Imagine Jupiter and its little asteroids as cosmic Halloween decor on the solar system’s front stoop. The planet itself—swirly, stormy, the largest in the solar system—is the pumpkin, while the tiny asteroids that accompany it are kind of like funky-shaped gourds, one cluster in front and the other behind. The pumpkin and these gourds have been on display like this for billions of years, strung together by a quirk of gravity, tracing the same loop around the sun.
Earlier this month, NASA launched a spacecraft named Lucy to admire this autumnal space display. Spacecraft have visited Jupiter before, but none has ever gone to inspect the planet’s little asteroids up close. After half a century of exploring most of the major planets and moons, though, NASA is starting to extend its reach to smaller, more niche objects that most people have never heard of. Everyone knows Jupiter the pumpkin, but who’s heard of these funky-shaped gourds? So far, the mission hasn’t gone exactly according to plan (more on that later), but NASA seems confident that the spacecraft will one day make it to these distant, mysterious objects.
Astronomers have known about the asteroids for quite some time. A German astronomer discovered the first one in 1906, and over the years, as telescopes became more sophisticated, astronomers found many more. They took to naming the objects after figures from Greek mythology’s most famous battle, the Trojan War: Achilles, Agamemnon, Hector. The asteroids, which today number in the thousands, are known together as the Trojans.
The Trojan asteroids are bigger than the last asteroid NASA visited last year. But they’re still small on a cosmic scale, and especially compared to Jupiter: most known Trojans are about the size of a small U.S. state, while more than 1,300 Earths could fit inside Jupiter. The Trojans are dark, with matte surfaces that reflect very little sunlight. The gravitational forces of the sun and Jupiter keep the Trojans where they are; one set always remains ahead of the planet on its path around the sun, and the other always trails behind. And the Trojans might be the most relatable asteroids in the solar system: “Like on a cold winter morning [when] it takes a lot more energy to leave your bed, the asteroids also don’t want to use the energy to move away,” Sierra Ferguson, a planetary scientist at the Southwest Research Institute, in Boulder, Colorado, told me.
But much about the asteroids is still unknown. We’ve had photos of Jupiter for decades, but even to powerful space telescopes like Hubble, the Trojans appear only as specks of light. From the ground, astronomers observe the Trojans through “occultations,” an inadvertently spooky-sounding term for an astronomical phenomenon that involves light and shadow. Sometimes, a Trojan asteroid will briefly pass in front of a distant star in Earth’s line of sight and cast a shadow on our planet for just a few seconds. Astronomers fan out around the world, trying to observe the star temporarily fading as the asteroid moves past. “If you have enough of these telescopes spread out over a large-enough area,” says Alessondra Springmann, a scientist at the University of Arizona, “you can actually get a silhouette of the asteroid by tracking how long it takes for the star to dim.”
The Lucy mission aims to turn these fuzzy silhouettes into textured worlds. The spacecraft will visit seven Trojans over the next 12 years, studying their composition and other properties, and checking whether any of them have their own tiny moon, or even a delicate set of rings. There’s also the very real chance that the team will stumble on something they never imagined. In 2019, when another NASA spacecraft visited a nearby asteroid, scientists discovered that the rock was ejecting hundreds of gravel-size pieces of itself into space, a scenario no one on the team had predicted.
The Trojans, small as they are, could reveal some big truths. Scientists believe that these asteroids are remnants of the early solar system, bits and pieces that weren’t swept up to create planets and moons. That’s why the team named the mission Lucy, after the famous skeleton of a human ancestor that archaeologists discovered in Ethiopia in 1974. Just as Lucy the fossil gave us new information about human evolution, astronomers hope that Lucy the spacecraft will provide insights into the formation of the solar system.
That’s because astronomers now believe that some of the outer planets—Saturn, Uranus, and Neptune—didn’t form where they are now, but instead originated much closer in. They came to this conclusion about 15 years ago, after astronomers around the world kept finding exoplanets the size of Jupiter orbiting very close to their suns. Such a configuration implied that the exoplanets had migrated. Perhaps the same thing happened in our own solar system? Scientists say that in the beginning, Jupiter and the other big planets jostled around the solar system, swinging their gravity about. Jupiter must have swept up some asteroids that were coasting well beyond Neptune, where many other space rocks reside, and brought them with it closer to the sun. Nature smoothed some rocks into planets and moons, but it left these asteroids untouched, suspended in the invisible amber of the universe.
Lucy will look for signs of this ancient scramble in the Trojans, Andy Rivkin, a planetary astronomer at the Johns Hopkins Applied Physics Laboratory, in Maryland, told me. Astronomers understand some minerals well enough to know where they should show up in the solar system, based on their distance from the warmth of the sun. Lucy’s instruments could detect some substances that you’d expect to find somewhere as far as Neptune’s orbit, but not near Jupiter’s current orbit—an indication that the asteroids indeed came in from the cold outer reaches.
But first, Lucy has to right itself. After the spacecraft launched into space, it struggled to unfurl one of its two giant, decagonal solar panels that, when deployed, resemble intricate spiderwebs. The wonky solar panel still hasn’t unfolded all the way, and engineers don’t yet know why. Lucy will fly farther from the sun, and for longer, than any other solar-powered spacecraft in history, and it needs all the sunlight it can get. Engineers are currently analyzing data and trying to come up with a fix. They could try to deploy the panels again sometime in mid-November, NASA said yesterday, or maybe just leave them as is. If all goes well, Lucy will sidle up to one of the solar system’s most delightful displays, at long last letting us admire the gourd-like asteroids that follow Jupiter, our great big pumpkin of a planet, around the sun.