The James Webb Space Telescope has a rare appointment tonight: look for a building-sized asteroid so faint that it has effectively vanished from Earth’s telescopes. If Webb can pick 2024 YR4 out from the background stars, one tiny dot could redraw its route through the Earth–Moon system in 2032.
The challenge is deliciously extreme. Webb’s view is narrow, YR4 is many millions of kilometers away, and even a small mismatch between its predicted and real position could leave it outside the frame. Astronomers are using accurately mapped Gaia stars as fixed landmarks, hoping the asteroid appears as the traveler among them.
The lunar-impact estimate going into this attempt is 4.3%, or about 1 chance in 23. A miss remains much more likely, and neither outcome would put Earth in danger. The real suspense is whether Webb can see the target at all. Tonight’s observation is less like taking a portrait and more like catching a single dark snowflake across a stadium—with a camera built to study galaxies.