How Didymos got a moon

Fast spin, a ring of debris and a rubble-pile moonlet: the leading theory of how binary asteroids like Didymos form.

Published 2026-10-23

Didymos is a binary: a roughly 780-meter asteroid with a 170-meter moon, Dimorphos.^[3][1] How does a small asteroid end up with a moon?

Spotting the moon

Suspicion came first from radar: NASA's Goldstone antenna recorded multiple echoes. The moon was confirmed in November 2003 using optical light curves — measurements of brightness over time — and Arecibo radar images.^[1] Petr Pravec and colleagues at Ondřejov Observatory in the Czech Republic made the photometric discovery on November 20, 2003.^[8]

A fast-spinning parent

Didymos rotates once every 2.26 hours.^[5] Sunlight can gradually change the spin of small asteroids through the YORP effect. A 2023 study proposed that YORP spun Didymos up until it shed material from its equator, forming a debris ring with about 25% of Didymos's mass.^[8]

From ring to moonlet

Part of that ring clumped together into Dimorphos, a low-density rubble pile; much of the rest fell back onto Didymos.^[8] The idea fits several observations: DART's final images showed Dimorphos as an egg-shaped body covered in boulders, and its surface is less than 0.3 million years old — much younger than the roughly 12.5 million years estimated for Didymos's surface.^[8]

Shaken up by DART

DART's 2022 impact changed Dimorphos from a roughly symmetrical oblate spheroid to a flatter, triaxial shape, and may have set it tumbling. If so, it should settle back into a tidally locked state within several decades.^[8] ESA's Hera, arriving in November 2026, will measure the result up close.^[4]

Why binaries matter

A binary system is a natural laboratory: the moon's orbit reveals the pair's mass and lets scientists measure tiny changes, like DART's 33-minute shift, from Earth.^[1]

Numbered citations link to the sources list on the Didymos overview page.