Triple-Lobed Asteroid Dinkinesh Is a World Unlike Any Other, Scientists Say

NASA's Lucy spacecraft has identified a small asteroid in the main belt that defies the usual expectations of what a space rock should look like. The asteroid, named Dinkinesh, presents a three-lobed silhouette that mission scientists say makes it a world unlike any other encountered in the solar system. The finding, from a flyby on Nov. 1, 2023, offers a rare record of the slow and gentle process by which many small planetary bodies may have come together.

The encounter was not the main goal of Lucy's mission. The spacecraft was launched in October 2021 to explore the Jupiter Trojan asteroids, a group of primitive bodies that share Jupiter's orbit around the sun. Dinkinesh was added to the itinerary as a test target, giving the probe's navigation system a chance to practice tracking a small, moving object. What scientists expected was a quick look at another ordinary asteroid. Instead, they got the most complicated small world they had ever seen at close range.

A Surprise in the Asteroid Belt

Dinkinesh is a tiny object by asteroid standards, measuring only a few hundred meters across at its widest point. It orbits the sun in the inner part of the main asteroid belt, between Mars and Jupiter, and its name comes from the Amharic word for "you are marvelous." When Lucy made its closest approach, the spacecraft passed within a few hundred kilometers of the asteroid, returning images with far better resolution than any telescope can provide.

The first close-up images showed something immediately odd. Instead of a single roundish rock, Dinkinesh appeared to be made of two lobes joined at a narrow neck, a type of object astronomers call a contact binary. Then further images showed that the asteroid had a small moon. As the spacecraft sent back more data over the following days, the story became stranger still: the moon, later named Selam, was also a contact binary, with two lobes of its own.

"This is confusing," said John Spencer, the Lucy deputy principal investigator at the Southwest Research Institute. "This is going to be very interesting to figure out." The mission team realized they were looking at a system with no close relative among the roughly one million known asteroids. It was not just an asteroid with a moon; it was an asteroid with a companion moon that was built the same bizarre way as the asteroid itself.

A Three-Lobed Anatomy

The unusual structure has made Dinkinesh a natural object for the word "three-lobed." In the highest-resolution frames, the asteroid's two large lobes sit side by side, and the moon appears as a smaller, rounded lobe attached at one end. Depending on the angle, the entire system looks like a slightly misshapen snowman: a large lobe in the middle, a second lobe at its base, and the small satellite lobe perched on one side. The arrangement is not random; the relative sizes and spacing of the lobes record the history of how the pieces came together.

Contact binaries are not rare in the solar system. Several asteroids and comets, including Comet 67P/Churyumov-Gerasimenko, have two lobes that touch at a narrow waist. They are thought to form when two separate bodies collide at very low speed, so slow that they do not break each other apart on impact. The two objects settle together, sometimes with a pile of loose rubble filling the gap between them. Dinkinesh's main body fits this picture. What puzzled scientists was the second pair of lobes on Selam, and the fact that both components of the system preserved their original shapes instead of being destroyed by tidal forces.

A Record of Gentle Collisions

The survival of so many distinct lobes imposes strict limits on how the system could have formed. If the components had collided with typical asteroid speeds, which can reach several kilometers per second, they would have shattered or melted on impact. The fact that they are intact means the collision speeds were probably only a few meters per second, roughly the pace of a slow walk. At such low speeds, gravity alone is enough to bring the pieces together and hold them there.

That kind of gentle assembly is exactly what scientists think happened during the first stages of planet formation. The solar system began as a cloud of gas and dust around the young sun. Tiny grains stuck together electrostatically, forming pebbles, and pebbles gradually built up into larger bodies. Most of those early building blocks were later destroyed by collisions or melted by radioactive heat. Only small asteroids like Dinkinesh have been able to preserve the original structure of those first accreted bodies.

"Small asteroids are time capsules," said a planetary scientist familiar with the Lucy data. "They are not big enough to separate into layers like planets, so every bump and every fragment is still visible on their surfaces." Dinkinesh, with its record of at least two separate low-speed meetings, is an unusually clear snapshot of that process.

The complexity of the system also suggests that the pieces did not all come together at once. Scientists believe the two lobes of Dinkinesh merged first. Later, material may have been lifted off the young asteroid by sunlight, changes in rotation, or a small collision, forming a debris disk around it. That disk could have clumped together to form Selam. Because Selam itself is a contact binary, it must have formed separately from two small pieces that came together in another gentle encounter. The moon then settled into orbit around the larger pair, preserving every joint in a kind of cosmic fossil.

Why Small Bodies Matter

Dinkinesh is not a large enough object to have become a planet. Its gravity is far too weak to shape it into a sphere, and it contains too little rock to generate internal heat or geological activity. But that small size is exactly what makes it scientifically valuable. Large worlds such as Earth and Mars have been recycled by plate tectonics, volcanism, and erosion, erasing nearly all evidence of how they originally formed. Asteroids have no such processes. Their surfaces preserve the history of the solar system's earliest days.

By studying Dinkinesh's lobes, scientists can test computer models that predict how planetesimals — the original building blocks of planets — merged and grew. The discovery also raises the possibility that contact binary pairs are more common than people realize. Most asteroids are too small and too distant to be resolved by telescopes. If a three-lobed system can survive in such a stable configuration, other similarly complex objects may be waiting to be discovered.

The find has implications beyond planetary science. NASA's planetary defense programs need to know how asteroids are put together. If an asteroid ever threatens Earth, astronauts or spacecraft would not be pushing a single solid boulder; they might be dealing with a loose collection of fragments, held together only by gravity and weak van der Waals forces. Dinkinesh shows that such structures can be even more complicated than simple rubble piles.

What Happens Next

Lucy has already moved on. The spacecraft is continuing toward Jupiter's Trojan asteroids, which it will begin visiting in 2027. Before that, it will fly past another small main-belt asteroid, 52246 Donaldjohanson, in 2025. The experience with Dinkinesh has taught the mission team to expect surprises, and the records from the encounter will be used to plan observations of the Trojans, many of which may also be contact binaries.

Researchers are now building detailed models of the Dinkinesh system from Lucy's images. Those models should reveal the exact sizes of the lobes, the density of the material, and the way the two bodies rotate around each other. The density, in particular, could help scientists determine whether the asteroid is a solid rock or a fluffy pile of rubble. The team's findings are expected to be refined in peer-reviewed studies and compared with observations of other small bodies made by telescopes and future spacecraft.

Until those studies are complete, the three-lobed asteroid stands as a reminder of how much remains unknown in the solar system. Even a small object, visited for only a few hours, can overturn assumptions that have lasted for decades. As one member of the Lucy team put it, "Nature was not done surprising us." For a mission whose main destination is still beyond Jupiter, that is a promising outlook.