The James Webb Space Telescope has taken the measure of 2024 YR4, and the result is wonderfully tangible: the asteroid is estimated to be 53 to 67 meters wide, roughly the height of a 15-story building laid on its side.
Webb can read the heat coming from the asteroid, which gives researchers a better size estimate than reflected sunlight alone. The observations also improve the orbit. NASA now calculates a 3.8% chance of a lunar impact on December 22, 2032, up from 1.7%. In everyday terms, that is about 1 chance in 26, with a 96.2% chance of a miss.
If YR4 did meet the Moon, it would add a fresh crater to a world already covered in them; it would not change the Moon’s orbit. The scientific opportunity would be extraordinary. Astronomers could know the arrival time years ahead, point telescopes at the expected region, and watch an impact process that usually leaves only ancient clues. First, though, Webb has another observing opportunity ahead—and one more chance to tighten the route.