Scientists may have learned the history of the small contact binary moon orbiting asteroid 152830 Dinkineshwhich was NASA’s first cosmic stop Lucy spacecraft. That moon may have split off from its larger parent asteroid when Dinkinesh was sent through space after absorbing and re-emitting sunlight.
The Lucy mission, launched in 2021, is on its way to explore the Trojan asteroids, which share the orbit of Jupiter – but to reach them Lucy has to go through the asteroid belt between Mars and Jupiter. On the plus side, this has given Lucy the chance to test her skills on a small world in the asteroid belt before she reaches the Trojans, which are located near Jupiter. L4 and L5 Lagrange points.
Related: Lucy Mission: NASA’s Journey to the Trojan Asteroids
On November 1, 2023, Lucy flew within 431 kilometers of Dinkinesh, affectionately known as ‘Dinky’. That may not sound like a remarkable distance until you consider that Dinkinesh is only 720 meters in diameter. Still, the spacecraft’s autonomous ranging and tracking system was able to track Dinkinesh, allowing the Lucy Long-Range Reconnaissance Imager (L’LORRI) to image the asteroid.
What it discovered was surprising: Dinkinesh is not alone!
L’LORRI a natural satellite discovered every 52.7 hours in an orbit around Dinky at a distance of 3.1 kilometers. That an asteroid has a moon is not surprising; astronomers find that about 15% of small asteroids do indeed have companions, such as Dimorphos, the small body orbiting the asteroid Didymos and which was subject to NASA’s DART planetary defense mission in 2022. However, what is particularly interesting about Dinky is that its small moon, called Selamis itself a contact binary: two objects stuck together as one.
Again, contact binaries themselves are not that rare; the comet 67P/Churyumov-Gerasimenkofor example visited by the European Space Agency Rosetta mission for two years between 2014 and 2016, was a contact binary. Arrokothwhich one is the Kuiper belt object to that New horizons flew by on New Year’s Day 2019 is also a contact binary.
However, Selam is the first contact binary-asteroid-moon situation.
More specifically, Selam appears to consist of two objects, or lobes, touching each other from end to end. They have the same dimensions: one lobe is about 210 meters wide and the other is 230 meters wide. Selam is tidally linked to Dinkinesh, meaning one lobe remains permanently closest to the larger asteroid. Frustratingly, the point of contact between the two lobes in L’LORRI’s images is hidden in shadow.
Naturally, astronomers tried to figure out how Selam formed this way. According to a team led by the mission’s principal investigator, Hal Levison of the Southwest Research Institute in Boulder, Colorado, the clues can be found on Dinkinesh’s surface.
The larger asteroid is distinguished by a large trough that runs lengthwise around it, and by an equatorial ridge that lies 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 disaster that occurred when the asteroid’s rotation was accelerated by a phenomenon called the YORP effect.
The YORP effect, short for Yarkovsky-O’Keefe-Radzievskii-Paddack after the scientists who first modeled it, describes how a modestly small object like Dinkinesh can be spun by the effects of absorbing and then re-emitting of sunlight. The momentum of solar photons hitting the surface, and then the thermal photons emitted by the surface as the surface heats up in sunlight, create small amounts of momentum that can push around an asteroid smaller than about 3 miles (5 kilometers) wide. Although such a thrust would be extremely gentle, it could build up enough force over centuries to greatly affect an asteroid’s rotation. In Dinkinesh’s terms, the YORP effect led to an increase in the rotation of the space rock; nowadays it runs once every 3.7 hours.
But that wasn’t all.
Scientists have now discovered that the resulting centrifugal force on Dinkinesh then caused material to rise from the surface of the spinning, loosely held asteroid. That material then settled into a ring of debris 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 spun by Dinkinesh fell back onto the asteroid to form the equatorial ridge, while the rest coalesced to form two satellites. By the way, this is the mechanism that is supposed to exist Didymos’ moon formed Dimorphosat.
However, Dimorphos is just one regular satellite – so to create a contact binary like Selam there has to be more going on.
Related stories:
— NASA’s Lucy spacecraft successfully completes first flyby of asteroid ‘Dinky’
– Surprise! Asteroid ‘Dinky’ is actually a double space rock, NASA’s Lucy probe reveals (pictured)
– Curious double moon discovered orbiting asteroid ‘Dinky’ now has a name
Levison is intrigued by the fact that the two lobes are nearly the same size, and wonders whether this tells us something about the process of satellite formation. In any case, once the two halves of Shelam formed, they must have gotten closer and closer together, at a very slow relative speed, until they were close enough to kiss. From there, gravity was able to keep them together.
It seems that nature performed an extremely delicate operation in creating Selam.
If these lobes had hit each other at a higher relative speed, they would have either pressed together into just a single lobe or, more likely, knocked each other apart. Instead, they bonded and are now literally stuck together.
The analysis of Lucy’s observations of Dinkinesh and Selam has been published in Nature.