Didymos and Dimorphos

Didymos is a pair of asteroids that orbit each other: the main body, about 805 m across, and a moonlet, Dimorphos, about 170 m across, a little longer than one and a half football fields.^[1] Neither poses any threat to Earth, but the pair passes relatively close to Earth, and the moonlet's orbit could be timed from the ground, which made it the target for the first test of asteroid deflection.^[1][2]

Discovery and a hidden moon

Didymos, Greek for twin, was discovered on 11 April 1996 by Joseph Montani of the Spacewatch survey at Kitt Peak National Observatory in Arizona.^[1] Multiple echoes in radar data from NASA's Goldstone antenna first suggested a moon, and observations in November 2003 confirmed it.^[1] That year the pair passed about 7.2 million km from Earth.^[1]

Didymos spins once every 2.26 hours and is shaped like a spinning top, with a ridge around its equator.^[1] Scientists think its fast spin flung material off its surface, which gathered into Dimorphos; one sign is the surfaces' ages, about 12.5 million years for Didymos and less than 300,000 years for Dimorphos.^[1] The pair circles the Sun once every 2.11 years, ranging from about 152 million to 340 million km from it.^[1]

The DART impact

On 26 September 2022, NASA's DART spacecraft struck Dimorphos at about 6.1 km per second.^[1] The impact shortened the moonlet's orbit around Didymos by 33 minutes, plus or minus 1 minute, from an original period of 11 hours and 55 minutes.^[3][2] It also reshaped Dimorphos, a loosely packed rubble pile, from a squashed ball into an elongated shape.^[1] Because the two bodies are bound together, the pair's 770-day orbit around the Sun changed too, by about 0.15 seconds: the first time a human-made object has measurably altered a celestial body's path around the Sun.^[4] How the test fits into Earth's defenses is covered in Planetary defense.

Hera's arrival

ESA's Hera spacecraft is due to reach the pair in November 2026, a month earlier than first planned, to survey the aftermath of the impact.^[5] It carries two small CubeSats, Milani and Juventas, that will make their own close-up studies.^[5]