The Lucy mission is on a 12-year journey to explore a peculiar set of asteroids that lead and follow Jupiter as it orbits the Sun. As it journeys to these Trojan asteroids, the probe is preparing for its second stop at the main belt, approaching a mysterious space rock to practice for the main event.
NASA’s Lucy spacecraft will carry out a flyby of the small main belt asteroid Donaldjohanson on April 20 at 1:51 p.m. ET, when it will come within an eerily close distance of 596 miles (960 kilometers) from the target. This is Lucy’s second encounter with an asteroid, but it is far more complicated than its first flyby, with NASA describing it as a “comprehensive dress rehearsal for Lucy’s main mission,” the space agency wrote in a news release.
Lucy first laid its robotic eyes on Donaldjohanson in February and has been tracking the asteroid over the past two months to prepare for its flyby. Donaldjohanson, named after the discoverer of the Lucy hominid fossil, is a carbonaceous asteroid that’s approximately 2.5 miles (4 km) wide. The asteroid is a fragment from a collision that took place around 150 million years ago, making it one of the youngest main belt asteroids ever visited by a spacecraft.
Although it’s not a Trojan asteroid, it is located in a convenient spot for Lucy to swing by it on its way to the Jovian system. The upcoming flyby will provide the team with an opportunity to test Lucy’s systems and carry out observations using all three of the spacecraft’s science instruments.
The spacecraft will orient itself to track the asteroid around 30 minutes before its close approach. During that time, Lucy’s high-gain antenna, which it uses to communicate with ground control, will be turned away from Earth, suspending communication. For the remainder of the flyby, Lucy will be on its own, autonomously rotating using its terminal tracking system to keep Donaldjohanson in view. Lucy will then use its science instruments to conduct observation sequences of the asteroid that are very similar to the one it will perform once it reaches the Trojans.
Around 40 seconds before its closest approach to the asteroid, however, Lucy will stop tracking Donaldjohanson to protect its sensitive instruments from intense sunlight. “If you were sitting on the asteroid watching the Lucy spacecraft approaching, you would have to shield your eyes staring at the Sun while waiting for Lucy to emerge from the glare. After Lucy passes the asteroid, the positions will be reversed, so we have to shield the instruments in the same way,” Michael Vincent of Southwest Research Institute (SwRI) in Boulder, Colorado, and encounter phase lead of the mission, said in a statement. “These instruments are designed to photograph objects illuminated by sunlight 25 times dimmer than at Earth, so looking toward the Sun could damage our cameras.”
After its closest approach, the spacecraft will reorient its solar arrays back toward the Sun. It will reestablish communication with Earth about an hour after its flyby, hopefully beaming back all the precious data it collected. The team, however, does have to be patient considering how far Lucy is from Earth, so the process will take several days.
“One of the weird things to wrap your brain around with these deep space missions is how slow the speed of light is,” Vincent said. “Lucy is 12.5 light minutes away from Earth, meaning it takes that long for any signal we send to reach the spacecraft. Then it takes another 12.5 minutes before we get Lucy’s response telling us we were heard. So, when we command the data playback after closest approach, it takes 25 minutes from when we ask to see the pictures before we get any of them to the ground.”
Lucy launched in October 2021, and pulled off a flyby of the tiny main belt asteroid Dinkinesh in November 2023. The mission’s first asteroid encounter was far more rewarding than initially anticipated as the brief rendezvous revealed a binary pair with the discovery of a tiny satellite closely orbiting Dinkinesh.
The mission’s primary focus is studying the Trojan asteroids, a group of rocky bodies that lead and follow Jupiter as it orbits the Sun. Lucy will begin its tour of the Trojan asteroids in 2027 by visiting Eurybates and its binary partner Queta, followed by Polymele and its binary partner, Leucus, Orus, and the binary pair Patroclus and Menoetius.
“Every asteroid has a different story to tell, and these stories weave together to paint the history of our solar system,” Tom Statler, Lucy mission program scientist at NASA, said in a statement. “The fact that each new asteroid we visit knocks our socks off means we’re only beginning to understand the depth and richness of that history. Telescopic observations are hinting that Donaldjohanson is going to have an interesting story, and I’m fully expecting to be surprised—again.”
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