Donaldjohanson
Key facts
- What it is
- Main-belt asteroid
- Size
- About 8 km long
- Discovered
- 1981
- Spacecraft
- Lucy fly-by, 2025
- Orbit group
- Main-belt Asteroid
- Average distance from the Sun
- 2.38 au (about 360 million km)
- Time to go round the Sun
- 3.7 years
- Closest and farthest from the Sun
- 1.94 to 2.83 au
- Tilt of its orbit
- 4.4°
An au (astronomical unit) is Earth's average distance from the Sun, about 150 million km. Orbit figures are from open tracking data, figures last updated 1 Nov 2026.
Donaldjohanson is a small asteroid in the inner part of the main belt, and the second target of NASA's Lucy mission on its long journey to the Trojan asteroids that share Jupiter's orbit. Lucy flew past it on 20 April 2025, at a distance of about 960 kilometres.
The pictures showed that the asteroid is larger than expected, about 8 kilometres long, and has a striking shape: two lobes joined by a narrow neck, like a peanut. This kind of body is called a contact binary. It probably formed when two smaller pieces, drifting slowly side by side after a collision, came together and stuck. The shape of the neck surprised the team and is still being studied.
Donaldjohanson belongs to a family of asteroids that seem to have formed when a larger body broke apart in the inner belt. Fragments from collisions like this are thought to be one of the sources of the meteorites and near-Earth asteroids that reach us, so close-up looks at family members help connect the rocks in our collections to their places of origin.
The asteroid is named after Donald Johanson, the American palaeoanthropologist who in 1974, with his team, discovered the fossil skeleton of a human ancestor nicknamed Lucy in Ethiopia. The space mission takes its name from that fossil, because just as the skeleton revealed something about human origins, the Trojan asteroids may reveal how the planets formed.
The fly-by also served as a full rehearsal for the Trojan encounters to come, starting with Eurybates in 2027.