Astronomers were in for a surprise when NASA's Lucy mission flew past an asteroid called Dinkinesh in November and spotted a contact binary star - two smaller space rocks touching each other - orbiting the asteroid like a moon.
It was the first time a contact binary star was 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 called Selam, are even more complex than expected.
The complexity of both space rocks could change the way astronomers understand how asteroids, and even planets like 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 to understanding how planets like Earth got here," said lead study author Hal Levison, Lucy's principal investigator at the Southwest Research Institute in Boulder, Colorado, in a statement. .
"Basically, the planets formed when countless smaller objects orbiting the sun, such as asteroids, collided. How objects behave when they touch each other, whether they fall apart or stick together, has a lot to do with their strength and internal structure."
Dinkinesh is located in the main asteroid belt, which is between Mars and Jupiter.
In addition to the discovery of Selam, Lucy's observations showed a ridge and a trough on Dinkinesh. At one point in Dinkinesh's history, a quarter of the asteroid suddenly shifted and broke off.
"The trough suggests an abrupt failure, more like an earthquake with a gradual build-up of stress and then a sudden release, rather than a slow process like the formation of a sand dune," said co-author Keith Noll, Lucy project scientist at the Goddard NASA's Space Flight Center. in Greenbelt, Maryland, in a statement.
The trough and Dinkinesh's earthquake-like response are helping scientists better understand the asteroid's internal structure.
The rocky history of Dinkinesh
Dinkinesh is not a perfect sphere, so the asteroid receives an uneven amount of sunlight on different sides.
"The solar radiation puts pressure on it, and over time the asteroid starts to spin, and as it gets fast enough, it sheds material," said co-author Jessica Sunshine, a professor of astronomy and geology at the University of Maryland, College Park. .
Dinkinesh's heating and faster spinning likely took millions of years, and centrifugal forces on the space rock caused part of the asteroid to shift into an elongated shape and release debris. The debris then entered a narrow orbit around Dinkinesh, with some of the material falling back onto the asteroid to form a ridge, while the remaining material likely formed Selam.
If Dinkinesh had been made of a weaker, sandier material, the asteroid's particles would have shifted toward the space rock's equator and ended up in space as it spun faster. But Lucy's footage shows that Dinkinesh's rocky body held together much longer and stronger, eventually breaking apart into large pieces.
"These features tell us that Dinkinesh has some power, and they let us do a little historical reconstruction to see how this asteroid evolved," Levison said. "It broke, things separated, and during that failure formed a disk of material, some of which rained back onto the surface to form the edge."
But Selam and the exact process behind its formation still baffles astronomers. There are currently no theories explaining how two nearly equal-sized pieces flew away from Dinkinesh and eventually came together as a contact binary, Sunshine said. But figuring out how Selam came to be is all part of the fun, she said.
Sunshine was also part of the research team for NASA's Double Asteroid Redirection Test. The September 2022 mission, also known as DART, deliberately sent a spacecraft hurtling into a small moon called Dimorphos into orbit around a larger near-Earth asteroid called Didymos to track the motion of a celestial body in space to change.
"Personally, I'm very excited to compare the Didymos binary system to (Dinkinesh), especially since they seem to have a lot of similarities, such as size, overall shape and possibly composition, despite being in completely different parts of the solar system, " she said. "They have very different characteristics, but we think they went through similar processes to become what we know them to be today."
The NASA Galileo mission discovered the first asteroid known to have a lunar satellite, photographing the asteroid 243 Ida and its moon on August 28, 1993.
Since then, scientists have discovered more asteroids with moons, also known as double stars.
"About 15% of the near-Earth asteroid population now has binary stars," 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 main goal: mapping the swarms of Trojan asteroids around Jupiter. The flight past Dinkinesh, which means "wonderful" in Ethiopia's Amharic language, was only added to Lucy's itinerary in January 2023.
Lucy's next encounter, expected in 2025, will be with another main belt asteroid named Donaldjohanson. And then the spacecraft will set off to see the Trojans.
The Trojan asteroids, which take their name from Greek mythology, orbit the sun in two swarms: one that lies in front of Jupiter, the largest planet in our solar system, and a second that remains behind it. The asteroids are too far away to see in detail with telescopes. They'll get a closer look when Lucy reaches the Trojans in 2027.
The mission takes 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 is named after the fossil of a 3.3 million year old small female toddler, which is considered the infant counterpart of the Lucy fossil. Selam means 'peace' in the Ethiopian-Amharic language.
The asteroids are themselves fossils and represent 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 the Earth formed? We know that large planets are formed by smaller bodies, so by studying these small asteroids we can see how materials behave and interact on smaller scales. With Dinky and the other asteroids we fly past, we are laying the foundation for a better understanding of how planets are made."
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