An Asteroid Has Its Own Moon

When NASA's Galileo spacecraft flew past the asteroid Ida in 1993 on its way to Jupiter, it returned images that stunned the scientific community. Trailing the 54-km-long asteroid was a tiny companion, a minuscule moon just 1.6 km across. This discovery, later named Dactyl, was the first definitive proof that asteroids, like planets, could have their own natural satellites. Before this encounter, the idea of an asteroid moon was purely theoretical, and finding one was a moment of pure serendipity for the mission team.

The existence of Dactyl was more than just a curiosity; it was a scientific breakthrough. By observing Dactyl's orbit around Ida, astronomers could use the laws of gravity to calculate Ida's mass and, subsequently, its density. The result was another surprise: Ida's density was far too low for it to be a solid, monolithic rock. This provided the first strong evidence for the "rubble pile" theory of asteroids, suggesting that many are not single chunks of stone but rather loose collections of smaller rocks and debris held together by their own weak gravity. This chance discovery fundamentally changed our understanding of what asteroids are and how they are formed.