NASA on Saturday launched a probe toward clusters of asteroids along Jupiter’s orbital path. They’re known as the Trojan swarms, and they represent the final unexplored regions of asteroids in the solar system. The spacecraft, a deep-space robotic archaeologist named Lucy, will seek to answer pressing questions about the origins of the solar system, how the planets migrated to their current orbits and how life might have emerged on Earth.
“We have never gone this far to study asteroids,” said Bill Nelson, the administrator of NASA. “In so doing, we’re going to be able to better understand the formation of the solar system, and better understand ourselves and our development.”
After a six-year cruise, Lucy will fly close to seven Trojan asteroids through 2033, completing wild circuits of the sun that conjure the outline of a Formula 1 racetrack in some graphic renderings.
The spacecraft will study the geology, composition, density and structure of the Trojans, which are small bodies locked in stable points along Jupiter’s orbit of the sun, fixed in their own orbits ahead or behind the massive planet.
“It’s always interesting to go somewhere for the first time,” said Cathy Olkin, the deputy principal investigator of the Lucy mission. “Every time we’ve done that, we’ve learned more and more about our solar system and the area in space that we live in.”
Humanity has explored a variety of small rocky bodies throughout the solar system. The NEAR mission landed on Eros in the inner asteroid belt. The Dawn mission orbited Ceres and Vesta, the two largest worlds in the belt between Mars and Jupiter. Japan’s Hayabusa missions and NASA’s OSIRIS-REX completed close encounters with near earth asteroids. And the New Horizons mission visited Arrokoth, an object in the solar system’s distant Kuiper belt.
But the Trojans in Jupiter’s vicinity have yet to be investigated. About 10,000 such objects have been discovered. When the first was spotted more than a century ago, astronomers began naming them after heroes of Homer’s Iliad. The result was the overall descriptor of “Trojan.”
The mission name “Lucy” is a nod to the 3.2 million-year-old australopithecine skeleton discovered in 1974, which revealed secrets of human evolution. The NASA team hopes that the robotic Lucy does the same for the solar system’s evolution, and prehistory is a recurring theme among the mission’s scientists.
Tom Statler, the Lucy program scientist at NASA, describes Lucy as “planetary archaeology” and compares it to the study of the pyramids in Egypt.
“If you want to understand how the pyramids were built, you can go and look at the outsides of them, you can climb all over them,” Dr. Statler said. Doing that, however, will answer very little about how they were actually built.
“If you can find and excavate the abandoned construction site that’s next to the pyramids, however, and you find the tools used to build them, and you find the leftover blocks — the things that got broken and shaped but didn’t get used — then you start getting insight into a pyramid’s interior and how it got there,” he explained.
“That’s what we’re doing with asteroids,” he said. “We’re excavating the leftovers from the construction site.”
The mission was born of necessity.
Thirty years ago, the concept of planetary formation was much more orderly than it is today. A star formed in the center of a rotating disc of protoplanetary material. Gradually, the material condensed and collected into eight planets in simple orbits (as well as Pluto).
However, when Hal Levison, a planetary scientist, and fellow theorists tried to simulate the formation of the solar system, they repeatedly ran into a problem: It was virtually impossible to build Uranus and Neptune in their present orbits. To account for those worlds, known as the ice giants, Dr. Levison, now the principal investigator of the Lucy mission, and three other researchers developed the Nice model of solar system evolution (named for the city in France).
The model suggests that the giant planets formed much closer to the sun than their current orbits, and that the increasingly eccentric orbits of a young Jupiter and Saturn destabilized and rearranged the solar system. In the process, as the giant planets moved, and Uranus and Neptune bounded outward, they scattered the small bodies of the solar system. Some comets and asteroids were flung to the deep outer solar system, and others were ejected entirely out into the Milky Way.
A small minority of scattered asteroids were ensnared in two of Jupiter’s permanent Lagrange points, which are regions of space where the gravitational and orbital influences of the planet and the sun are balanced. The regions both lead and follow Jupiter in its orbit. Those asteroids are the Trojan swarms.
Today, the Nice model offers the predominant understanding of how a disc of dust and gas about 4.6 billion years ago became a system of planets circling a sun. Moreover, telescope observations of exoplanets prompted a broader scientific re-evaluation of how star systems, including our own, can form. Some distant stars are orbited by giant planets that are closer to them than Mercury is to our sun.
Dr. Levison came to believe that the planetary science community’s ideas about planetary formation outpaced the data it had available. The best way to constrain variables in the Nice model would be to account for the origins of the Trojans.
“One of the surprising things about the Trojan population is that they are physically very different from one another but occupy a really small region of space,” he said. “That diversity in that small region is telling us something important about the early evolution of the solar system.”
