Scientists may have learned the history of the tiny contact binary month orbiting asteroid 152830 Dinkineshwhich was the first space station for NASA Lucy space ship. The marigold may have separated 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 that share an orbit Jupiter – but to get to them, Lucy has to swim through asteroid belt between Mars and Jupiter. However, this gave Lucy an opportunity to test her mettle on a smaller world in the asteroid belt before reaching the Trojans, who are located on Jupiter. L4 and L5 Lagrange points.
Related: Mission Lucy: 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 remarkable distance until you consider that Dinkinesh averages just 787 yards (720 meters). The spacecraft’s autonomous ranging and tracking system was still able to zero in on 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 discovered a natural satellite orbiting Dinky every 52.7 hours at a distance of 1.9 miles (3.1 kilometers). That the asteroid has a moon is not that surprising; astronomers find that about 15% of small asteroids actually have companions like Dimorphos, which is a small body that orbits the asteroid. Didymos and which was subject to NASA ARROW planetary defense mission in 2022. But what is particularly interesting about Dinky is that its small moon, the so-called Hellois itself a contact binary — two objects stuck together as one.
Again, contact binaries themselves are not that rare; Comet 67P/Churyumov–Gerasimenkovisited, for example, by the European Space Agency Rosetta the mission for two years between 2014 and 2016 was a contact binary star. Arrokothwhich is Kuiper belt object to it New horizons flew by on New Year’s Day 2019, it is also a contact binary star.
However, Selam is the first contact-binary-asteroid-moon situation.
More specifically, Selam appears to be two objects or lobes that touch end to end. They are similar in size: one lobe is about 230 yards (210 meters) in diameter and the other 250 yards (230 meters). Selam is tidally locked to Dinkinesh, meaning that one lobe permanently remains closest to the larger asteroid. Frustratingly, the contact point between the two lobes is hidden in shadow in the L’LORRI images.
Astronomers naturally tried to find out how Selam came to be like this. The clues can be found on Dinkinesh’s surface, according to a team led by mission principal investigator Hal Levison of the Southwest Research Institute in Boulder, Colorado.
A larger asteroid is characterized by a large trough that runs lengthwise around it and an equatorial ridge that is superimposed over 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 rotation was spun up by a phenomenon called the YORP effect.
Short for Yarkovsky–O’Keefe–Radzievskii–Paddack, according to the scientists who first modeled it, the YORP effect describes how a modestly small object like Dinkinesh can spin up the effects of absorbing and 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, create a small amount of thrust that can push around an asteroid less than about 5 kilometers across. Although such a pull would be extremely subtle, over eons it would be able to generate enough energy to significantly affect the asteroid’s rotation. As for Dinkinesh, the YORP effect has 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 now know that the resulting centrifugal force on Dinkinesh then caused some material to lift off the surface of the spinning, loosely held asteroid. This material then settled into a debris ring around the asteroid’s equator. Structurally destabilized, a crack literally appeared in Dinkinesh’s surface—the great trough we see today.
Some of the material that broke away from Dinkinesh fell back on the asteroid to form the equatorial ridge, while the rest coalesced to form two satellites. By the way, this is a mechanism that is trusted created Didymos’ moon Dimorphosalso.
However, Dimorphos is just one ordinary satellite — so there must be more to the story to create a contact binary like Selam.
Levison is intrigued by the fact that the two lobes are nearly identical in size, and wonders if this tells us something about the process of satellite formation. Regardless, once 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 was able to hold them together.
Nature seems to have performed a most delicate operation in the creation of Selam.
If these lobes had touched at a higher relative velocity, they would have either collided together to form only a single lobe, or, more likely, they would have smashed into each other. Instead, they bonded and are now literally glued to each other.
An analysis of Lucy’s observations by Dinkinesh and Selam is published in Nature.