52246 Donaldjohanson is an asteroid located in the inner main asteroid belt, between the orbits of Mars and Jupiter. It is an elongated contact binary consisting of two conjoined lobes spanning in total length. The asteroid was discovered on 2 March 1981 by astronomer Schelte Bus at Siding Spring Observatory in Australia and was named after American paleoanthropologist Donald Johanson. It was visited by NASA's Lucy mission on 20 April 2025.
Donaldjohanson is classified as a C-type asteroid, because it has a dark surface made of carbonaceous chondrite-like minerals. Both of its lobes are heavily cratered by past asteroid impacts, whereas the neck connecting them appears smoothed out by relatively recent landslides less than 20 million years ago. Several geological features on Donaldjohanson, including boulders, craters, and ridges, have been named after various paleoanthropological sites and hominin fossils.
Instead of rotating around a single axis, Donaldjohanson wobbles chaotically. It slowly rotates end-over-end once every , and wobbles back and forth around its long axis once every . The slow, tumbling rotation is believed to have been caused by the YORP effect, which is a gradual torque exerted by sunlight. Donaldjohanson is a member of the Erigone family, a collisional family of asteroids with physical and orbital properties similar to 163 Erigone. Donaldjohanson and the Erigone family are believed to have formed within the last 270 million years, in the aftermath of a catastrophic collision between two once-larger asteroids.
History Discovery Donaldjohanson was discovered by astronomer Schelte J. Bus while scanning observations made by the U.K. Schmidt–Caltech Asteroid Survey. The observations were made on 2 March 1981 using the U.K. Schmidt Telescope at Siding Spring Observatory, Australia. The Minor Planet Center (MPC) published the asteroid's discovery in August 1982 and gave it the provisional designation .
From 1983 to 1985, the MPC published additional Siding Spring observations of Donaldjohanson from 14 February, 1 March, and 3 May 1981. However, the asteroid was not observed again until 16 December 1998, when it was rediscovered as a seemingly new asteroid by the Lincoln Near-Earth Asteroid Research (LINEAR) project in New Mexico. The MPC published LINEAR's observations with the provisional designation in January 1999. In February 1999, Arno Gnädig reported that was the same object as .
Naming and exploration
Donaldjohanson has the minor planet number 52246, which was given by the MPC on 16 February 2003. The asteroid was named in honor of Donald C. Johanson, an American paleoanthropologist who discovered the "Lucy" hominid fossil in 1974. For the asteroid, Donald Johanson's first and last names were concatenated to conform with the International Astronomical Union's naming rules. The asteroid's name is commonly abbreviated as "DJ".
The Lucy fossil lends its name to NASA's eponymous spacecraft, which visited Donaldjohanson on 20 April 2025. The Lucy spacecraft's primary objective is to explore Jupiter's trojan asteroids, which NASA has described as "fossils" of the Solar System. Donaldjohanson received its name from Lucy mission scientists after they found that the asteroid coincided with the spacecraft's planned trajectory. The MPC announced the asteroid's name on 25 December 2015, nearly three months after NASA's selection of Lucy for the Discovery Program and almost six years before the spacecraft's launch. In a 2021 interview with Space.com, Donald Johanson remarked that he was "enormously excited" to watch the launch of the spacecraft and said that the naming was "something that was utterly and totally unanticipated in my life".
During the years before Lucys encounter with Donaldjohanson, astronomers studied the asteroid with telescopes on Earth. They were able to determine that the asteroid was highly elongated, slowly rotating, and carbon-rich, although much of its exact details were uncertain. On 20 and 22 February 2025, the Lucy spacecraft took its first images of Donaldjohanson from a distance of approximately . Over the two months leading up to its encounter, the spacecraft continued observing the asteroid from afar. Lucy made its closest approach to Donaldjohanson on 20 April 2025 17:51 UTC, at a distance of and a relative speed of . The spacecraft photographed the asteroid at resolutions of up to per pixel, but stopped 40 seconds before closest approach in order to avoid pointing its instruments too close to the Sun. During this time, Johanson was at the Southwest Research Institute in Colorado to watch the encounter live. This was the first time a spacecraft has visited an asteroid named after a living person.
Orbit Donaldjohanson is located in the inner region of the main asteroid belt between the orbits of Mars and Jupiter. It follows an elliptical orbit around the Sun, with an average distance (semi-major axis) of 2.38 astronomical units (AU) and an orbital period of 3.68 Earth years. It has an eccentricity of 0.19 and an inclination of 4.4° with respect to the ecliptic, although over long periods of time they can change and average out to 0.21 and 5.3°. Throughout its elliptical orbit, the asteroid's distance from the Sun varies from at perihelion to at aphelion.
