Spacecraft have visited most pockets of the solar system by now, but a new region is about to come under the robotic inspection: the two clumps of asteroids that flank mighty Jupiter in its orbit.
Although scientists have spotted thousands of so-called Trojan asteroids in these two swarms, no mission has ever seen one up close. That will change in 2027, when a mission dubbed Lucy makes its first of five flybys that will turn a select few Trojans from fuzzy specks of light into unique worlds. And it isn't just about space rocks: the resulting portraits could help scientists piece together a better picture of the early days of the solar system.
"I've been dreaming of sending a spacecraft to the Trojan asteroids for more than a decade," Cathy Olkin, the mission's deputy principal investigator and a planetary scientist at the Southwest Research Institute (SwRI) in Colorado, told Space.com. "This opportunity is just outstanding."
NASA's $981 million Lucy mission, which is scheduled to launch on Saturday (Oct. 16) at 5:34 a.m. EDT (0934 GMT), is a daring expedition that will make six carefully orchestrated flybys: one in the main asteroid belt, the rest among Jupiter's Trojans. The mission's agenda relies on the nimble data gathering necessitated by flyby visits and a convenient celestial alignment that offers scientists a veritable treasure trove of eight intriguing space rocks.
"We were amazingly lucky about being able to get such a rich set of targets," Hal Levison, the mission's principal investigator and another planetary scientist at SwRI, told Space.com. "Some of these objects, they would be interesting objects to send spacecraft to even if that were the only target the spacecraft were going to."
That's a model that recent asteroid missions like Japan's Hayabusa2 and NASA's OSIRIS-REx, followed; each spacecraft spent months studying its near-Earth asteroid before eventually snagging a sample to deliver to scientists.
But Lucy can't stay at any one space rock without breaking the bank, since orbiting an asteroid requires much more fuel than flying past one. Instead, Lucy will become the first mission to make so many flybys in the outer solar system, and it will approach its targets at a hurtling pace of 3 to 6 miles (5 to 9 kilometers) per second. That's a whopping 10,800 to 21,600 mph (18,000 to 32,400 kph).
To grasp the enormity of that challenge, Olkin suggested imagining running a 10K race — but in just a second or two. "I really love the challenge of it, because it makes the science return so much richer," Olkin said. "With a flyby, every moment is precious."
Similarly, although the mission stretches over 12 years, Lucy will do the bulk of its work in a total of about 24 hours, Levison said.
The allure of the Trojans
From Earth, scientists can't see much detail about any given Trojan. But between the two clumps of asteroids at Jupiter, astronomers have identified more than 10,000 bodies stuck in the gravitational parking spaces ahead of and behind the gas giant planet in its orbital track around the sun. (These to clusters are called L4 and L5 swarms, a nod to the technical term for such gravitational sweet spots, Lagrange points.)
And from a distance, scientists have noticed unusual diversity among the Trojan asteroids, particularly in terms of their color, which ranges from gray to quite red and is tied to expected differences in chemical makeup.
"They occupy a very small region of space, and yet, they're very different from one another," Levison said. "They really are a mystery."
Levison has long suspected that better understanding the Trojans can sharpen scientists' picture of how the solar system came to be the way it is, with planets that seem impossible to form where they are found today. A model he helped develop in 2005 suggests that the four giant planets formed relatively close to the sun then migrated outward, in the process scattering small pieces of debris like asteroids into the inner solar system.
He said he hopes that the data Lucy gathers will give scientists a better understanding of how closely this and other theories match the real past of the solar system. When he entered the field in the 1980s, he said, scientists were short on theories, but now that's changed. "We've gotten to the point where I think we have more ideas floating around and don't have the data to be able to figure out which one's right," Levison said.
His hunch is that the different colors on display among the Trojans represent formation at different distances from the sun, with the migrating planets kicking them into the gravitationally stable regions ahead of and behind Jupiter.
Comparison among the Trojans and among asteroids in general is at the heart of Lucy's science. "These guys have been sitting in the same place and so they've had the same evolutionary pathway since they got trapped in their swarm," Levison said. "So the differences we see really are going to be fundamental differences on the bodies."
To that end, the flybys are coordinated so that scientists can easily compare the details of observations gathered at different asteroids. Among others, the observations include color photographs of the Trojans' surface, infrared spectrometry to identify different compounds on their surfaces and analysis of Lucy's communication signal from Earth that will tell scientists how dense each asteroid is.
A rich itinerary
The team built the spacecraft's itinerary around two asteroids that were generally quite similar in size and orbit, but one gray and one red.
"If we found a pair like that, we know that they've had the same collisional history because they're on the same orbits, they're the same size, the same solar radiation — all that's been the same for the last 4 billion years, give or take," Levison said. "So if we saw differences between the two, we would know that was telling us something important about their intrinsic properties."
