Asteroids sometimes exhibit unexpected behaviors. Recent images show that some of them gently exchange fragments in a slow cosmic waltz.
In 2022, the DART spacecraft deliberately impacted the moon-asteroid Dimorphos, which is part of the binary system with Didymos. Just before impact, the cameras captured detailed images of the surface. These photos, initially examined to study the deflection, revealed strange fan-shaped patterns that puzzled scientists.
Left: Dimorphos seen 8.55 seconds before the DART impact. Right: The same image after correcting for lighting conditions, revealing a fan-like pattern of streaks highlighted in color.
Credit: NASA/JHU-APL/UMD
The research team, led by Jessica Sunshine from the University of Maryland, initially suspected technical anomalies. By applying advanced image processing methods to eliminate shadows cast by boulders and adjust the lighting, the streaks became more visible. This clarification confirmed they were real structures, not artifacts. Scientists were then able to analyze their formation, discovering that the debris came from the companion asteroid Didymos.
These streaks result from very slow impacts, at speeds of about 1 foot per second (30 centimeters per second), where debris from Didymos gently settles onto Dimorphos. Instead of creating craters, they form fan-shaped deposits, mainly concentrated near the equator. This distribution matches modeled predictions about the trajectory of ejected material, showing a transfer of matter between the two asteroids over long periods, perhaps millions of years.
This process is linked to the YORP effect, a phenomenon where an asteroid's absorption and re-emission of sunlight generates a thermal thrust. This force can gradually accelerate its rotation, eventually releasing surface materials. Similar observations by NASA's Lucy mission on the asteroids Dinkinesh and Selam show equatorial ridges, indicating that this material migration is common among small celestial bodies in the Solar System.
Beyond debris exchange, DART also altered Dimorphos's orbit around Didymos, and even slightly that of the binary system around the Sun. These changes, although minimal, demonstrate the effectiveness of deflection techniques for planetary defense. They help predict how asteroids could be deflected in case of a future threat to Earth, as noted by researchers in
Science Advances.
Artist's illustration of NASA's DART probe in the Didymos binary system.
Credit: NASA/Johns Hopkins APL/Steve Gribben
To deepen these discoveries, the European Space Agency's Hera mission is expected to arrive in the Dimorphos-Didymos system next December. It will closely examine post-impact transformations, check for the persistence of the streaks, and search for new patterns. These data will be valuable for improving models on asteroid evolution and strengthening our planet's protection strategies, building on the lessons from DART.
The YORP Effect
The YORP effect, an acronym for Yarkovsky-O'Keefe-Radzievskii-Paddack, is a physical phenomenon that influences the rotation of small asteroids. It occurs when these bodies absorb sunlight and re-emit it as thermal radiation. This re-emission is not uniform, creating a tiny but constant force that can alter the asteroid's rotation speed over long timescales, often thousands or millions of years.
Over time, this rotational acceleration can become significant. For small asteroids, it can make them spin faster until the centrifugal force exceeds their surface gravity. When this happens, materials like rocks or dust can be ejected into the surrounding space, contributing to a slow reshaping of their form and surface.
These ejections explain why some asteroids develop features like equatorial ridges or elongated shapes. In the case of binary systems, debris released by one asteroid can migrate to its companion, as observed with Dimorphos and Didymos. This process shows how asteroids dynamically interact with their environment.
Understanding the YORP effect is important for space missions and planetary defense. It helps scientists predict how asteroids evolve and allows them to determine potential risks to Earth, integrating these mechanisms into trajectory and deflection models.