In 2005, Japan’s unmanned Haya-busa spacecraft visited an “impossible” asteroid. Images and data indicate that the asteroid, named Itokawa, is twice as porous as loose sand. This has astonished scientists, who believe that asteroids make repeated impacts with other space rocks and hence should be very dense. As they make additional discoveries, scientists may learn why Itokawa is different. But for now, we have an asteroid that challenges scientific understanding.