NASA's Lucy Mission Reveals Asteroid's Strange Moon

When NASA’s Lucy mission passed within 430 kilometers (270 miles) of asteroid 152830 Dinkinesh last November, Hal Levison (Southwest Research Institute) expected it to be “just a rock in space.” It turned out to be more than that.

The Lucy team had diverted the spacecraft not for science but because its orbit in the inner main belt made it a good place to test the spacecraft’s tracking and range-finding system on its way to scientific targets among Jupiter’s Trojan asteroids. But Levison, Lucy's principal investigator, says that once the team saw a contact binary moon orbiting 720-meter Dinkinesh, what had started out as routine “was anything but boring.”

Dinkinesh and Selam

The asteroid and its two-lobed moon represent something of an oddity: Some 15% of small asteroid are known to be contact binaries but none previously had been seen orbiting around another object. Moreover, unlike other contact binaries, Dinkinesh’s moon (dubbed Selam) consists of twin nodes almost equal in size — one about 210 meters across, the other about 230 meters. And Dinkinesh itself is the smallest asteroid in the main belt that has ever been studied close up. The more the researchers looked at the images collected during the brief flyby, the more they became intrigued.

NASA launched Lucy on October 16, 2021, on a 12-year exploration of five asteroids that orbit beyond the main belt, in the L₄ and L₅ Lagrange points of Jupiter’s orbit. The spacecraft sped within 300 km of Earth a year later for a gravity boost. Then, in January 2023, the Lucy team added the Dinkinesh flyby for November 1, 2023, to test Lucy's autonomous system developed to track and image the target asteroids.

The team dubbed the previously unnamed asteroid Dinkinesh, the Amharic name for the famed hominid fossil that gave the mission its name. (It means “you are marvelous” in Amharic, one of the languages spoken in Ethiopia where the fossil was found.) The binary moon’s name, Selam, means “peace” in Amharic.

Lucy's Shot

Lucy’s original path would have taken it 64,000 km from Dinkinesh, but its redirection allowed it to pass within 431 km. The binary satellite wasn’t visible until the final approach on November 1st. Only for the few minutes when Lucy was closest to the satellite — and speeding past it at 4.5 kilometers per second — did the images show any detail.

The photos published in Nature show Dinkinesh and Selam separated by about 3.1 km, with some frames showing both in full. While some photos seem to show one lobe of Selam right next to Dinkinesh, this is a projection effect. One wide photo even shows the system in full, with the two lobes of Selam aligned end-to-end with Dinkinesh.

Essentially all of the light in the photos comes from the Sun, Levison says, so the “neck” where the two lobes meet is hidden in shadow. The difference in the angles of shadow on Dinkinesh come about from changes in Lucy’s viewing angle as it sped past. The apparent linear alignment of Selam’s two lobes and Dinkinesh indicates the bodies are tidally locked, with Selam rotating synchronously in its 52.7-hour orbit around Dinkinesh.

Origin Story

The unique system calls for what Levison calls a “complex” history, something we now know is common for asteroids smaller than about 5 km. At that size their gravity is so low that they are vulnerable to the YORP (Yarkovsky–O'Keefe–Radzievskii–Paddack) effect: The absorption of incident sunlight by a body’s surface and the ensuing emission of heat result in a force that spins up that body. If an asteroid loses surface material due to this effect, that debris can over time recombine to form a moon.

Dozens of near-Earth asteroids are known to have moons, yet we know of no other small asteroids with contact binary companions. “None of the models have predicted anything like that," Levison says.

Dinkinesh has the diamond shape of 101955 Bennu, 162173 Ryugu, and other near-Earth asteroids like them. But unlike such asteroids, it doesn’t appear to be a soft “rubble pile,” the term for the structure of boulders and grains loosely bound by gravity.

“The thing is fractured,” Levison says, “and seems to have a piece broken away. That is something no one would have expected.” That finding suggests Dinkinesh has internal strength that isn’t present in some rubble piles. Whatever broke off could have become the two cores of Selam.

“The discovery of the contact-binary satellite (Selam) and all the details provided by this ‘encounter of opportunity’ are exciting and very impressive,” says David Vokrouhlický (Charles University, Czech Republic).

The unexpected complexity of the Dinkinesh system suggests that small asteroids in the main belt may be more complex than expected. Understanding the formation of this system might also help explain the origin of other bi-lobed asteroids found in isolation. Levinson and colleagues write in Nature that they may have been pieces of a system such as Dinkinesh that fell apart.