Lucy is a NASA mission designed to fly past a series of Trojan asteroids that share Jupiter’s orbit, visiting a more diverse set of small bodies than any previous spacecraft. Launched in October 2021, it is on a twelve-year journey that will take it to eight Trojan targets plus two bonus asteroids encountered along the way, sampling objects that range from roughly one kilometer to a hundred kilometers across and span several compositional classes. The Trojans have never been visited before, and because they are thought to be preserved remnants from the earliest stages of the solar system, Lucy’s findings bear directly on how the giant planets formed and migrated to their current positions.
What the Trojan Asteroids Are and Why They Matter
Jupiter’s Trojan asteroids are two enormous swarms of small bodies trapped near Jupiter’s L4 and L5 Lagrange points, gravitational sweet spots that sit roughly 60 degrees ahead of and behind Jupiter in its orbit around the Sun. Thousands of Trojans have been cataloged, and the total population likely numbers in the hundreds of thousands for objects larger than a kilometer. They are not a uniform group. Ground-based telescopes have shown that Trojans come in at least two broad spectral types, loosely called P-type and D-type, with surfaces that range from moderately dark to extremely dark and whose colors hint at different mixes of silicates, organics, and possibly ice.
What makes the Trojans so scientifically valuable is their location and their likely origin. Planetary migration models suggest that the Trojans were not born where they sit today. Instead, they were probably swept into Jupiter’s Lagrange points during a dramatic reshuffling of the outer planets early in solar system history. If that picture is correct, the Trojans are a grab bag of planetesimals that originally formed across a wide range of distances from the Sun, then got captured and frozen in place. They are, in a sense, a fossil record of the building blocks that went into making the outer solar system.
Lucy’s science goals flow directly from that idea. The mission aims to determine surface composition, assess surface geology, measure bulk physical properties like density, and search for satellites and rings around each target. By sampling multiple Trojans of different types, the mission can test whether the diversity seen from Earth holds up close, and whether these objects really do look like they came from different parts of the primordial solar system.
Targets and the Flight Plan
Lucy’s trajectory is unusually ambitious. Rather than orbiting a single body, the spacecraft uses a series of Earth gravity assists to loop between the inner solar system and Jupiter’s orbit, threading a path that lets it visit targets in both the L4 (leading) and L5 (trailing) Trojan swarms. The primary Trojan targets include Eurybates, Polymele, Leucus, and Orus in the L4 swarm, and the Patroclus-Menoetius binary in the L5 swarm. Along the way, Lucy also flew past two objects that were not part of the original mission plan but added enormous value: the small main-belt asteroid Dinkinesh and the somewhat larger main-belt asteroid Donaldjohanson.
The targets were chosen to capture as much diversity as possible. They include all the recognized taxonomic classes found among the Trojans, a member of a collisional family (Eurybates), and a near-equal-mass binary (Patroclus-Menoetius). Their diameters span two orders of magnitude. That spread is deliberate: a mission that visits only one type of Trojan would leave open the question of whether its findings are representative.
Dinkinesh and Selam, the First Surprise
Lucy’s first close encounter came on November 1, 2023, when it flew past the small main-belt asteroid Dinkinesh, a body roughly 738 meters across. The flyby was originally intended as an engineering checkout for the spacecraft’s tracking systems, but it delivered a genuine scientific surprise. Dinkinesh turned out to have a moon, and that moon, now named Selam, is a contact binary: two lobes stuck together like a lumpy snowman. Selam was the first confirmed contact binary satellite ever observed.
Researchers have since modeled Selam’s formation and concluded it likely assembled through a series of low-speed collisions among several small moonlets that once orbited Dinkinesh. The idea is that Dinkinesh shed material, perhaps through rotational spin-up caused by sunlight pressure, and the resulting debris clumped together in stages rather than all at once. That process has been hypothesized for other small asteroid systems, but Selam gave scientists the clearest physical evidence yet.