That backbone pair is Eurybates and Orus. Scientists estimate both asteroids at about 40 miles (64 kilometers) wide; Eurybates is quite gray and likely rich in carbon while Orus is quite red and likely rich in organic materials. Lucy will make these flybys on Aug. 12, 2027, and Nov. 11, 2028, the other flybys on the itinerary are coincidences, objects that Lucy would fly close enough to that the spacecraft could afford a slight sightseeing detour.
"That was where we started and the rest was luck, basically," Levison said.
The luck begins with Lucy's very first flyby, in April 2025 of a main belt asteroid now dubbed Donaldjohanson in honor of the anthropologist who discovered the fossil that the Lucy mission is named for, a hominin who lived in what is now Ethiopia 3.2 million years ago.
The flyby is a bonus — the asteroid will be near the path that Lucy had to travel anyway on its journey out to the Trojans — but the object is still intriguing. It's one member of a clump of asteroids that are the fragments of a much larger space rock that was smashed to bits, a type of asteroid that scientists haven't seen before. And astronomers even know about when the collision in question occurred.
"This object is very young," Levison said. "It's estimated to have an age between 100 and 200 million years, which makes it one of the youngest things in the solar system."
On to the Trojans
Lucy's first Trojan target is Eurybates, the gray component of the pair at the heart of the mission's design. It, like Donaldjohanson, is a fragment from an impact. "It's the largest asteroid from a collisional family, and that makes it very interesting," Olkin said.
Scientists know that history because Eurybates is surrounded by a host of smaller pieces that trudge along a nearly identical orbit, the marker of a collisional family. Eurybates has already surprised scientists during their work planning the mission.
In 2019, astronomers using the Hubble Space Telescope spotted a much smaller asteroid circling the main body of Eurybates — a tiny moon — with the discovery confirmed in 2020. Now dubbed Queta, the satellite is perhaps 0.6 miles (1 kilometer) across and circles Eurybates every 84 days or so. Queta is a much smaller piece of the same asteroid that Eurybates once belonged to.
Levison said he hopes the visit will help scientists understand collisions, which are a crucial step of planet formation. "We see the biggest member of this population, the brightest of the guys in the clump, and then around it is one of the smallest things, so comparing those two objects is going to be interesting too," he said.
Next, Lucy will bang out three quick flybys. First, in September 2027, Polymele, a smallish redder asteroid that may also represent a fragment of a larger lost rock. Seven months later comes Leucus, with a weirdly slow spin that likely affects the asteroid's temperature. Next comes Orus, the red member of the color-comparison pair that Lucy is built around.
That schedule means that the bulk of Lucy's flybys will come during just 15 months in late 2027 and 2028, Olkin noted. "It's going to be a very busy time."
A grand finale
All of these Trojans are in the L4 swarm, which runs ahead of Jupiter in its orbit. After this burst of flybys, Lucy will head back toward the sun to complete another flyby of Earth, which will put the spacecraft on track to visit the L5 swarm following behind Jupiter in the early 2030s.
And in that swarm is another prize of the mission: a pair of rocks nearly the same size orbiting each other dubbed Patroclus and Menoetius, which Lucy will fly past in 2033.
"This is, I must admit, my favorite — I know you're not supposed to have favorite kids, but this one is my favorite," Levison said of the pair, called a binary.
Such evenly sized binaries are rare in the inner solar system and main asteroid belt, where scientists can most easily study such rubble. But more recent observations out in the Kuiper Belt of small bodies beyond Neptune show that in this outer neighborhood, well-balanced binaries are quite common.
And out at that distance, the objects are essentially untouched. "These things are far enough away from the planetary system that the violence of planet formation never affected them," Levison said. The theory, then, is that Patroclus and Menoetius is one of just a few formerly plentiful equal-mass binaries in the heart of the solar system to survive the havoc of the solar system's early days.
"It's one of the lone survivors that we can easily get to with a spacecraft," Levison said. "To me, that's very profound."
And, in accordance with Lucy's comparison work, Patroclus and Menoetius will serve as a key contrast to the spacecraft's first Trojan, Eurybates. "We're going by one object that underwent a really massive collision and therefore sort of was beginning to be involved in planet formation, and an object that we think is pristine," Levison said. "Being able to compare those two things are going to be really, really important."
But both Olkin and Levison expect that Lucy will do far more science and see much stranger rocks than they can outline from here on Earth.
"I don't know what we're going to see, so I can't say that I have an expectation of this or that," Olkin said. "We're going to send a spacecraft there and we're going to learn about it because that's the process of how we do science."