Dinkinesh's Moonlet is Only 2-3 Million Years Old

Last November, NASA’s Lucy mission conducted a flyby of the asteroid Dinkinish, one of the Main Belt asteroids it will investigate as it makes its way to Jupiter. In the process, the spacecraft spotted a small moonlet orbiting the larger asteroid, now named Selam (aka. “Lucy’s baby”). The moonlet’s name, an Ethiopian name that means “peace,” pays homage to the ancient human remains dubbed “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 moonlet is only 2-3 million years old.

The research was led by Colby Merrill, a graduate student from the Department of Mechanical and Aerospace Engineering at Cornell. He was joined by Alexia Kubas, a researcher from the Department of Astronomy at Cornell; Alex J. Meyer, a Ph.D. student at the UC Boulder College of Engineering & Applied Science; and Sabina D. Raducan, a Postdoctoral Researcher at the University of Bern. Their paper, “Age of (152830) Dinkinesh-Selam Constrained by Secular Tidal-BYORP Theory,” recently appeared on April 19th in Astronomy & Astrophysics.

Merrill was also part of the NASA Double Asteroid Redirection Test (DART) mission, which collided with the moonlet Dimorphos on September 26th, 2022. As part of the Lucy mission, Merrill was surprised to discover that Dinkinesh was also a binary asteroid when the spacecraft flew past it on November 1st, 2023. They were also fascinated to learn that the small moonlet was a “contact binary,” consisting of two lobes that are piles of rubble that became stuck together long ago.

While astronomers have observed contact binaries before – a good example is the KBO Arrokoth that the New Horizons spacecraft flew past on January 1st, 2019 – this is the first time one has been observed orbiting a larger asteroid. Along with Kubas, the two began modeling the system as part of their studies at Cornell to determine the age of the moonlet. Their results agreed with one performed by the Lucy mission based on an analysis of surface craters, the more traditional method for estimating the age of asteroids. As Merrill said in a recent Cornell Chronicle release:

“Finding the ages of asteroids is important to understanding them, and this one is remarkably young when compared to the age of the Solar System, meaning it formed somewhat recently. Obtaining the age of this one body can help us to understand the population as a whole.”

Binary asteroids are a subject of fascination to astronomers because of the complex dynamics that go into creating them. On the one hand, there are the gravitational forces working on them that cause them to bulge and lose energy. At the same time, binary systems will also experience what is known as the Binary Yarkovsky–O’Keefe–Radzievskii–Paddack (BYORP) effect, where exposure to solar radiation alters the rotation rate of the bodies. Eventually, 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 ejected from Dinkinesh before the BYORP effect slowed its rotation down. They also assumed that the system had since reached a state of equilibrium and that the density of both objects was comparable. They then integrated asteroid data obtained by the Lucy mission to calculate how long it would take Selam to reach its current state. After performing about 1 million calculations with varying parameters, they obtained a median age estimate of 3 million years old, with 2 million being the most likely result.

This new method complements the previous age estimates of the Lucy mission and has several advantages. As their paper indicates, this method can yield 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 experienced recent changes and can be applied to the moonlets of other known binary systems, which account for 15% of near-Earth asteroids (NEAs). This includes Didymos and Dimorphos, which are even younger.

The researchers hope to apply their new method to this and other binary systems where the dynamics are well-characterized, even without close flybys. Said Kubas:

“Used in tandem with crater counting, this method could help better constrain a system’s age. If we use two methods and they agree with each other, we can be more confident that we’re getting a meaningful age that describes the current state of the system.”

Further Reading: Cornell Chronicle