Has an asteroid ever been spotted before it hit Earth?
Yes. Since 2008 about a dozen small asteroids have been discovered hours before striking Earth, and meteorites were recovered from a few of them.
The first time it happened was a milestone in astronomy. On 6 October 2008, a telescope of the Catalina Sky Survey in Arizona spotted a faint object heading straight for Earth. Named 2008 TC3, it was only about 4 metres across. Astronomers around the world tracked it, calculated that it would hit northern Sudan, and issued a prediction. About 19 hours after discovery, early on 7 October, it exploded high over the Nubian Desert exactly as forecast.
A search team of scientists and students later combed the desert and found hundreds of fragments, named Almahata Sitta. Most are a rare type of stony meteorite called a ureilite, but the collection also contains pieces of many other meteorite types, suggesting the asteroid was a jumbled mixture of debris. It was the first time a meteorite had been linked to an asteroid that was actually observed in space.
Since then, as sky surveys have grown more sensitive, about a dozen more small asteroids have been caught before impact. Most hit the ocean or remote areas, but several produced recovered meteorites, including 2018 LA over Botswana, 2023 CX1 over northern France and 2024 BX1, which lit up the sky west of Berlin in January 2024.
All these objects were tiny, a few metres at most, and harmless, burning up or breaking apart high in the atmosphere. Their real value is as practice: each one tests the global system of telescopes, orbit calculations and alerts that would be essential if a larger, dangerous asteroid were ever found on a collision course. They also show that we are now finding more of the small, faint bodies that until recently arrived completely unannounced.