Donaldjohanson's orbit is similar to that of the large asteroid 163 Erigone, and it has been identified as a member of the Erigone family. Like the majority of Erigone family members, Donaldjohanson's orbit is not influenced by any meaningfully strong orbital resonances with another planet. However, its high eccentricity leads to chaotic changes in its orbit over relatively short timescales, with a Lyapunov time of about 12,000 years. Over a period of 10 million years, Donaldjohanson's perihelion distance may drop to , which would bring it close enough to Mars to weakly influence the asteroid's orbit. N-body simulations predict that the asteroid's orbit will remain stable for at least 1 billion years, but its semi-major axis will undergo small random variations on the order of . Donaldjohanson's semi-major axis is also predicted to be slowly drifting outward at a rate of per million years due to the Yarkovsky effect. The Yarkovsky effect is a phenomenon involving the asteroid absorbing sunlight and asymmetrically radiating it away, which pushes the asteroid away from the Sun if it rotates in the same direction as its orbit (see section ).
Physical characteristics
Geology
Donaldjohanson is an elongated contact binary asteroid. It consists of two lobes connected by a smooth, narrow neck, giving it a peanut-shaped appearance. The asteroid has overall dimensions of and a volume of approximately . Its mass has not been measured directly. Assuming it has a density of similar to those of carbonaceous asteroids Bennu and Ryugu, Donaldjohanson is estimated to have a mass of . Its surface is estimated to have a cohesive strength of up to 40,000 pascals, probably much higher than those of loosely-bound rubble pile asteroids like Bennu and Ryugu (cohesive strengths <).
Only 40% of Donaldjohanson's surface has been imaged in detail by Lucy. The International Astronomical Union (IAU) and the Lucy team have given formal names to several geological features on the asteroid, in honor of significant paleoanthropological sites and hominin discoveries. Named features include boulders (saxa), craters, and ridges (dorsa). The first eleven of these names were approved by the IAU on 27 August 2025.
Lobes The smaller lobe of Donaldjohanson is named Afar Lobus, after the Afar Triangle in Ethiopia, while the larger lobe is named Olduvai Lobus, after Olduvai Gorge in Tanzania. Afar Lobus has a length of about , whereas Olduvai Lobus has a length of about . Because Olduvai Lobus is larger, the entire asteroid's center of mass lies inside it. The lobes are thought to be former individual asteroids that formed together, gravitated toward each other, and then merged gently.
Both lobes are heavily covered with impact craters. The largest known crater on Afar Lobus has a diameter between , whereas Olduvai Lobus's largest known crater has a diameter between . The number density of craters larger than appears to have reached a steady state where it remains nearly constant over time. In contrast, craters with diameters smaller than 400 m appear scarcer on both lobes. Crater counts indicate that small craters were erased within the last 40 million years. The Lucy team proposed that they were erased by the flow of loose material, when a large impact event triggered seismic shaking across Donaldjohanson's surface.
Neck
The neck connecting the two lobes is named Windover Collum, after the Windover Archeological Site in Florida. The middle of Windover Collum is encircled by a ridge named Luzia Dorsum (named after the Luzia Woman), which divides the neck into a larger Hadar Regio at Afar Lobus and a smaller Minatogawa Regio at Olduvai Lobus (named after Hadar, Ethiopia and the Minatogawa Man, respectively). Due to the ridged appearance of Windover Collum, researchers of the Lucy team have compared Donaldjohanson's shape to that of an hourglass, or a pair of nested ice cream cones.
At Windover Collum, the surface gravity points toward Donaldjohanson's center of mass at Olduvai Lobus, so loose material is expected to preferentially slide downward from Afar Lobus to Olduvai Lobus. This is evident from the smooth terrain seen in Hadar Regio, which is sparsely covered by degraded and typically flat-floored craters. The slope is also steep and uniform, with an average angle of 25° with respect to the direction of Donaldjohanson's surface gravity. The side of Luzia Dorsum facing Afar Lobus has a shallow slope of less than 10°, which indicates that material has accumulated there. These characteristics of Windover Collum suggest that it has undergone resurfacing relatively recently, less than 20 million years ago, most likely as a result of landslides triggered by large impact events.
Other features One boulder (Boxgrove Saxum) and two small craters (Mungo and Narmada) were named in August 2025 with the purpose of defining Donaldjohanson's prime meridian. They were selected as coordinate markers because they were identified as the smallest and most visible features in Lucys close-up photographs of Donaldjohanson. Two other larger boulders, Cashel Saxum and Kennewick Saxum (diameters , respectively), have also been named. All named boulders and craters are located on Olduvai Lobus.
Rotation
Donaldjohanson exhibits a very slow, tumbling rotation, known as non-principal axis rotation. Instead of rotating about a fixed axis, it wobbles chaotically without ever exactly repeating its orientation. The asteroid wobbles around its long axis with a roll precession period of and wobbles around its angular momentum vector with a yaw precession period of . The asteroid's short axis is not aligned with the angular momentum vector and can stray as far as 40° away from it. These two precession periods make Donaldjohanson rotate end-over-end with a primary period of . Observations by the Lucy spacecraft indicate that Donaldjohanson also displays a secondary rotation period of .
Due to Donaldjohanson's elongated shape, its apparent brightness varies greatly as it rotates. Astronomers first reported Donaldjohanson's slow rotation in 2021, after ground-based telescopes detected periodic brightness variations ranging approximately 1 magnitude. The Lucy spacecraft confirmed these results and discovered the asteroid's tumbling rotation while monitoring its brightness over the two months leading up to its encounter.