defence planners because knowing an asteroid is on a collision course is only part of the challenge; the world also currently lacks deployable systems capable of deflecting or neutralising a dangerous object on short notice. Dr. Nancy Chabot, a leading planetary scientist who heads NASA’s Double Asteroid Redirection Test (DART) mission, acknowledged that while the 2022 DART mission successfully altered the path of a small asteroid’s moon to demonstrate deflection technology, agencies do not have spacecraft on standby that could be immediately dispatched to intercept and redirect a threatening “city killer.”
Chabot and other experts have noted that planetary defence infrastructure is underfunded and not yet at a level where it could respond rapidly if a large, previously unseen object were discovered on a collision trajectory toward Earth. “We would not have any way to go and actively deflect one right now,” she said, outlining a stark reality in which the capability to detect exists, but the capability to act decisively in the face of short-notice danger does not.
Efforts are underway to improve detection. NASA and international partners are preparing to launch the Near-Earth Object Surveyor space telescope, which is designed to use infrared observations to detect dark, elusive asteroids that evade traditional optical telescopes. Planners hope this next generation of technology will dramatically increase the catalogue of known near-Earth objects and provide earlier warning of possible threats.
The warning from NASA comes amid continuing interest in specific asteroids that have previously raised concern. One such object, designated 2024 YR4, was discovered in late 2024 and has been monitored because early projections indicated a small chance of it striking the Moon or even the Earth within the next decade. While further observations have reduced the likelihood of an impact, its existence underscores the uncertainties that remain in tracking and characterising near-Earth bodies of this size.
Experts agree that increased investment in both detection technologies and deflection capabilities would strengthen global preparedness. Without such advances, the risk posed by undetected mid-sized asteroids remains a sobering reminder that, for all of humanity’s scientific progress, Earth’s defence against some natural cosmic hazards is still limited.
