NASA’s Lucy spacecraft was all set to fly by what was its first asteroid on Wednesday. But as it turns out, the mission flew past its first two asteroids as the spacecraft found that the main belt asteroid Dinkinesh is actually two asteroids bound together as a binary pair.
NASA launched the Lucy mission on October 16, 2021, and on its six-billion-kilometre journey, the spacecraft’s main aim is to observe the Jupiter Trojan asteroids. The Trojans are a group of small bodies that orbit the Sun in two “swarms” along the orbit of Jupiter. One swarm leads Jupiter in its orbit while another trails behind the gas giant.
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Dinkinesh means “marvellous” in the Amharic language. Hal Levison, principal investigator for the Lucy mission, asserts that the asteroid has lived up to its name. “When Lucy was originally selected for flight, we planned to fly by seven asteroids. With the addition of Dinkinesh, two Trojan moons, and now this satellite, we’ve turned it up to 11,” said Levison in a NASA press statement Thursday announcing the discovery.
But the discovery of the binary pair is not a complete surprise. Lucy’s instruments were observing the asteroid’s brightness changing over time. This led Lucy’s team members to wonder if Dinkinesh might be a binary system in the weeks leading to the encounter. Now, based on preliminary analysis of the first available images, they estimate that the larger body is about 790 metres at its widest while the smaller one is about 220 metres in size.
Even though it led to the interesting discovery, Lucy’s encounter with Dinkinesh was mainly intended as an in-flight test of the spacecraft. In particular, a test of the systems that allow Lucy to autonomously track an asteroid as it flies past at thousands of kilometres per hour.
This image shows the smaller asteroid of the binary system emerging from the shadow of the larger. (NASA/Goddard/SwRI/Johns Hopkins APL/NOAO) 
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