Data from the L’Ralph instrument confirmed that both Dinkinesh and Selam are consistent with an S-type or Sq-type asteroid classification, as expected for a body in that part of the main belt. The thermal emission spectrometer measured Dinkinesh’s thermal inertia at about 91 joules per square meter per kelvin per root-second, with a surface roughness of around 35 degrees of root-mean-square slope, providing a first detailed thermal portrait of such a small asteroid.
Donaldjohanson, a Bilobed Fragment
On April 20, 2025, Lucy flew past (52246) Donaldjohanson, a main-belt asteroid named after the paleoanthropologist who discovered the famous “Lucy” fossil. This was Lucy’s second bonus target before reaching the Trojans. The flyby revealed Donaldjohanson to be a bilobed body, two roughly connected masses with overall dimensions of about 8.8 by 4.4 by 3.1 kilometers, joined by a smoother neck. Both lobes are heavily cratered, and the crater density is consistent with the estimated age of the Erigone asteroid family to which Donaldjohanson belongs, about 155 million years. However, craters smaller than about 400 meters were notably scarcer, suggesting some process has been erasing small craters over time.
Donaldjohanson rotates slowly and in a tumbling state, meaning it does not spin neatly around a single axis but wobbles. Researchers attribute the slow, tumbling rotation to gradual spin-down driven by radiative forces, a phenomenon called the YORP effect. Surface spectra revealed iron-bearing phyllosilicates, minerals that form in the presence of liquid water. That finding implies Donaldjohanson’s parent body, the larger asteroid that broke apart to create the Erigone family, experienced moderate aqueous alteration at some point in its history.
The Donaldjohanson encounter also has a broader context. The Erigone family sits near the source regions of asteroids Bennu and Ryugu, both of which have been visited and sampled by other missions. Lucy’s close-up look at a member of that same neighborhood lets scientists compare a flyby dataset with the detailed sample-return results from those other missions.
Eurybates and Its Satellite Queta
Eurybates is the parent body of the only major collisional family among the Jupiter Trojans, meaning it is the largest surviving fragment of a catastrophic impact that shattered a much bigger asteroid. It is classified as a C-type object, which in the Trojan context groups it with the “less red” spectral population. In 2018, the Hubble Space Telescope discovered that Eurybates has a small satellite, now named Queta, with an estimated diameter of about 1.2 kilometers.
Follow-up observations refined Queta’s orbit. It circles Eurybates at a distance of roughly 2,350 kilometers with a period of about 82 days. From that orbital data, researchers calculated Eurybates’s mass and derived a density of about 1.1 grams per cubic centimeter, broadly in line with densities measured for other Trojans, C-type asteroids in the outer main belt, and small icy objects from the Kuiper Belt. That low density is a clue: it suggests Eurybates, despite being the remnant of a violent collision, is a fairly typical Trojan composition-wise, possibly containing a significant fraction of ice mixed with rock.
Lucy’s flyby of the Eurybates-Queta system in 2027 is expected to offer direct insight into the collisional processes that shaped the Trojan population. Understanding how much material was excavated, what the interior structure looks like, and whether Queta is a fragment from the same impact will help constrain models of how often and how violently Trojan asteroids collide.
The Slow Spinner Leucus and the Red Surface of Orus
Leucus is one of the more unusual targets. It is a D-type Trojan, part of the “red” spectral population, and it is an extremely slow rotator. Its rotation period has been measured at roughly 446 hours, or about 18.5 Earth days per single spin. That makes it one of the slowest-turning asteroids known among bodies of its size. Its shape is irregular, with maximum dimensions of about 61 by 39 by 28 kilometers, and its geometric albedo is only about 0.043, making it exceptionally dark. A phase curve analysis supports its D-type classification.
Orus, another D-type Trojan, shares Leucus’s red spectral character but has not been studied in as much ground-based detail. Near-infrared spectroscopy from the ground has confirmed that both Orus and Leucus sit firmly on the “red” end of the Trojan spectral continuum. James Webb Space Telescope observations have gone further, detecting absorption features at 3.3 to 3.6 micrometers on both objects that are indicative of aliphatic organics, carbon-rich compounds arranged in chain-like molecular structures. Those organic signatures were systematically deeper on Orus and Leucus than on the less-red Trojans, suggesting the red color and the organic content are linked.
