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| Panels a, b, and c each show stereographic image pairs of asteroid Dinkinesh taken by NASA’s Lucy spacecraft’s L’LORRI Instrument around closest approach on Nov. 1, 2023. Yellow and rose dots indicate the trough and ridge features, respectively. These images have been sharpened and processed to enhance contrast. Panel d shows a side view of Dinkinesh and its satellite Selam taken a few minutes after closest approach. |
Images from the November 2023 flyby of asteroid Dinkinesh by NASA’s Lucy spacecraft reveal a trough on Dinkinesh where a large piece, approximately a quarter of the asteroid, shifted suddenly. The images also show a ridge and a separate contact binary satellite, now known as Selam. Scientists interpret this complex structure as evidence of Dinkinesh and Selam’s significant internal strength and dynamic history.
“We want to understand the strengths of small bodies in our solar system because that’s critical for understanding how planets like Earth got here,” explained Hal Levison, Lucy principal investigator at the Boulder, Colorado, branch of the Southwest Research Institute in San Antonio, Texas. “Basically, the planets formed when zillions of smaller objects orbiting the Sun, like asteroids, ran into each other. How objects behave when they hit each other, whether they break apart or stick together, has a lot to do with their strength and internal structure.” Levison is the lead author of a paper on these observations published on May 29 in Nature.
Researchers believe that Dinkinesh is revealing its internal structure by how it has responded to stress. Over millions of years, the asteroid’s rotation in sunlight caused small torques due to thermal radiation emitted from its warm surface. These torques gradually increased Dinkinesh’s rotational speed, building up centrifugal stresses until part of the asteroid shifted into a more elongated shape. This event likely led to debris entering a close orbit, which then formed the ridge and the satellite Selam.
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| Stereo movie of asteroid Dinkinesh from NASA’s Lucy spacecraft flyby on Nov. 1, 2023. |
If Dinkinesh were much weaker, more like a fluid pile of sand, its particles would have gradually moved toward the equator and flown off into orbit as it spun faster. However, the images suggest that it held together longer, more like a rock, with more strength than a fluid, eventually fragmenting into large pieces under stress. (Although the strength needed to fragment a small asteroid like Dinkinesh is minuscule compared to most rocks on Earth.)
“The trough suggests an abrupt failure, more like an earthquake with a gradual buildup of stress and then a sudden release, instead of a slow process like a sand dune forming,” said Keith Noll of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, project scientist for Lucy and a co-author of the paper.
“These features tell us that Dinkinesh has some strength, and they let us do a little historical reconstruction to see how this asteroid evolved,” said Levison. “It broke, things moved apart and formed a disk of material during that failure, some of which rained back onto the surface to make the ridge.”
Researchers believe some of the material in the disk formed the moon Selam, which is actually two objects touching each other, a configuration called a contact binary. Details of how this unusual moon formed remain mysterious.
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| Stereo movie of Selam from NASA’s Lucy spacecraft flyby on Nov. 1, 2023. |
Dinkinesh and its satellite are the first two of 11 asteroids that Lucy’s team 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. This close flyby will propel the spacecraft back through the main asteroid belt to observe asteroid Donaldjohanson in 2025, and then on to the first encounters with the Trojan asteroids that lead and trail Jupiter in its orbit of the Sun, beginning in 2027.
Lucy’s principal investigator is based at the Boulder, Colorado, branch of Southwest Research Institute, headquartered 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, which is managed by NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the Science Mission Directorate at NASA Headquarters in Washington.
