‘Lucy’ discovers a double moon around asteroid Dinkinesh

On 1 November 2023, NASA’s Lucy space probe flew past the asteroid Dinkinesh at a distance of 431 kilometres. A satellite appeared behind the main belt asteroid, which is only 720 metres across, in the first image data received (large image).

However, the real surprise came with images taken during the departure from Dinkinesh: The satellite is a double body, a so-called contact binary asteroid, whose two 210 and 230-metre sub-bodies are connected to each other, rotate around a common centre of gravity and orbit Dinkinesh (small image). Like Dinkinesh (‘the marvellous one’), the companion was given an Amharic name: Selam (‘peace’, ሰላም).

Astrophysicists had hypothesized the existence of a satellite based on irregularities observed in light curves captured by ground-based telescopes. The reality exceeded expectations, as confirmed by the first images from Lucy. The discovery was significant enough to prompt an immediate designation by the International Astronomical Union (IAU), naming the moon (152830) Dinkinesh I Selam.

The Lucy team observed that the contact binary consists of two parts, with diameters of 210 and 230 meters, respectively. These bodies are tethered together, orbiting Dinkinesh at a distance of 3.1 kilometers with an orbital period of nearly 53 hours. The discovery, supported by data from the German Aerospace Center (DLR), provides a unique insight into the complexities of asteroid formations.

The flyby was captured by Lucy’s L’LORRI (Lucy Long Range Reconnaissance Imager) camera, producing a half-minute video sequence from 48 high-resolution images. This footage, available on NASA’s Quickchannel, showcases the binary nature of Dinkinesh’s satellite and confirms the earlier suspicions of a companion.

“The Lucy science team has been collecting data on Dinkinesh using telescopes since January 2023. Our preliminary telescope data provided a glimpse of Dinkinesh, but the flyby revealed much more than we anticipated. The detection of a contact binary was completely unforeseen,”

stated Simone Marchi, Deputy Project Investigator for the NASA Lucy mission at the Southwest Research Institute in Boulder, Colorado.

Stefano Mottola, from the DLR Institute of Planetary Research, along with his colleague Frank Preusker, was instrumental in analyzing the photogrammetric data, generating digital terrain models of the bodies.

Mottola explained, “The light curves we studied showed irregular patterns, indicating the possible presence of a companion. However, discovering it to be a contact binary was an astonishing surprise.”

NASA’s Lucy mission, part of the Discovery program launched on October 16, 2021, aims to study asteroids in Jupiter’s orbit, known as Trojans. These asteroids will be explored more extensively between 2027 and 2028, with follow-up observations planned for 2033. Lucy is named after the fossilized hominid skeleton discovered in Ethiopia in 1974, reflecting the mission’s goal of uncovering ancient solar system secrets.

The flyby of Dinkinesh, located in the inner main asteroid belt, was not initially part of the 2023 mission plan. In January, Raphael Marschall of the Lucy team identified a unique opportunity for a close flyby, made feasible by minor trajectory adjustments. This encounter provided a critical test of Lucy’s terminal tracking system, simulating the mission’s future encounters with Jupiter’s Trojan asteroids. The spacecraft’s instruments were also subjected to rigorous experimental testing during the flyby.

The Lucy probe approached Dinkinesh at a velocity of 4.5 kilometers per second, navigating approximately 470 million kilometers from Earth. The autonomous maneuvers were pre-programmed, as real-time adjustments were impractical due to a 26-minute data transmission delay.

Dinkinesh, an S-type asteroid characterized by silicate minerals, revealed a complex structure during the flyby. A prominent groove and an equatorial bulge on its surface suggest a dramatic past, likely involving a significant structural disruption that formed the contact binary. The study, published in Nature, highlights the potential influence of the YORP effect—a phenomenon where thermal emissions from sunlight alter an asteroid’s rotation—on the creation of such binary systems.

Dinkinesh’s density of 2.4 grams per cubic centimeter, lower than the average for S-type asteroids, suggests considerable porosity, with voids comprising up to a quarter of its volume. This discovery enriches our understanding of asteroid composition and behavior, setting the stage for future explorations by Lucy.