Both Leucus and Orus sit in the L4 swarm and will be visited during the same phase of Lucy’s trajectory. Comparing them side by side with the less-red Eurybates and the intermediate-colored Polymele will test whether the color differences seen from Earth correspond to genuinely different bulk compositions or are just skin-deep surface effects.
Polymele and Its Surprise Companion
Polymele is the smallest of Lucy’s named Trojan targets and a P-type object, putting it spectrally between the redder D-types and the less-red C-types. Ground-based observations have measured its rotation period at about 5.76 hours, making it a relatively fast spinner compared to Leucus. The light curve is double-peaked and low-amplitude, consistent with a modestly elongated body viewed at a geometry close to pole-on.
In 2022, a ground-based stellar occultation campaign made an unexpected discovery: Polymele has a small companion satellite, roughly 5 kilometers in diameter, orbiting at a distance of about 200 kilometers. The discovery was announced by the Lucy team and added yet another satellite system to the mission’s growing list of binary and multi-body targets. Lucy’s close approach will give scientists their first direct images of this system and a chance to determine whether the satellite is a captured fragment or a product of Polymele’s own rotational shedding.
Patroclus-Menoetius, the Near-Equal Binary
The final primary target on Lucy’s itinerary is the Patroclus-Menoetius system, a near-equal-mass binary in the L5 (trailing) Trojan swarm. The two components are roughly 113 and 104 kilometers across, respectively, and orbit each other with a period of about 4.3 days. This binary is unusual because the two bodies are so similar in size; most known asteroid binaries consist of a large primary and a much smaller secondary.
Patroclus-Menoetius has a measured density close to 0.8 grams per cubic centimeter, lower than water ice, which strongly implies a porous, ice-rich interior. In some planetary migration models, the survival of a loosely bound binary like this one through the chaotic reshuffling of the outer solar system places tight constraints on how gentle or violent that migration was. If the binary survived, the capture process could not have been too energetic. Lucy’s encounter with this system, planned for 2033, will refine the density measurement, map the surfaces of both components, and look for evidence of how and when the pair formed.
Lucy’s Science Instruments
Lucy carries three main instruments, each tailored to a different aspect of asteroid science. The first is L’LORRI, a long-range reconnaissance imager derived from the camera that flew on New Horizons to Pluto. L’LORRI is a panchromatic visible-light camera with a narrow field of view and high resolution, designed to map the sunlit surfaces of each target to about 10-meter resolution after image processing. It handles crater counting, shape modeling, stereo imaging, and satellite searches.
The second instrument is L’Ralph, a combined visible and infrared spectral imager. Its job is to map surface geology and composition by measuring how each asteroid reflects light at different wavelengths, from visible colors through the near-infrared. L’Ralph can identify minerals, ices, and organic compounds on the surface, and it will also be used to search for any tenuous atmosphere or exosphere around the Trojans. At Dinkinesh, L’Ralph confirmed the S-type classification and picked up a 3-micrometer absorption feature that may relate to hydroxyl-bearing minerals or adsorbed water.
The third is L’TES, a thermal emission spectrometer that measures the heat radiating from each asteroid’s surface. Thermal data reveal properties like thermal inertia, which tells scientists whether a surface is covered in loose dust or bare rock, and surface roughness, which affects how sunlight heats the ground. At Dinkinesh, L’TES provided the thermal inertia and roughness measurements mentioned earlier. For the much darker and colder Trojans, L’TES will face a tougher measurement challenge, but the data it collects will be the only direct thermal information ever obtained for these objects.
In addition to the three primary instruments, Lucy carries a terminal tracking camera system (TTCam) whose main role is navigation: it autonomously locates each target asteroid during the final approach phase and feeds position updates to the spacecraft’s guidance system. Once the tracking job is done, the TTCam’s wide-field images also contribute to science by providing broadband views of each target that are useful for shape modeling and geological assessment.
