Try guessing the size of a car at night using only the light reflected from its paint. A small white car and a large black SUV might send back the same amount of light. Asteroids create a similar puzzle.
Early estimates for YR4 used visible brightness and covered a broad range: about 40 to 90 meters. Webb added infrared measurements—the asteroid’s heat glow—which helped separate size from shininess. The improved estimate narrowed to 53–67 meters.
That is why asteroid sizes often arrive as ranges rather than neat answers. Astronomers are working from a tiny amount of light across an enormous distance. Infrared observations give them another clue, like finally feeling the warmth of the mystery car’s engine—and a better idea of what will race through the Earth–Moon system in 2032.