When you think of moons, you probably picture Earth's familiar companion or the dozens orbiting Jupiter and Saturn. But here's something most people don't realize: asteroids can have moons too. Hundreds of these space rocks are cruising through the solar system with their own tiny satellites tagging along.

Scientists call these systems "binary asteroids," and they're far more common than astronomers initially expected. An estimated 2% of all asteroids have at least one moon, with some asteroids even hosting multiple companions. In 2024, ESA's Gaia spacecraft spotted potential moons around more than 350 asteroids that weren't previously known to have companions—nearly doubling the count overnight.

The First Confirmed Asteroid Moon

The breakthrough came in 1993 when NASA's Galileo spacecraft flew past asteroid 243 Ida on its way to Jupiter. Scientists were stunned to discover a tiny moon—later named Dactyl—orbiting the potato-shaped rock. Dactyl is only about a mile wide, circling Ida like a cosmic puppy following its owner. This discovery shattered the assumption that only planets could have natural satellites.

Since then, astronomers have identified hundreds more. Among near-Earth asteroids, 85 are known to have at least one moon, and three actually have two moons. The asteroid 130 Elektra takes the prize with three moons, making it the solar system's only known quadruple asteroid system.

How Do Asteroids Get Moons?

Most asteroid moons form when a "parent" asteroid spins so fast that chunks of material fly off into orbit. This happens through something called the YORP effect—a physics phenomenon where sunlight gradually speeds up an asteroid's rotation over millions of years.

Many of these asteroids are "rubble piles"—loose collections of rocks barely held together by weak gravity. When they spin fast enough, pieces break away and settle into orbit instead of drifting off into space. It's like a cosmic centrifuge creating its own satellite system.

Famous Asteroid Pairs

In September 2022, NASA deliberately crashed a spacecraft into Dimorphos, a 160-meter-wide moonlet orbiting the larger asteroid Didymos. This DART mission was humanity's first test of planetary defense—proving we could nudge a dangerous asteroid off course by hitting its moon. The European Space Agency's Hera mission will visit the impact site in December 2026 to study the aftermath.

Binary asteroids aren't just cool—they're scientifically invaluable. When a moon orbits an asteroid, astronomers can calculate the system's mass and density by watching how they interact. This reveals whether an asteroid is solid rock, a loose rubble pile, or something in between—critical information if we ever need to deflect one heading toward Earth.

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