JWST Observations and What They Preview
While Lucy is still en route to its Trojan targets, the James Webb Space Telescope has already begun studying them from afar. JWST near-infrared spectroscopy of several Lucy targets detected absorption features consistent with hydroxyl (OH) groups, aliphatic organic compounds, and carbon dioxide on some of the Trojans’ surfaces. The OH feature, if confirmed by Lucy’s up-close measurements, would be a significant finding because it would suggest the presence of hydrated minerals or even trapped water ice beneath the surface.
The organic signatures are especially intriguing. Aliphatic organics, the kind detected at 3.3 to 3.6 micrometers, are carbon-hydrogen chains that can form through irradiation of ice-rich surfaces or through primordial chemistry in the outer solar disk. Their detection on Trojans supports the idea that these objects formed in the outer solar system where volatile-rich chemistry was common. Lucy’s instruments will be able to map where on each asteroid’s surface those organics concentrate, whether in fresh crater interiors, old weathered plains, or everywhere uniformly, which will help distinguish between formation and processing scenarios.
What Makes Lucy’s Trajectory Unusual
Most asteroid missions visit one target, or at most two. Lucy visits ten distinct objects across both Trojan swarms and the main belt. Achieving that required an intricate trajectory built around multiple Earth gravity assists, a technique where the spacecraft swings past Earth to reshape its orbit without burning fuel. Lucy returns to Earth’s vicinity several times over its twelve-year mission, picking up speed and redirecting itself each time. The trajectory works because of a fortunate alignment of the target asteroids’ orbits during the mission window, an alignment that will not repeat for decades.
Each flyby is brief. Lucy does not slow down and orbit its targets; it sweeps past at several kilometers per second, collecting data during a window that lasts only hours for close-approach science. That constraint puts heavy demands on the instruments’ ability to track, image, and spectroscopically measure a fast-moving target, which is why the autonomous terminal tracking camera system is so central to the mission design. A small error in pointing at closest approach could mean the difference between a sharp geological map and a blurry streak.
The Contact Binary Question
One recurring theme from Lucy’s encounters so far is the prevalence of contact binaries and multi-lobed shapes. Selam, Dinkinesh’s moon, is a contact binary. Donaldjohanson is bilobed. Both Patroclus-Menoetius and Eurybates-Queta are binary systems, and Polymele has a satellite of its own. This is not entirely surprising; surveys from other missions and telescopic campaigns have shown that binary and contact-binary configurations are common among small solar system bodies. But the sheer variety of configurations Lucy is sampling, from a tiny contact-binary moon to a near-equal-mass pair of hundred-kilometer worlds, will give scientists an unprecedented comparative dataset on how binaries form and evolve under very different conditions of size, composition, and dynamical environment.
The formation mechanisms are thought to differ depending on the size regime. Small asteroids like Dinkinesh can spin up through the YORP effect until they shed material, which then reaccumulates into moonlets. Larger binaries like Patroclus-Menoetius may have formed very early, possibly through gravitational collapse of a pebble cloud in the protoplanetary disk. Lucy’s density and shape measurements across multiple binary systems will help sort out which formation pathway dominates at which scale.
Organic Chemistry on Primitive Asteroids
The detection of organics on Trojan surfaces by JWST ties into a broader question in planetary science: how much organic material was delivered to the inner solar system, including Earth, by small bodies during the late stages of planet formation? If the Trojans are indeed transplanted objects from the outer solar system, their organic inventories represent the kind of material that could have been scattered inward during planetary migration. Lucy will not answer the question of whether Trojan organics contributed to Earth’s prebiotic chemistry, but it will provide the first ground-truth measurements of what those organics actually are, how abundant they are, and how they are distributed on the surface.
The comparison between Trojan organics and the organics found in returned samples from Bennu and Ryugu will be particularly telling. Bennu and Ryugu are carbonaceous near-Earth asteroids whose samples contain amino acids and other complex organic molecules. Trojans, being more distant and presumably less thermally processed, might preserve an even more primitive organic signature. Whether that turns out to be the case is one of the open questions Lucy was designed to help address.