Scientists estimate that more than 25,000 asteroids large enough to destroy a city are near Earth, but fewer than half have been identified so far, according to NASA. These mid-sized objects, around 140 meters (460 feet) wide, are considered among the most dangerous because they can cause severe regional damage but are harder to detect, as reported by USA Today on April 4.
Experts say many of these asteroids remain hidden due to their small size, dark surfaces, and positioning in sunlight, which can obscure them from Earth-based telescopes. While no known threat is currently on a collision course with Earth, scientists warn that undetected objects could still pose a risk.
To improve detection, NASA is developing the Near-Earth Object Surveyor, a space-based telescope set to launch in 2027. The mission aims to identify most potentially hazardous asteroids years or even decades before they approach Earth.
Asteroids are rock-like objects that orbit the sun, and meteors are pieces of asteroids (or comets) that enter the Earth's atmosphere. While small meteors occur every day, large meteors or asteroids are far rarer. According to NASA, a car-sized asteroid enters our atmosphere about once a year, burning up as a fireball, while an object the size of an American football field hits Earth roughly every 2,000 years, causing major damage.
Intrigued? Click through this gallery to discover some of the close calls from the last 20 years.