Scientists may have learned the story of the little binary moon contact asteroid in orbit 152830 Dinkineshwhich was NASA’s first cosmic shutdown Lucy spatialship. This moon may have come from its larger parent asteroid when Dinkinesh was sent spinning through space after absorbing and re-emitting sunlight.
Launched in 2021, the Lucy mission is on its way to explore the Trojan asteroids, which share the orbit of Jupiter – but, to reach them, Lucy must cross the Asteroid belt between March and Jupiter. Fortunately, this gave Lucy the opportunity to test her prowess on a minor world in the asteroid belt before reaching the Trojans, which are positioned at Jupiter. Lagrange points L4 and L5.
Related: Mission Lucy: NASA’s journey to the Trojan asteroids
On November 1, 2023, Lucy flew within 270 miles of Dinkinesh, affectionately known as “Dinky.” This may not seem like a notable distance until you consider that Dinkinesh is only 787 yards (720 yards) in diameter. Still, the spacecraft’s autonomous telemetry and tracking system was able to lock onto Dinkinesh, allowing the Lucy Long-Range Reconnaissance Imager (L’LORRI) to image the asteroid.
What he discovered was surprising: Dinkinesh is not alone!
THE LORRI discovered a natural satellite orbiting Dinky every 52.7 hours at a distance of 1.9 miles (3.1 kilometers). That an asteroid has a moon is not that surprising; astronomers find that about 15% of small asteroids actually have companions, such as Dimorphos, which is the small body that orbits the asteroid. Didymos and who was subject to DART planetary defense mission in 2022. What’s particularly interesting about Dinky, however, is that its small moon, called Hiis itself a binary contact: two objects stuck together to form 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 binary contact. Arrokothwho is the Kuiper Belt object that New Horizons hovered on New Year’s Day 2019, is also a contact binary.
However, Selam is the first binary-asteroid-moon contact situation.
Specifically, Selam appears to be two objects, or lobes, touching end to end. They are similar in size: one lobe is about 230 yards (210 yards) in diameter and the other is 250 yards (230 yards) in diameter. Selam is tidally locked to Dinkinesh, meaning one lobe permanently remains closest to the larger asteroid. Unfortunately, the contact point between the two lobes is hidden in shadow in L’LORRI’s images.
Naturally, astronomers sought to know how Selam was formed in 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 lie on Dinkinesh’s surface.
The larger asteroid is distinguished by a large trough that extends longitudinally around it, as well as an equatorial ridge that superimposes above this trough and wraps around its axis of rotation. Levison’s team says these features are the result of a massive structural calamity 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 such as Dinkinesh can be rotated by the effects of light. absorption and then re-emission of sunlight. The momentum of solar photons striking the surface, and then thermal photons emitted from the surface as the surface heats under sunlight, create small amounts of thrust that can bypass an asteroid less than about 5 kilometers wide. Although such a push would be extremely gentle, it would be capable, over eons, of accumulating enough power to greatly affect the rotation of an asteroid. As for Dinkinesh, the YORP effect led to an increase in the rotation of the space rock: today, it rotates once every 3.7 hours.
But that wasn’t all.
Scientists have now learned that the resulting centrifugal force on Dinkinesh then caused some material to lift off from the surface of the spinning asteroid, loosely held together. This material was then deposited in a ring of debris around the asteroid’s equator. Structurally destabilized, a crack literally appeared in the surface of Dinkinesh – the great hollow we see today.
Some of the material from Dinkinesh fell back onto the asteroid, forming the equatorial ridge, while the rest merged to form two satellites. This is, moreover, the mechanism which would have formed Didymos’ moon DimorphosAlso.
Dimorphos, however, is just an ordinary satellite – so to create a binary contact such as 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 anything about the process of satellite formation. Regardless, once the two halves of Selam formed, they must have moved closer and closer together, moving at very low relative speed, until they were close enough to s ‘to kiss. From there, gravity was able to hold them together.
Nature, it seems, had accomplished a most delicate operation in creating Selam.
If these lobes had hit with a higher relative velocity, they would have either crushed into a single lobe or, more likely, broken apart. Instead, they bonded and are now literally glued to each other.
Lucy’s analysis of Dinkinesh and Selam’s observations is published in Nature.