You Are Beautiful: Lucy at Dinkinesh

Since its launch in 2021, exactly two years ago to the day, the Lucy mission has had an eventful, and sometimes stressful, journey through space. In particular, mission teams had to deal with an unlatched solar panel, an issue which could seriously affect the ability of the mission to continue nominally, which it thankfully could. Last October, Lucy performed its first Earth Gravity Assist, one of two which will give Lucy its boost towards Jupiter’s trojan camps. During this assist, Lucy came closer to Earth than the International Space Station, and as it left Earth once more the mission began another coast phase: a period of calm where Lucy will uneventfully drift through the asteroid belt before looping back for its second assist off Earth in 2024. Or so we thought. In January 2023, it was announced that Lucy had a new target, 1999 VD57, now known as Dinkinesh, which would be encountered just 10 months later, this November. We now find ourselves rapidly approaching Lucy’s first encounter with an unexplored target; previously this wouldn’t have happened until 2025.

Dinkinesh was not the first new world to be added to Lucy’s mission manifest, only the latest. In fact Dinkinesh is not even the first new target identified since its launch. When Lucy was approved, there were seven targets. These were the main belt asteroid Donaldjohanson (named for the paleoanthropologist who discovered the fossil “Lucy”), the large trojan asteroids Eurybates, Polymele, Leucus, and Orus, which occupy Jupiter’s L4 trojan camp, and the binary asteroids Patroclus and Menoetius, which orbit each other in Jupiter’s L5 camp. The eighth target of the mission is Queta, a moon discovered around Eurybates by Hubble the year after Lucy was approved, and named for Olympic athlete Enriqueta Basilio. Being a moon of Eurybates, which Lucy was already targeting, this little asteroid is a sort of freebee, requiring no major adjustments to the mission’s trajectory. Similarly, a satellite was discovered around Polymele in March of 2022, the year after Lucy launched, becoming the ninth target added to the mission. Even asteroid Orus has a candidate satellite, tentatively seen in Hubble Observations back in 2018; though it remains unconfirmed, it would become the eleventh target. Dinkinesh, however, is different: it is not a member of a previously targeted system, and is not a new discovery.

Dinkinesh was first discovered in 1999, by the Lincoln Near-Earth Asteroid Research (LINEAR), and was not named. Then known only as 1999 VD57, the asteroid was just one of thousands discovered by LINEAR, and was not particularly interesting. Being a small, stony asteroid orbiting in the main belt, 1999 VD57 was not even a member of the asteroid populations LINEAR aimed to catalog, that being Near-Earth Asteroids. 1999 VD57 would remain this way, just a number in the minor planet catalogs, little more than a size estimate and an orbit around the Sun. There was absolutely no reason to look closer, so there it sat. The outlook for the small asteroid changed suddenly in January of 2023, simply by coincidence.

Lucy has an experimental solution to imaging its asteroid targets. Rather than a preprogrammed sequence aiming its cameras over a wide area to ensure images of a target during approach, Lucy’s Terminal Imaging System allows the spacecraft to automatically track and image asteroids during flybys, maximizing the amount of data that can be collected without requiring intensive calculations on where the cameras need to be pointed. Being such an important part of the mission’s hardware, a chance to test this unique equipment was sought after following Lucy’s first Earth assist. Raphael Marschall, Lucy collaborator of the Nice Observatory in France, was tasked with identifying any asteroids that Lucy may incidentally pass by. Even a relatively distant encounter would allow for system tests, even if the asteroid was little more than distant speck. Marschall took half a million asteroids with well defined orbits, a batch that just so happened to include 1999 VD57, to see if any got close enough. “This asteroid really stood out,” said Marschall in regards to VD57. “Lucy’s trajectory as originally designed will take it within 40,000 miles of the asteroid, at least three times closer than the next closest asteroid.” However, this could be taken a step further. A small series of maneuvers which began in May of 2023 have adjusted the path of the Lucy Spacecraft, and now an encounter where Lucy would pass 40,000 miles away from the asteroid has become a mere 280 miles. The encounter will occur on November 1st, 2023, and assuming all goes well should yield some very impressive imagery. One thing quickly changed about VD57 in response to this announcement: 24 years after its discovery, 1999 VD57 was given a proper name. The Lucy team nicknamed the asteroid “Dinkinesh,” a name which was then approved officially by the IAU in early February. Dinkinesh comes from the Ethiopian name of the fossil which the Lucy mission is named for.

Dinkinesh means “you are beautiful/marvelous.”

As for Dinkinesh itself, aside from being the first new asteroid system added to Lucy’s minor planet tour, very little is known about the asteroid. It’s incredibly small; size estimates have narrowed to around ~700 meters, making it not only the smallest target of the mission, but also the smallest main belt asteroid ever explored. This small size is closer to that of explored Near-Earth Asteroids, than previously explored members of the main belt. There is hope that Dinkinesh may act as a geologic link between NEAs and the main belt, potentially yielding insights on the origins of NEAs like Bennu and Ryugu. Dinkinesh has now been identified as an “S-type” asteroid, a stony asteroid with light traces of metal. An S-type asteroid, while not considered particularly interesting compared to other spectral types, is new to the mission. Other asteroids targeted by Lucy are dark, carbonaceous C-type asteroids, or the reddish D and P-type asteroids, asteroids with more organic compounds and potentially trace amounts of water ice. Dinkinesh could provide a brilliant baseline for comparison to other asteroids Lucy encounters on its mission, representing an average stony main belt asteroid. The finer details, whether Dinkinesh is a solid unit or rubble pile, its shape, and its crater counts will remain largely unanswered until its close-up in November. When we get our first images of this diminutive world in the coming months, keep in mind that Dinkinesh is one in half a million.