'Moonlet' asteroid named Selam is practically a baby

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In a solar system billions of years old, a recently discovered 'baby' asteroid and its moonlet stand out as remarkably young. A Cornell University-led research team has developed a new way to estimate the ages of asteroid systems.

Rather than relying solely on crater analysis, they employ precise calculations of orbital dynamics - how the bodies interact in space. This method offers unique insights and could be applied to numerous known binary asteroid systems.

Baby asteroids Dinkinesh and Selam

The baby asteroid Dinkinesh, located in the main asteroid belt between Mars and Jupiter, initially seemed unremarkable. However, in November 2023, NASA's Lucy mission revealed a surprise companion: a smaller moonlet named Selam.

While the existence of binary asteroids, where two asteroids orbit each other, is not uncommon, Selam's peculiar structure intrigued researchers.

Termed a "contact binary," Selam has a unique configuration. Selam is not a typical binary asteroid with clear separation. It consists of two distinct, irregularly shaped objects that are touching.

This structure indicates that both components are mainly loose collections of rocks. They resemble rubble piles, loosely bound by their mutual gravity.

Gravity, tides, and sunlight

Baby asteroids Dinkinesh and Selam exist in a delicate gravitational dance. Their mutual attraction creates tidal forces within both bodies.

These forces cause subtle deformations, stretching and reshaping the asteroids over time. This interaction also impacts their orbital energy, influencing how they move around their common center of mass.

Additionally, the sun's radiation plays a role in the system's evolution through a phenomenon called the BYORP effect (short for Binary Yarkovsky-O'Keefe-Radzievskii-Paddack effect). Sunlight subtly alters the asteroids' rotation rates and the way they reflect heat. This, in turn, modifies their orbital energy and spin states.

Over an extended period, these complex forces - gravity, tides, and the BYORP effect - interact in a way that drives the system toward a state of equilibrium. This equilibrium represents a balance where the strength of the tidal forces matches the influence of the BYORP effect.

Rewinding the cosmic clock on asteroid Selam

The study of Dinkinesh and Selam illustrates a fascinating application of celestial mechanics to trace the history of an asteroid pair. Researchers employed a combination of detailed observations of the asteroids' current orbits and advanced simulations incorporating the BYORP effect.

This effect plays a significant role in shaping the dynamics of binary asteroid systems by altering their rotational and orbital energy over time.

By simulating the orbital dynamics and the influence of the BYORP effect, the team was able to "rewind" the orbital evolution of these celestial bodies. This reverse simulation provided insights into how Dinkinesh and Selam might have evolved from their formation to their current configuration.

The accuracy of this model rests on the precise understanding of the physical and dynamical properties of the asteroids, which are often difficult to measure directly.

The outcome of this reverse modeling was a robust estimation of Selam's age, placing it at around 2 to 3 million years. This is remarkably young, especially when compared to the age of the solar system, which is about 4.5 billion years.

These findings are vital for multiple reasons. They enhance our understanding of individual asteroids' characteristics. They also broaden our knowledge of how asteroids form and evolve.

This information aids scientists in predicting the trajectories and potential impacts of these and other celestial bodies in our solar system.

Estimated age of baby asteroids and implications

"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," says Colby Merrill, a doctoral student at Cornell University. The team estimates that Selam is approximately 2-3 million years old.

This novel method of "dynamical dating" complements traditional crater counting techniques. It offers advantages, such as not requiring close-up spacecraft imagery, and may be more accurate in cases where asteroid surfaces have changed recently.

"Used in tandem with crater counting, this method could help better constrain a system's age," says Alexia Kubas, a doctoral student and co-author of the study.

The potential to apply this technique to other known binary asteroids promises to broaden our understanding of these intriguing objects and the evolution of our solar system.

The study is published in the journal Astronomy and Astrophysics.

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