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When NASA\u2019s Lucy spacecraft<\/a> flew past its first official target Dinkinesh<\/a> in November 2023, researchers discovered that the asteroid\u2014known as \u201cDinky\u201d\u2014was not alone in space. A satellite asteroid, which the team named \u201cSelam,\u201d was orbiting Dinky. As Lucy sent more data back to Earth, the researchers discovered something surprising: Selam was not just one moon, it was a contact binary\u2014or two moons melded together.<\/p>\n The Lucy team, which includes University of Maryland Professor of Astronomy and Geology Jessica Sunshine<\/a>, detailed the unexpected finding in a paper<\/a> published in the journal Nature<\/em> on May 29, 2024. The researchers noted that the unusual arrangement challenges existing theories about how asteroids and other celestial bodies formed over time and provides additional insight into the internal structure, dynamics and evolutionary history of both Dinky and Selam.<\/p>\n \u201cThere\u2019s a lot more complexity in these small bodies than we originally thought,\u201d said Sunshine, a co-author of the paper. \u201cWith the additional observations taken by the spacecraft, we were able to better analyze features such as Dinkinesh\u2019s rotation speed and Selam\u2019s orbit pattern. We also have a better understanding of what materials they\u2019re possibly made of, bringing us a step closer to learning just how terrestrial bodies are created.\u201d<\/p>\n Images taken by the Lucy spacecraft revealed a trough on Dinkinesh where about a quarter of the asteroid broke off from its main body, a ridge that formed after the asteroid\u2019s structural failure and the contact binary now known as Selam (which was named after the child counterpart of the Lucy hominin fossil discovered in 1974<\/a>). The team theorized that Dinky\u2019s fast spinning motion\u2014boosted by the uneven reflection of sunlight <\/a>off the asteroid\u2019s surface\u2014caused it to shed and eject rocky debris into orbit. Some of the debris could have aggregated to form Selam, while another portion of the fragments rained back down on Dinky as boulders and created the ridges photographed by the Lucy spacecraft.<\/p>\n \u201cOne of the things that\u2019s critical to understanding how planets like Earth got here is understanding how objects behave when they hit each other, and to understand that we need to understand their strength,\u201d said lead scientist Hal Levison<\/a> of Southwest Research Institute, Boulder, Colorado, principal investigator for the Lucy mission. \u201cBasically, the planets formed when [smaller objects like asteroids] orbiting the Sun 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.\u201d<\/p>\n The team deduced that Dinky likely had some internal strength, which allowed it to maintain most of its form.<\/p>\n Just how Dinky\u2019s unusual dual moons formed remains a mystery, but Sunshine said that the team\u2019s findings open the door to comparative studies with similar celestial bodies.<\/p>\n