Scientists may have learned the story of the little one contact-binary moon asteroid in orbit 152830 Dinkineshwhich was the first space station for NASA Lucy spacecraft. This moonbird may have separated from its larger parent an asteroid when Dinkinesh was sent spinning through space after absorbing and re-emitting sunlight.
After launching in 2021, the Lucy mission is on track to explore the Trojan asteroids that share the orbit of Jupiter — but to get to them, Lucy has to go through asteroid belt between Mars and Jupiter. In a silver lining, however, it gave Lucy the chance to test her might on a small world in the asteroid belt before reaching the Trojans, who are stationed on Jupiter Lagrange points L4 and L5.
Connected: Mission Lucy: NASA’s Journey to the Trojan Asteroids
On November 1, 2023, Lucy flew within 268 miles (431 kilometers) of Dinkinesh, which is affectionately called “Dinky.” That might not sound like a remarkable distance until you consider that Dinkinesh is only 787 yards (720 meters) in diameter. However, the spacecraft’s autonomous detection and tracking system managed to fix Dinkinesh, allowing the Lucy Long-Range Reconnaissance Imager (L’LORRI) to image the asteroid.
What he found was surprising: Dinkinesh is not alone!
L’LORRI found a natural companion orbiting Dinky every 52.7 hours at a distance of 1.9 miles (3.1 kilometers). An asteroid having a moon isn’t that surprising; astronomers find that about 15% of small asteroids do have moons, such as Dimorphos, which is the small body that orbits the asteroid Didymos and which was subject to NASA ARROW planetary defense mission in 2022. What is particularly interesting about Dinky, however, is that its small moon, the so-called Greetingsitself is a contact binary — two objects glued together as one.
Again, contact binaries themselves are not that rare; the comet 67P/Churyumov–Gerasimenkofor example, visited by the European Space Agency Rosette mission for two years between 2014 and 2016, was a contact binary. ArrokotWho is it The Kuiper Belt object that New horizons flew on New Year 2019 is also a contact binary.
However, Selam is the first contact-binary asteroid-moon situation.
Specifically, Selam looks like two objects or lobes touching each other. They are of similar size: one section is about 230 yards (210 meters) and the other is 250 yards (230 meters). Selam is tidally locked to Dinkinesh, meaning that one lobe constantly remains closest to the larger asteroid. Disappointingly, the point of contact between the two lobes is hidden in shadow in the L’LORRI images.
Naturally, astronomers tried to learn how Selam was formed in this way. According to a team led by mission principal investigator Hal Levison of the Southwest Research Institute in Boulder, Colorado, the clues can be found on Dinkinesh’s surface.
The larger asteroid features a large trough that runs longitudinally around it, as well as an equatorial ridge that superimposes this trough and wraps around its axis of rotation. Levison’s team argues that these features are the result of a massive structural catastrophe that occurred when the asteroid’s spin was spun by a phenomenon called the YORP effect.
Short for Yarkovsky–O’Keefe–Radzievskii–Paddack, after the scientists who first modeled it, the YORP effect describes how a modestly small object like Dinkinesh can be rotated by the effects of absorbing and then re-emitting sunlight. The momentum of solar photons hitting the surface, and then the heat photons emitted from the surface as the surface warms in the sunlight, creates small amounts of thrust that can push around an asteroid less than about 3.1 miles wide (5 kilometers). Although such a pull would be extremely gentle, over centuries it would be able to build up enough power to significantly affect the asteroid’s rotation. Regarding Dinkinesh, the YORP effect has caused the space rock to spin up – today it rotates once every 3.7 hours.
But that wasn’t all.
The resulting centrifugal force on Dinkinesh, scientists now know, then caused some material to lift off the surface of the spinning, loosely held together asteroid. This material then settled into a debris ring around the asteroid’s equator. Structurally destabilized, a crack literally appeared on the surface of Dinkinesh – the great trough we see today.
Some of the material shed by Dinkinesh fell back onto the asteroid, forming the equatorial ridge, while the rest coalesced to form two moons. This, by the way, is the mechanism it is believed to have forms the moon of Didymus Dimorphostoo.
However, Dimorphos is just an ordinary companion – so to create a binary contact like Selam, there must be more to the story.
Levison is intrigued by the fact that the two lobes are almost the same size and wonders if this tells us something about the satellite formation process. Nevertheless, once the two halves of Selam had formed, they must have moved closer and closer together, moving at very low relative speeds, until they were close enough to kissed. From there, gravity was able to hold them together.
Nature seems to have performed a most delicate operation when she created Selam.
If these lobes had touched at a higher relative velocity, they would have either crushed to form just one lobe or, more likely, smashed into each other. Instead, they bonded and are now literally glued to each other.
Analysis of Lucy’s observations of Dinkinesh and Selam are published in Nature.
