Asteroid · Near-Earth asteroid (binary) · orbits Sun
Didymos is a 780-metre near-Earth asteroid orbited by a 151-metre moonlet, Dimorphos - and that little moon is now famous: in September 2022 NASA’s DART spacecraft deliberately crashed into it, the first-ever test of deflecting an asteroid.
Dimorphos is the first celestial body whose orbit humans have measurably changed.
Didymos spins in just 2.26 hours - near the limit before a rubble pile flies apart.
ESA’s Hera spacecraft is en route to survey the aftermath up close in late 2026.
Planetary defence, tested for real
DART hit Dimorphos at 6.1 km/s and shortened its orbit around Didymos by about 32 minutes - far more than predicted, because the ejected debris acted like a rocket plume. It proved that with enough warning, a small impactor really can nudge an asteroid off a collision course.
Physical characteristics
Diameter
0.78 km
Mass
5.300e+11 kg
Rotation period
2.26 h
Geometric albedo
0.15
Known moons
1
Atmosphere and composition
Atmosphere
None.
Composition
Silicate rock (S-type); a fast-spinning top shape with a tiny moon, Dimorphos.
Discovery
Discovered by
Spacewatch (Kitt Peak)
Year
1996
Missions
DARTNASA · Impactor · 2021–2022 · completed
The first planetary-defence test: DART deliberately crashed into the small asteroid moon Dimorphos in September 2022 and shortened its orbit around Didymos by 32 minutes. Humanity has now demonstrably changed the motion of a celestial body.