SAN ANTONIO — May 30, 2024 —When NASA's Lucy spacecraft flew past the tiny main belt asteroid Dinkinesh last November, the Southwest Research Institute-led mission discovered a trough and ridge structure on the main asteroid as well as the first-ever-encountered contact binary satellite. The flyby data of this half-mile-wide object revealed a dramatic history of sudden breakups and transformation.
Scientists think a big chunk of Dinkinesh suddenly shifted, excavating the trough and flinging debris into its vicinity. Some materials fell back to the asteroid body, forming the ridge, while others coalesced to form a contact binary satellite known as Selam. The complex shapes show that Dinkinesh and Selam have significant internal strength and a complex, dynamic history.
"To understand the history of planets like Earth, we need to understand how objects behave when they hit each other, which is affected by the strength of the planetary materials," said SwRI's Hal Levison, principal investigator for the Lucy mission and lead author of May 29 paper in Nature discussing this research. "We think the planets formed as zillions of objects orbiting the Sun, like asteroids, ran into each other. Whether objects break apart when they hit or stick together has a lot to do with their strength and internal structure."
Researchers think that Dinkinesh is revealing its internal structure in how it has responded to stress. Over millions of years, its surface was unevenly heated by the Sun. This slight imbalance caused Dinkinesh to gradually rotate faster. Stress built over time and was suddenly released as a large piece of the asteroid shifted into a more elongated shape.
"The Lucy science team started gathering data about Dinkinesh using telescopes in January 2023, when it was added to our list of targets," said SwRI's Simone Marchi, Lucy deputy principal investigator and the paper's second author. "Thanks to the telescopic data, we thought we had quite a good picture of what Dinkinesh would look like, and we were thrilled to make so many unexpected discoveries."
If the structure of Dinkinesh were weaker, more like the rubble-pile asteroid Bennu, the fragmented materials would have gradually moved toward the equator and flown off into orbit as it spun faster. However, images suggest Dinkinesh has more cohesive strength, because it could hold together longer, more like a rock that suddenly gives way under stress, fragmenting into large pieces.
"This flyby showed us Dinkinesh has some strength and allowed us to do a little 'archeology' to see how this tiny asteroid evolved," Levison said. "When it broke apart, a disk of material formed, some of which rained back onto the surface, creating the ridge."
The rest of the disk materials likely formed the double-lobed moon Selam, a contact binary. How this unusual moon ultimately formed remains a mystery, one that the scientists are already digging into.
"We see ridges around asteroids' equators regularly among near-Earth asteroids, but seeing one up close, around an asteroid with a satellite, helps to unravel some of the possible histories of these binary asteroids," said SwRI's Kevin Walsh, an astrophysicist specializing in planetary formation.
Dinkinesh and its satellite are the first two of 11 asteroids that Lucy plans to explore over its 12-year journey. After skimming the inner edge of the main asteroid belt, Lucy is now heading back toward Earth for a gravity assist in December 2024. That close flyby will slingshot the spacecraft back through the main asteroid belt, where it will observe asteroid Donaldjohanson in 2025 en route to the Trojan asteroids, two swarms of ancient bodies that lead and trail Jupiter in its orbit around the Sun. Starting in 2027, Lucy is scheduled to fly past eight Trojans in both asteroid swarms.
Lucy's principal investigator is from SwRI's Solar System Science and Exploration Division in Boulder, Colorado. SwRI is based in San Antonio. NASA's Goddard Space Flight Center in Greenbelt, Maryland, provides overall mission management, systems engineering, and safety and mission assurance. Lockheed Martin Space in Littleton, Colorado, built and operates the spacecraft. Lucy is the 13th mission in NASA's Discovery Program. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Discovery Program for the Science Mission Directorate at NASA Headquarters in Washington.
For a movie about the Dinkinesh-Selam encounter, visit: https://youtu.be/aE3ixq2yrcw?si=ICxGRZeZFDhYO5Nq
To read the May 29 paper in Nature discussing Dinkinesh research, visit https://www.nature.com/articles/s41586-024-07378-0.