To understand the secrets locked in the Trojans’ orbits, Dr. Levison needed to persuade NASA to build a spacecraft to study them and determine what formed where. Lucy is the result. It was chosen for flight in 2014 through NASA’s Discovery Program, in which scientists compete over smaller mission proposals.
Studying camera imagery will be an important part of the Lucy team’s scientific efforts. By counting the number of craters spotted on each asteroid, an object’s surface age is revealed. (Older surfaces will have been hit by more impactors and thus show more craters.) The scientists will also analyze the returned images for the distribution of color across the asteroids’ surfaces, which can be an indicator of what the rocks are made of: Thermal measurements will help identify the compositions and structures of the asteroids. They will also use infrared spectra to measure the presence of minerals, ices and organic molecules.
NASA is interested in finding primordial organic material on asteroids because billions of years ago, they may have seeded Earth with the chemical ingredients necessary for life.
Despite the Trojans sharing an orbit with Jupiter, Lucy will not visit Jupiter. Before the spacecraft even launches from Earth, it will be closer to Jupiter than when it visits the Trojans.
During its 12-year mission it will be powered by two giant solar arrays that are stowed at launch and gradually expand outward like folding fans. Lucy’s roller coaster-like trajectory will carry it farther than any solar-powered spacecraft has ever flown. And it will be moving at about six miles per second at its fastest clip.
“Its speed will be like running a 10K every second,” Dr. Olkin said.
The spacecraft will be in a sophisticated orbit of loop-the-loops across the solar system, circling the sun, borrowing gravity from Earth for free propulsion to Jupiter’s orbital path at a point known as Lagrange 4. Gravity will take it back around the sun to the Earth, whose gravity will again hurl it outward, this time to Lagrange 5, and back, the process repeating. The trajectory is driven by the positions of planets and gravity assists, meaning the spacecraft, if nothing stops it, will continue to do this for hundreds of thousands — if not millions — of years.
Each encounter will be at an altitude of 600 miles or less from the Trojan’s surface. After the final flyby, depending on Lucy’s health, NASA can target future asteroids and other celestial objects for analyses.
“As we are flying past a Trojan asteroid, we are taking data,” Dr. Olkin said. Lucy’s science instruments are mounted to a movable pointing platform connected to the spacecraft. Tracking cameras feed images to onboard computers that keep the science instruments locked onto the target regardless of the spacecraft’s position. Lucy will collect data for the complete rotation of each asteroid, some of which spin faster than others.
The probe will arrive at the first target — an asteroid between Mars and Jupiter named 52246 Donaldjohanson, after the discoverer of the Lucy skeleton — in 2025. This is not a Trojan, and is more like a rehearsal flyby for the mission. During observation campaigns, scientists discovered that Donaldjohanson is most likely only 100 million years old, making it one of the youngest objects in the solar system, and a worthy target of exploration in its own right.
Two years later, the spacecraft will fly by the first Trojan asteroid, 3548 Eurybates, which also has a tiny moon, Queta. Eurybates was once part of another destroyed asteroid. Such an origin might also explain its moon. The second target, 15094 Polymele, is the smallest of the non-moon targets for Lucy. Planetary scientists will be looking closely at its surface features and density. In general, objects that formed closer to the sun are denser than objects that formed farther away.
The spacecraft will next fly by the asteroid 11351 Leucus, which is a “slow rotator,” whose day lasts about 400 hours. Its shape is of particular interest. The final asteroid of the first loop of Lucy’s trajectory is 21900 Orus. Scientists are interested in differences between its terrain and that of Eurybates.
In 2028, Lucy will begin its sweep from one side of the solar system to the other to visit the opposite Trojan swarm. After swinging past Earth to pick up speed, the spacecraft will, in 2033, fly to 617 Patroclus and Menoetius, binary asteroids that rotate around a common center of mass. These are among the largest known Trojan bodies, and scientists want to know whether their pairing is a sign that they came from farther out in the solar system, where such binaries are more common.
After that final planned encounter, the mission could be extended by NASA to study more small bodies. But even if the mission ends in the 2030s, there could be another act: Lucy will still be soaring through Jovian Lagrange points and swinging back around Earth, there and back, there and back. As such, the agency has equipped Lucy with a “time capsule” of poetry, quotes and song lyrics, in the hope that future space-faring humans may one day recover the spacecraft and share with our descendants a hint of what life must have been like in the prehistoric ages of the 21st century.
If Lucy returns data suggesting that the Trojans formed in different places at different distances from the sun and were then swept into their current orbits, it would be significant corroborating evidence for the Nice model.
On the other hand, when Lucy’s prime mission is completed and all data is returned, it might have revealed something entirely unexpected about how the solar system evolved. That, says the mission’s leader, would be a good thing.
“My hope,” Dr. Levison said, “will be to look at the current models of solar system formation — including my own work — and say: ‘Nope, this is all wrong. It wasn’t that simple, and we have to start again.’”