‘Dinky’ asteroid has a tiny companion that baffles astronomers

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Astronomers were in for a surprise when NASA’s Lucy mission flew by an asteroid named Dinkinesh in November and spotted a contact binary — two smaller space rocks that touch each other — orbiting the asteroid like a moon.

It was the first time a contact binary has been discovered orbiting an asteroid.

Now, researchers have had a chance to study Lucy’s observations, and the findings, published Wednesday in the journal Nature, reveal that “Dinky” and its contact binary, now named Selam, are even more complex than expected.

The complexities of both space rocks could change the way astronomers understand how asteroids, and even planets such as Earth, formed during the early days of our solar system.

“We want to understand the strengths of small bodies in our solar system because that’s critical for understanding how planets like Earth got here,” said lead study author Hal Levison, Lucy principal investigator at the Southwest Research Institute in Boulder, Colorado, in a statement.

“Basically, the planets formed when zillions of smaller objects orbiting the Sun, like asteroids, ran into each other. How objects behave when they hit each other, whether they break apart or stick together, has a lot to do with their strength and internal structure.”

Dinkinesh is in the main asteroid belt, which exists between Mars and Jupiter.

In addition to the discovery of Selam, Lucy’s observations showed a ridge and a trough on Dinkinesh. At some point in Dinkinesh’s history, one-quarter of the asteroid suddenly shifted and broke off.

The Lucy mission captured additional imagery revealing that the asteroid Dinkinesh’s moon is actually two space rocks that are touching one another. - NASA/Goddard/SwRI/Johns Hopkins APL
The Lucy mission captured additional imagery revealing that the asteroid Dinkinesh’s moon is actually two space rocks that are touching one another. - NASA/Goddard/SwRI/Johns Hopkins APL

“The trough suggests an abrupt failure, more an earthquake with a gradual buildup of stress and then a sudden release, instead of a slow process like a sand dune forming,” said study coauthor Keith Noll, Lucy project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement.

The trough, and Dinkinesh’s earthquake-like response, is helping scientists to better understand the internal structure of the asteroid.

Dinkinesh’s rocky history

Dinkinesh is not a perfect sphere, so the asteroid receives an unequal amount of sunlight on different sides.

“The solar radiation puts pressure on it, and over time, the asteroid starts spinning up, and when it gets fast enough, material sheds off,” said study coauthor Jessica Sunshine, professor of astronomy and geology at the University of Maryland, College Park.

Yellow and rose dots pinpoint the trough and ridge features, respectively. - NASA/GSFC/SwRI/Johns Hopkins APL/NOIRLab
Yellow and rose dots pinpoint the trough and ridge features, respectively. - NASA/GSFC/SwRI/Johns Hopkins APL/NOIRLab

The heating and faster spinning of Dinkinesh likely took place over millions of years, and the centrifugal forces on the space rock caused part of the asteroid to shift into an elongated shape and shed debris. Then, the debris entered into a close orbit around Dinkinesh, with some of the material falling back on the asteroid to form a ridge, while the remaining material likely formed Selam.

If Dinkinesh was made of weaker, sandier material, particles of the asteroid would have shifted toward the space rock’s equator and released into space as it rotated more rapidly. But Lucy’s images show that Dinkinesh’s rocky body held together for much longer and more strongly, eventually fragmenting into large pieces.

“These features tell us that Dinkinesh has some strength, and they let us do a little historical reconstruction to see how this asteroid evolved,” Levison said. “It broke, things moved apart and formed a disk of material during that failure, some of which rained back onto the surface to make the ridge.”

But Selam and the exact process behind how it formed still baffle astronomers. No current theories explain how two pieces of virtually equal size flew off Dinkinesh and then eventually came together as a contact binary, Sunshine said. But figuring out how Selam formed is “all part of the fun,” she said.

Sunshine was also part of the research team of NASA’s Double Asteroid Redirection Test. Also known as DART, the September 2022 mission intentionally sent a spacecraft hurtling into a moonlet, named Dimorphos, orbiting a larger near-Earth asteroid named Didymos to change the motion of a celestial object in space.

“I’m personally very excited to compare the Didymos binary system with (Dinkinesh), especially as they appear to share many similarities such as size, general shape and possibly composition despite being in totally different parts of the solar system,” she said. “They have very different features, but we think they may have undergone similar processes to become what we know of them today.”

The NASA Galileo mission spotted the first asteroid known to have a lunar satellite, snapping a photo of the asteroid 243 Ida and its moon on August 28, 1993.

Since then, scientists have discovered more asteroids with moons, referred to as binaries.

“Something like 15% of the near-Earth asteroid population now have binaries,” Sunshine said.

Unlocking the secrets of asteroids

Lucy’s flyby of Dinkinesh was part of a test of the spacecraft’s equipment before tackling the mission’s primary goal: surveying the swarms of Trojan asteroids around Jupiter. The flyby of Dinkinesh, which means “marvelous” in the Amharic language of Ethiopia, wasn’t even added to Lucy’s itinerary until January 2023.

Lucy’s next close encounter, set to occur in 2025, will be with another main belt asteroid called Donaldjohanson. And then, the spacecraft will set off to see the Trojans.

The Trojan asteroids, which borrow their name from Greek mythology, orbit the sun in two swarms — one that’s ahead of Jupiter, the largest planet in our solar system, and a second one that lags behind it. Too distant to be seen in detail with telescopes, the asteroids will get their close-up when Lucy reaches the Trojans in 2027.

The mission borrows its name from the Lucy fossil, the remains of an ancient human ancestor discovered in Ethiopia in 1974. The skeleton has helped researchers piece together aspects of human evolution, and NASA Lucy team members hope their mission will achieve a similar feat regarding the history of our solar system.

Selam was named after the fossil of a 3.3 million-year-old tiny female toddler, considered to be the child counterpart of the Lucy fossil. Selam means “peace” in the Ethiopian Amharic language.

The asteroids are like fossils themselves, representing the leftover material hanging around after the formation of giant planets in our solar system, including Jupiter, Saturn, Uranus and Neptune.

“Our ultimate goal is to understand the formation of celestial bodies,” Sunshine said. “How do planets form? How was Earth formed? We know that big planets are formed by smaller bodies, so studying these little asteroids lets us see how materials behave and interact on a smaller scale. With Dinky and the other asteroids we’re flying by, we’re laying the groundwork for understanding how planets are made.”

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