Scientists may have learned the history of the tiny contact-binary month orbiting asteroid 152830 Dinkineshwhich was the first space station for NASA Lucy spacecraft. That moon may have separated from its larger parent asteroid when Dinkinesh was sent spinning through space after absorbing and re-emitting sunlight.
After launching in 2021, the Lucy mission is on its way to survey the orbit-sharing Trojan asteroids Jupiter – but to get to them, Lucy has to cruise through asteroid belt between Mars and Jupiter. However, on the silver lining, this gave Lucy a chance to test her power on a smaller world in the asteroid belt before reaching the Trojans, who are on Jupiter L4 and L5 Lagrange points.
Related: The Lucy mission: NASA’s journey to the Trojan asteroids
On November 1, 2023, Lucy flew within 268 miles (431 kilometers) of Dinkinesh, affectionately nicknamed ‘Dinky’. That might not sound like a significant distance until you consider that Dinkinesh is only 787 yards (720 meters) in diameter. However, the spacecraft’s autonomous ranging and tracking system managed to latch on to Dinkinesh, allowing the Lucy Long-Range Reconnaissance Imager (L’LORRI) to image the asteroid.
It was revealed surprisingly: Dinkinesh is not alone!
L’LORRI discovered a natural satellite circling Dinky every 52.7 hours at a distance of 1.9 miles (3.1 kilometers). That an asteroid has a moon is not so surprising; astronomers find that about 15% of small asteroids actually have companions, such as Dimorphos, a small body orbiting the asteroid Didymos and which was subject to NASA’s DART planetary defense mission in 2022. What is particularly interesting about Dinky is that its small moon, the so-called Salaamcontact itself is binary — two objects joined as one.
Again, contact binaries themselves are not that rare; a comet 67P/Churyumov–Gerasimenkofor example, visited by the European Space Agency Rosetta the mission for two years between 2014 and 2016 was a contact binary. Arrokothwhich is Kuiper belt object that New horizons flew by on New Year 2019, is also a contact binary.
However, Selam is the first contact-binary situation-asteroid-moon.
More precisely, the salam appears as two objects, or lobes, touching end to end. They are similar in size: one lobe is about 230 yards (210 meters) and the other is 250 yards (230 meters). Selam is tidally bound to Dinkinesh, meaning that one lobe permanently remains closest to the larger asteroid. Frustratingly, the point of contact between the two lobes is hidden in shadow in L’LORRI’s images.
Of course, astronomers tried to find out how Salam came to be like this. The clues, according to a team led by the mission’s principal investigator, Hal Levison of the Southwest Research Institute in Boulder, Colorado, can be found on Dinkinesh’s surface.
A larger asteroid is distinguished by a large trough that runs longitudinally around it, as well as an equatorial ridge that sits on top of that trough and wraps around its axis of rotation. Levison’s team claims that these features are the result of a massive structural accident that occurred when the asteroid’s rotation was accelerated 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 spun by the effects of absorbing and then re-emitting sunlight. The momentum of solar photons hitting the surface, and then thermal photons emitted from the surface as the surface heats up in sunlight, creates small amounts of thrust that can push around an asteroid smaller than about 5 kilometers (5 miles). Although such a thrust would be extremely mild, over eons, it could build up enough force to greatly affect the asteroid’s rotation. As for Dinkinesh, the YORP effect has caused the space rock to spin up — today it rotates once every 3.7 hours.
But that was not all.
The resulting centrifugal force on Dinkinesh, scientists have now learned, caused loosely held together material to lift from the surface of the rotating asteroid. This material then settled into a debris ring around the equator of the asteroid. Structurally destabilized, the crack literally appeared on the Dinkinesh surface—the great trough we see today.
Some of the material that broke away from Dinkinesh fell back onto the asteroid, forming the equatorial ridge, while the rest coalesced into two satellites. By the way, this is a trusted mechanism formed Didymus’s moon Dimorphosalso.
Dimorphos, however, is just one ordinary satellite – so to create a contact binary like Selam, there must be more to the story.
Levison is intrigued by the fact that the two lobes are almost identical in size and wonders if this tells us something about the process of satellite formation. Regardless, when the two halves of Selam formed, they had to get closer and closer, moving at a very low relative speed, until they were close enough to kiss. From there, gravity could hold them together.
Nature seems to have done the finest work creating Selam.
If these lobes had touched at a higher relative speed, they would either have crushed to form a single lobe, or, more likely, they would have smashed into each other. Instead, they grew closer and are now literally stuck with each other.
An analysis of Lucy’s observations of Dinkinesh and Selam was published in the journal Nature.