Last November, NASA's Lucy mission conducted a flyby of asteroid Dinkinish, one of the main-belt asteroids it will study on its way to Jupiter. The spacecraft discovered a small moon orbiting the larger asteroid, now called Selam (also known as “Lucy’s Baby”). The moon's name, an Ethiopian name meaning “peace,” pays homage to ancient human remains called “Lucy” (or Dinkinish) that were unearthed in Ethiopia in 1974. Using novel statistical calculations based on how the two bodies orbit each other, a Cornell-led research team estimates that the moon is only 2 to 3 million years old.
The research was led by Colby Merrill, a graduate student in the Department of Mechanical and Aerospace Engineering at Cornell University. He was joined by Alexia Kubas, a researcher from the Department of Astronomy at Cornell; Alex J. Meyer, Ph.D. student at UC Boulder College of Engineering & Applied Science; and Sabina D. Raducan, postdoctoral researcher at the University of Bern. Their article “Age of (152830) Dinkinesh-Selam Constrained by Secular Tidal-BYORP Theory” recently appeared in Astronomy & Astrophysics on April 19th.
Merrill was also part of NASA's Double Asteroid Redirection Test (DART) mission, which collided with the lunar moon Dimorphos on September 26, 2022. As part of the Lucy mission, Merrill was surprised to find that Dinkinesh was also a binary asteroid when the spacecraft flew past it on November 1, 2023. They were also intrigued to learn that the small moon was a “contact binary star” made up of two lobes that were piles of debris stuck together long ago.
Artist's impression of NASA's Lucy mission, which will study asteroids in the main belt and Jupiter's Trojan population. Photo credit: Southwest Research Institute
While astronomers have observed contact binaries before – a good example is the KBO Arrokoth, which the New Horizons spacecraft flew by on January 1, 2019 – this is the first time one has been observed orbiting a larger asteroid. Together with Kubas, the two began modeling the system to determine the age of the moon as part of their studies at Cornell University. Their results were consistent with those of the Lucy mission, which were based on an analysis of surface craters, the more traditional method of estimating the age of asteroids. As Merrill said in a recent Cornell Chronicle publication:
“It's important to determine the age of asteroids to understand them, and this one is remarkably young compared to the age of the solar system, meaning it was formed relatively recently.” To determine the age of this one body , can help us understand the population as a whole.”
Binary asteroids are a fascinating subject for astronomers because of the complex dynamics involved in their formation. On the one hand, it is the gravitational forces acting on them that cause them to bulge and lose energy. At the same time, binary star systems will also experience the so-called binary Yarkovsky-O'Keefe-Radzievskii-Paddack effect (BYORP), in which the action of solar radiation changes the rotation speed of the bodies. Ultimately, these forces will balance out and reach a state of equilibrium for the system.
For their study, Merril and his team assumed that Selam formed from material expelled from Dinkinesh before the BYORP effect slowed its rotation. They also assumed that the system had now reached a state of equilibrium and that the density of both objects was comparable. They then integrated the asteroid data obtained from the Lucy mission to calculate how long it would take Selam to reach its current state. After running about a million calculations using different parameters, they came up with a median age estimate of 3 million years, with 2 million being the most likely result.
Artist's impression of the DART mission hitting the moon Dimorphos. Photo credit: ESA
This new method complements the Lucy mission's previous age estimates and has several advantages. As their work shows, this method can provide age estimates based on asteroid dynamics alone and does not require close-up images taken by spacecraft. It could also be more accurate where asteroid surfaces have recently changed and can be applied to the lunar moons of other known binary star systems, which make up 15% of near-Earth asteroids (NEAs). These include Didymos and Dimorphos, who are even younger.
The researchers hope to apply their new method to this and other binary systems whose dynamics are well characterized even without close flybys. Kubas said:
“In conjunction with crater counting, this method could help to better estimate the age of a system. If we use two methods and they agree with each other, we can be more confident that we are getting a meaningful age that describes the current state of the system.”
Further reading: Cornell Chronicle
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