LEARN – What Are Contact Binary Asteroids?
LEARN – What Are Contact Binary Asteroids? https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2025/07/10150416/unnamed-2-1.png 512 421 Asteroid Day https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2025/07/10150416/unnamed-2-1.png
Learning Objective:
To discover that there is an explanation for some peculiar shapes of asteroids and other smaller bodies.
Overview:
The shape characteristics of many asteroids can provide insight into their origin and offer important clues about how various members of the Solar System formed.
Specifics:
Contact binary asteroids are systems composed of two bodies that, originally orbiting a common centre of mass, collided gently and remained joined. When observed as a single structure, their shapes are distinctive, typically elongated and often resembling peanuts, snowmen, or bones. These features indicate the union of two distinct lobes.
Their formation can result from low-velocity collisions or from the gravitational reaccumulation of debris following an impact. In addition, the Yarkovsky–O’Keefe–Radzievskii–Paddack effect (commonly abbreviated as YORP), a force caused by solar radiation that alters the rotation of small bodies — can lead to fragmentation and the eventual merging of components.
Recent estimates suggest that contact binaries may account for as much as 30% of all small bodies in the Solar System, including comet nuclei and trans-Neptunian objects. This implies that if an as-yet-undiscovered potentially hazardous object exists, there is a significant chance that it could be a contact binary.
Most notable contact binaries:
25143 Itokawa – the first contact binary clearly identified. This was confirmed through detailed images captured by the Japanese Hayabusa probe in 2005 and subsequent analysis of the object’s structural density.
52246 Donaldjohanson – It had previously been observed to show large variations in brightness, which can be explained by its shape as seen in the images obtained by the Lucy probe in 2025.
4179 Toutatis – Images from China’s Chang’e 2 mission clearly show a bilobed shape, suggesting that the object was formed from the merger of two bodies of different sizes.
486958 Arrokoth – Images taken by the New Horizons mission in 2019 demonstrate that contact binaries are not exclusive to asteroids. This object, located in the Kuiper Belt, displays two well-defined lobes.
67P/Churyumov-Gerasimenko – Comet nuclei can also exhibit binary characteristics, as seen in images from ESA’s Rosetta probe in 2016.
Identifying contact binaries without probes – The small bodies mentioned above have been visited by probes in recent decades. However, before visual confirmation of their characteristics by a space probe, contact binary asteroids can often be identified indirectly, primarily through light curve analysis and radar observations.
The light curves, created by measuring an asteroid’s brightness as it rotates — can reveal unusual variations that suggest elongated or bilobed shapes, indicating the object may consist of two joined components.
Animated view of asteroid 216 Kleopatra, whose elongated shape resembles a bone. No spacecraft has flown past this asteroid, but its shape was determined using ground-based observations and equipment. (JPL/NASA Echo)
Additionally, radar imaging can be used to map the three-dimensional shape of nearby asteroids, producing echoes that also point to double or irregular structures. This data, combined with mathematical modelling, allows scientists to hypothesise the presence of contact binaries years before their shapes are confirmed by probes.
Although these techniques are effective, they are not without limitations, which can lead to unexpected findings, as was the case with Selam, recorded by the Lucy probe in 2024.
An image of the Dinkinesh system, taken by the Lucy spacecraft, shows a main body and a contact binary satellite, now named Selam. The satellite consists of two nearly equal-sized lobes, measuring 210 m and 230 m in diameter. It orbits Dinkinesh at a distance of 3.1 km, with an orbital period of approximately 52.7 hours. (NASA/GSFC/SwRI/Johns Hopkins APL/NOIRLab/Sci-News)
Learn more about this subject by visiting these websites:
What are binary asteroids? (Jason Davis – The Planetary Society)
Contact Binary Asteroids are Common, but We’ve Never Seen One Form. So Let’s Make One (Fraser Cain – Universe Today)
Contact Binary Asteroids, TNOs, and Comets – Wm. Robert Johnston
LEARN – Why Aren’t Asteroids Round?
