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Astronomers were in for a surprise when NASA’s Lucy mission flew past an asteroid named Dinkinesh in November and spotted a contact binary system — two smaller space rocks touching each other — orbiting the asteroid like a moon.
This was the first time a contact binary system was detected in orbit around an asteroid.
Now researchers have had the 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 the two space rocks could change the way astronomers understand how asteroids and even planets like Earth formed in the early days of our solar system.
“We want to understand the strengths of small bodies in our solar system because it’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 millions of smaller objects orbiting the Sun, such as asteroids, collided with 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 that exists between Mars and Jupiter.
In addition to Selam’s discovery, Lucy’s observations showed Dinkinesh’s crest and trough. At some point in Dinkinesh’s history, a quarter of the asteroid suddenly shifted and broke off.
NASA/Goddard/SwRI/Johns Hopkins APL
The Lucy mission captured additional images revealing that asteroid Dinkinesh’s moon is actually two space rocks touching each other.
“The indentation suggests a sudden failure, rather an earthquake with a gradual build-up of stress and then a sudden release, rather than a slow process like the formation of sand dunes,” said study co-author Keith Knoll, a Lucy project scientist at NASA’s Goddard Space Flight Center. in Greenbelt, Md., in a statement.
Dinkinesh’s trough and earthquake-like response are helping scientists better understand the asteroid’s internal structure.
Dinkinesh is not a perfect sphere, so the asteroid receives different amounts of sunlight from different sides.
“The sun’s radiation puts pressure on it, and over time the asteroid starts to spin up, and when it gets fast enough, the material is released,” said study co-author Jessica Sunshine, a professor of astronomy and geology at the University of Maryland, College Park.
NASA/GSFC/SwRI/Johns Hopkins APL/NOIRLab
Yellow and pink dots outline trough and ridge features, respectively.
Dinkinesh’s heating and faster rotation probably took place over millions of years, and centrifugal forces on the space rock caused some of the asteroid to shift into an oblong shape and eject debris. The debris then entered a close 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, grittier material, particles from the asteroid would have shifted equatorward of the space rock and been released into space as it spun faster. But Lucy’s images show Dinkinesh’s rocky body held together much longer and stronger, eventually disintegrating into large pieces.
“These features tell us that Dinkinesh has some power and allow us to 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 poured back to the surface to make the ridge.”
But Selam and the exact process behind how it formed still baffle astronomers. No current theory explains how two pieces of roughly the same size flew out of Dinkinesh and then eventually came together as a contact binary, Sunshine said. But understanding how Selam formed is “part of the fun,” she said.
Sunshine was also part of the research team on NASA’s Double Asteroid Redirect Test. Also known as DART, the September 2022 mission intentionally sent a spacecraft hurtling into a lunar cavity called Dimorphos orbiting a larger near-Earth asteroid called Didymos to alter the motion of a celestial object in space.
“I’m personally very excited to compare the Didymos binary system to (Dinkinesh), especially since they seem to share a lot of similarities in size, general shape and possibly composition, even though they’re in completely different parts of the solar system,” she said. “They have very different characteristics, but we think they may have undergone similar processes to become what we know about them today.”
NASA’s Galileo mission spotted the first asteroid known to have a lunar moon, snapping a picture of asteroid 243 Ida and its moon on August 28, 1993.
Since then, scientists have discovered more asteroids with moons, called binary systems.
“About 15 percent of the near-Earth asteroid population now has binary systems,” Sunshine said.
Lucy’s flyby of Dinkinesh was part of a test of the spacecraft’s equipment before it tackled the mission’s main objective: studying the Trojan asteroid swarms around Jupiter. A flyover over Dinkinesh, which means “wonderful” in Ethiopia’s Amharic language, wasn’t even added to Lucy’s itinerary until January 2023.
Lucy’s next close encounter, due in 2025, will be with another main belt asteroid called Donald Johansson. And then the spaceship will leave to see the Trojans.
The Trojan asteroids, which take their name from Greek mythology, orbit the sun in two clusters – one that is ahead of Jupiter, the largest planet in our solar system, and a second that lags behind it. Too distant to see in detail with telescopes, the asteroids will get their close-up 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 members of the NASA Lucy team hope their mission will achieve a similar a feat in terms of our solar system’s history.
Selam is named after the fossil of a 3.3-million-year-old small female child, believed to be a child counterpart of the Lucy fossil. Selam means “peace” in the Ethiopian Amharic language.
Asteroids themselves are like fossils, representing the leftover material hanging around after the formation of the 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 form from smaller bodies, so studying these small asteroids allows us to 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.”