NASA has revealed that Earth is vulnerable to thousands of 'city-killer' asteroids that are currently undetected and pose a significant threat to urban areas. Experts estimate around 15,000 medium-sized asteroids, measuring over 140 meters in diameter, are in close proximity to Earth, with only about 40% of their locations known. NASA officials, including planetary defense officer Kelly Fast, emphasized that current space telescopes struggle to detect these dark asteroids because they orbit near the Sun and reflect little sunlight. The upcoming Near-Earth Object Surveyor telescope, launching next year, aims to improve detection by using thermal signatures. Despite recent success with the 2022 DART mission, which demonstrated that a spacecraft could alter an asteroid's path, NASA admits it is unprepared to launch another similar mission if an asteroid were detected on a collision course. Dr. Nancy Chabot, who led the DART mission, expressed concern that no other spacecraft are ready for immediate deployment. Additionally, NASA is tracking a potential threat from asteroid YR4, which initially had a 3.2% chance of hitting Earth in 2032, but was later downgraded to zero. The agency is also developing the Surveyor mission to identify at least 90% of asteroids larger than 140 meters within a decade. Experts warn that the most imminent danger comes from mid-sized asteroids, which could cause regional destruction, and highlight the need for increased investment in planetary defense technology to prevent future catastrophic impacts.