Jupiter is a planetary bully, tossing other bodies aside with its outsize gravitational tug. But two clusters of asteroids precede and follow Jupiter at a respectful distance, like cowering children, held in a stable place by the pull of the gas giant and the Sun. Astronomers have long been puzzled by these rocks, called the Trojans, which look very different from the objects in the main asteroid belt and settled in the oddest of places, 374 million kilometers in front of and behind Jupiter.
Now, a $981 million robotic NASA mission called Lucy, to be launched on 16 October from Cape Canaveral, Florida, will explore six of these Trojans. During a 12-year tour, Lucy—named after the famed human-ancestor fossil—will test whether the Trojans themselves are fossils of the early Solar System that were dragged inward from the frigid far reaches beyond Neptune during a period of gravitational turmoil. Details of the Trojans’ surfaces, shapes, and composition could flesh out not only how this ancient upheaval occurred, but when. Hal Levison, Lucy’s principal investigator (PI) at the Southwest Research Institute (SwRI) in Boulder, Colorado, likens their promise to clues in a murder mystery. “Sometimes blood splatter on the walls can tell you more than blood on the corpse. [Asteroids] are really the splatter on the walls.”
Decades ago, researchers pictured the formation of the planets as a gentle affair, with Jupiter and the Trojans coalescing near their present location from a disk of gas and dust surrounding the Sun. But telescopes have shown that the Trojans are not uniform in color, as would be expected if they formed from the same local raw material. And rather than sitting perfectly in the plane of the planetary disk, as the old model would have predicted, their orbits are slightly inclined.
In 2005, using computer simulations of how newborn planets would have tugged on one another, Levison and others proposed the Trojans were a legacy of early Solar System turmoil. As the idea now goes, gas from the protoplanetary disk initially dampened the effects of the gravitational tugs of the gas giants and kept them in stable orbits much closer to one another than today. Once the gas dissipated, chaos erupted, launching Jupiter inward and the other giants outward. Pluto and other distant icy bodies were flung even farther out into what’s called the Kuiper belt, a region beyond Neptune. But the model also predicts that some of those icy bodies would have been scattered inward instead and captured as Trojans by Jupiter.
Although the idea of an instability can explain much, it hasn’t really been observationally tested, says Mike Brown, an astronomer at the California Institute of Technology and member of the Lucy team. “The Trojans are where those concrete tests are,” Brown says. If they have the same composition as Kuiper belt objects, “there’s no way the Solar System was put together without dynamical instability.”
To take a close look, Lucy will travel for 6 years, making two Earth flybys to build speed before reaching the leading group of Trojans in 2027. Two circular solar arrays will power the spacecraft, each spanning 7 meters to collect the dim light available in Jupiter’s orbit—farther from the Sun than any other solar-powered mission has ventured.
Lucy’s first target will be an oddball even for the Trojans: the 64-kilometer-wide Eurybates and its small moonlet, which resemble from afar the dark, carbon-rich asteroids found in the main asteroid belt. Within 1 year, it will then pass Polymele, Leucus, and Orus, which seem similar to icy Kuiper belt objects. Next, Lucy will swoop past Earth for a third gravity assist before flying back to the trailing Trojans. In 2033, it will survey Patroclus and Menoetius, 100-kilometer-wide asteroids orbiting each other. Binaries are oddities in the asteroid belt but common farther out, in the Kuiper belt, says SwRI’s Cathy Olkin, the mission’s deputy PI. “Could that pair have been captured from the Kuiper belt?” she asks.
A first task will be determining whether the Trojans contain substances that can only form out in the cold twilight of the outer Solar System, like a glassy form of water ice that lacks a crystalline structure. Brown also wants to understand the Trojans’ reddish color, a subdued version of the hue seen on many Kuiper belt objects. Brown’s best guess for the source of this redness is hydrogen sulfide, which grows red when irradiated by cosmic rays or other sources, but can also evaporate away if brought too close to the Sun. Some Kuiper belt objects on elongated orbits that bring them closer to the Sun “look a lot like Trojans,” Brown says.
Another task will be counting the number and size of craters on the Trojan surfaces and comparing them with those seen on Pluto’s moon Charon and Arrokoth, Kuiper belt objects visited by the New Horizons mission in 2015 and 2019. A similar distribution will suggest they formed in the same spot, and small differences could reveal when the Trojans were torn from their remote homeland, Levison says. Scientists once thought the instability shook up the Solar System some 700 million years after its formation, but some now think it could have happened after only 100 million years, or even earlier (Science, 24 January 2020, p. 350).
The Trojans might also provide clues about an even earlier time, when the planets coalesced from primordial gas and dust. Arrokoth’s lobed shape fits a model suggesting planetesimals formed when clouds of pebbles in the gassy disk collapsed to form city-size lumps; those might then have merged with others into progressively larger structures. If the Trojans are similarly lumpy, that would be an indication that cloud collapse is the standard way to assemble planetesimals, says Jessica Sunshine, a planetary scientist at the University of Maryland, College Park. The Trojans’ relative proximity—just one-sixth as far from the Sun as the Kuiper belt—will make them a ripe target for further exploration, including by NASA’s James Webb Space Telescope, which will be launched late this year.
Levison is hoping for surprises. “It’d be damn boring if we just saw what we expected,” he says. Lucy’s sheer endurance and number of targets will give it plenty of time to follow up on any surprises, Sunshine adds. “We have the opportunity to not only come up with new questions, but maybe even answers.”