Nasa’s Lucy spacecraft set for Easter Day flyby of Donaldjohanson asteroid

In key pre-Jupiter rehearsal

Nasa’s Lucy spacecraft is set to perform a high-speed flyby of asteroid (52246) Donaldjohanson on Easter Day, April 20, 2025, marking a critical rehearsal for future encounters with the Trojan asteroids orbiting Jupiter. The close approach, which will occur at 19:51 CEST, places the spacecraft just 960 kilometres from the asteroid at a staggering speed of 13.4 kilometres per second—nearly 50,000 kilometres per hour.

The Donaldjohanson asteroid, named after palaeoanthropologist Donald Johanson—the discoverer of the fossilised hominin skeleton known as ‘Lucy’—is situated in the central asteroid belt between Mars and Jupiter.

The C-type asteroid, measuring nearly four kilometres in diameter, will be observed in unprecedented detail during the encounter. Thanks to an advanced terminal tracking system that keeps the asteroid in precise view, scientists expect to capture surface features as small as 50 metres using Lucy’s L’LORRI camera system.

The manoeuvre is a pivotal step in the Lucy mission’s long journey towards Jupiter’s Trojan asteroids, which it will begin to explore in 2027. These clusters of celestial bodies, which share Jupiter’s orbit 60 degrees ahead and behind the gas giant, represent untouched remnants from the early Solar System.

Complete systems check

The April 2025 encounter with Donaldjohanson serves as a complete systems check for Lucy’s instruments and navigation capabilities before it ventures into deep space for more complex operations.

In the lead-up to the flyby, Lucy’s L’LORRI camera captured a series of long-range images of Donaldjohanson from February to March 2025. Initially, just a faint point of light from over 70 million kilometres away, the asteroid gradually brightened as Lucy closed in. These early images, spanning a 39-day observation period, allowed the mission team to confirm the spacecraft’s trajectory and further characterise the asteroid.

According to Stefano Mottola of the German Aerospace Centre (DLR), a member of the Lucy science team, the spacecraft required no mid-course corrections. “Lucy remained precisely on course,” he said, adding that the successful tracking and brightness data gathered during the approach will play a vital role in the mission’s scientific returns.

The close encounter will occur approximately 225 million kilometres from Earth, with radio signals from Lucy taking around 12.5 minutes to reach mission control. The spacecraft’s terminal tracking system (TTS) will keep the asteroid centred in the camera’s field of view to ensure maximum image quality. The TTS, which performed reliably during Lucy’s previous flyby of asteroid Dinkinesh in November 2023, will again be crucial in aligning the instruments with the fast-moving target.

Scientific suite

Lucy’s scientific suite includes four other instruments alongside L’LORRI, all of which will be activated during the flyby. The data collected will be analysed by teams including those at DLR, which will construct digital terrain models of the asteroid and study its topography, structure, and composition.

The approach observations also revealed that Donaldjohanson exhibits periodic variations in brightness, likely caused by its rotation and elongated shape. These findings, however, remain preliminary and will be validated by close-range imaging during the flyby. Scientists are eager to determine whether the asteroid might be a binary system, similar to what was observed with Lucy’s encounter with Dinkinesh.

During that 2023 flyby, Lucy discovered that Dinkinesh was accompanied by a 220-metre satellite—later named Selam—that turned out to be a contact binary. This unexpected discovery has raised the possibility that binary or even trinary systems may be more prevalent in the asteroid belt than previously believed. A similar revelation about Donald Johanson could significantly enhance current models of asteroid formation and evolution.

Donald Johanson was first identified in 1981 by astronomer Schelte John Bus at the Siding Spring Observatory in Australia. Its namesake, Donald Johanson, made global headlines in 1974 after unearthing a 3.2-million-year-old Australopithecus afarensis skeleton in Ethiopia’s Afar Triangle.

The fossil was named ‘Lucy’ after the Beatles’ song “Lucy in the Sky with Diamonds,” playing at the excavation camp. The spacecraft, launched in 2021, draws inspiration and its name from that significant anthropological discovery.

Orbital trajectory

Asteroid Donaldjohanson orbits the Sun every three years, travelling between 291 and 420 million kilometres from it. Its path is inclined at an angle of 4.3 degrees relative to the plane of Earth’s orbit, making it a typical but scientifically valuable representative of main-belt asteroids.

Lucy’s long-term goal is to unravel the mysteries of the Trojan asteroids around Jupiter, which may hold clues about the formation of the early Solar System. These bodies are suspected to have originated far beyond their current positions, possibly from the Kuiper Belt—a remote, icy region located billions of kilometres from the Sun. If this theory holds, Lucy’s observations could provide rare insights into material formed during the Solar System’s infancy.

Between 2027 and 2028, Lucy will conduct detailed flybys of six Trojan asteroids—some in the ‘Greek camp’ leading Jupiter, and others in the ‘Trojan camp’ trailing behind. In 2033, it will return to observe two more, completing a mission expected to span at least twelve years and include close encounters with eleven asteroids.

To function effectively at these vast distances—up to 800 million kilometres from the Sun—Lucy is equipped with two giant circular solar panels, each 7.3 metres in diameter. These panels generate up to 504 watts of electricity, a feat made possible by modern advancements in spacecraft power systems. By contrast, Earth receives roughly 100 times more solar energy than Lucy at Jupiter’s orbit.

Nasa’s Lucy mission is part of the agency’s Discovery programme, which supports focused and cost-effective planetary science missions. With its next primary objective only two years away, the successful flyby of Donaldjohanson will mark a key validation of Lucy’s design, planning, and execution.

For planetary scientists and space enthusiasts, the Easter Sunday flyby represents more than a fleeting moment of cosmic choreography. It represents a crucial milestone in humanity’s attempt to decode the building blocks of the Solar System—one asteroid at a time.

Image: CGI of Nasa’s Lucy spacecraft approaching the Donald Johanson asteroid, measuring nearly four kilometres across. Lucy’s flyby on April 20, 2025, will occur at a distance of 960 kilometres and a speed of 13.4 kilometres per second, serving as a dress rehearsal for the study of the Trojan asteroids in Jupiter’s orbit, which the probe will reach in 2027/28 and again in 2033. Credit: Nasa/GSFC