Could an asteroid wipe out life on Earth?
A very large impact could cause a mass extinction, as one did 66 million years ago, but sterilising the whole planet would need an object far larger than any known to be on a collision course; no known large asteroid threatens Earth in the foreseeable future.
It helps to separate wiping out all life from causing a mass extinction. Earth's history shows the second can happen. About 66 million years ago an asteroid roughly 10 km across struck what is now the Yucatán Peninsula in Mexico, creating the Chicxulub crater, around 180 km wide. Dust, soot and sulfur-rich aerosols darkened and cooled the planet, food chains collapsed, and about three-quarters of species vanished, including all the dinosaurs except birds.
Yet life as a whole came through. Mammals, birds, insects, plants and ocean plankton survived and diversified. Microbes living deep in rock or around volcanic vents on the sea floor are especially hard to kill. To sterilise the entire planet, an impactor would probably need to be hundreds of kilometres across, large enough to boil the oceans. Objects of that size struck the young Earth over 4 billion years ago, but none now crosses Earth's path.
The comforting news is that astronomers have searched the sky systematically since the 1990s. They believe they have found the great majority of near-Earth asteroids larger than 1 km, the size that could cause global effects, and none of them is on course to hit us in the coming century or more. Smaller objects, a few tens to hundreds of metres across, are more numerous and less completely catalogued; these could devastate a region but not end civilisation.
That is why space agencies now treat planetary defence seriously: find hazardous objects early, track their orbits precisely and, if needed, nudge one aside. NASA's DART mission showed in 2022 that striking a small asteroid can change its orbit. With decades of warning, a dangerous impact could very likely be prevented, something the dinosaurs could never manage.