NASA’s Lucy spacecraft is gearing up for a close encounter with a small asteroid this weekend, offering scientists a glimpse into the early days of the solar system—and a test run for a bigger journey to come.

Launched in 2021, Lucy is on a 12-year mission to study a total of 11 asteroids, including a rare group known as the Trojans that orbit near Jupiter. Sunday’s flyby marks the second asteroid encounter for the spacecraft and will serve as a trial ahead of its first Trojan asteroid rendezvous in 2027.

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This weekend, Lucy will pass wihin 596 miles (960 kilometers) of an asteroid named Donaldjohanson, located in the main asteroid belt between Mars and Jupiter. The asteroid is roughly 2.5 miles (4 kilometers) long, though its exact dimensions and shape remain uncertain—a mystery Lucy aims to help solve as it speeds by at over 30,000 mph (48,000 kph).

Donaldjohanson is believed to be a fragment of a much larger object shattered in a cosmic collision around 150 million years ago. Unlike typical round space rocks, scientists suspect this one could have an unusual shape—possibly elongated like a bowling pin or snowman, similar to Arrokoth, the distant Kuiper Belt object NASA explored in 2019. There’s also a chance it could be two separate elongated asteroids.

“We don’t know what to expect. That’s what makes this so cool,” said Hal Levison, the mission’s lead scientist at the Southwest Research Institute. “It’s not going to be a basic potato. We already know that.”

Lucy will power up all three of its science instruments during the flyby to collect images and data. But because the spacecraft must rotate its antenna away from Earth to track the asteroid, no live communication will be possible during the encounter. Scientists at Lockheed Martin’s Mission Control in Colorado expect to begin receiving data roughly 12 minutes after it’s transmitted across the 139 million miles (223 million kilometers) separating Earth from the asteroid.

Among those watching closely will be Donald Johanson, the paleontologist for whom the asteroid is named. He discovered the famous human ancestor “Lucy” in Ethiopia 50 years ago—after whom the spacecraft is named—and plans to be at Mission Control for the flyby.

If all goes as planned, Lucy’s brief encounter with the asteroid will offer valuable practice and insights before it moves on to its ultimate target: the mysterious Trojan asteroids near Jupiter, remnants from the solar system’s formation more than 4 billion years ago.