On Wednesday, NASA’s Lucy mission got one step closer to investigating the enigmatic asteroids near Jupiter. This story, however, really began in another corner of the cosmos more than three decades ago.
In 1992 astronomers Dave Jewitt and Jane Luu were searching for a theoretical sea of ice beyond Pluto. If they found a frigid reservoir far from the Sun, Luu said in 2013, it might explain why comets were still flying around billions of years after the Solar System finished forming. For five years they searched the outer Solar System using the University of Hawaii's 2.2 meter telescope with no luck until they finally spotted something — a reddish-speck known today as "Albion."
They had found the first icy member of the no-longer-theoretical Kuiper Belt.
Named for the Dutch-American astronomer who theorized its existence back in 1951, this region is a cradle of asteroid-sized objects and dwarf planets. It also sources comets that take less than 200 years to orbit the Sun, distant travelers that decorate our skies with their glimmering tails, as ice sublimates off their surface.
Although Kuiper Belt comets sojourns to the inner Solar System, many keep their secrets closely guarded and never leave the icy outer reaches of our Solar System. This immense distance can make studying the Kuiper Belt extremely difficult. Luckily, scientists think that rocky interlopers known as Trojan asteroids, which can be found flanking Jupiter in the gas giants’ Lagrangian points just ahead and behind the planet in its orbit around the Sun, are very similar to trans-Neptunian objects (TNOs) found in the icy Kuiper Belt.
To reach the targets so close to home (relatively speaking), NASA launched a $1 billion spacecraft — the Lucy mission — toward seven Trojan asteroids back in October 2021. With the spacecraft’s official dress rehearsal arriving Wednesday, by analyzing a main belt asteroid named Dinkinesh, NASA hopes Lucy will uncover mysteries about the farthest reaches of our Solar System.
A faraway place
Decades before Jewitt and Luu’s fateful discovery, the dwarf planet Pluto was technically the first-known Kuiper Belt object (though at the time, astronomers thought the pint-sized planet was a lone traveler). It wasn’t until the 1970s that its moon, Charon, was found nestled in its orbit. But it was Albion, also known as 1992 QB1, that really changed minds about what was really going on out there hundreds of millions of miles beyond Neptune.
Today, thousands of trans-Neptunian objects (TNOs) have been spotted in the Kuiper Belt though countless more are waiting to be discovered.
“The Kuiper Belt basically changed the scale of the Solar System by a factor of four,” Hal Levison, principal investigator of NASA’s Lucy mission and co-author of the Nice model, which attempts to explain the Kuiper Belt’s existence, tells Inverse. “It was a fundamental change.”
Kuiper Belt objects are some of the last remaining unexplored objects in the universe. Although they are much closer to Earth than the plethora of galaxies shining millions of light-years away, Kuiper Belt objects are very small and don’t emit light. And because they are so far from the Sun, they have little to reflect back towards our telescopes. Finding them is very hard. It’s why the James Webb Space Telescope, NASA’s latest foray into deep space research, is tasked with studying Kuiper Belt objects, too.
When more and more Kuiper Belt objects were discovered, it spawned a new view of the Solar System. This created more questions. Chief among them: why is there so much stuff out there? One possible answer lies with Uranus and Neptune, two of the Solar System’s largest — and also farthest — planets. At first those two planetary superlatives appear in conflict because, as Lucy team members told Inverse, the longer it takes a planet to orbit its star, the less likely it is to experience the phenomena that bulks it up.
Then in 2005, astronomers (including Levison) developed the Nice model, named after the French Mediterranean city where it was developed, suggesting that the gas giants — Jupiter, Saturn, Uranus, and Neptune — formed much closer to one another and to the Sun but eventually migrated outward to their current orbits. During this incredible journey, trillions of icy objects began to ricochet into new places. Some careened into planets, others got slingshot into deep space never to be seen again.
But around one percent survived, and either nestled into the Kuiper Belt or were captured by the gravitational pull of the mightiest gas giant of them all — Jupiter.
Lucy in the sky (with asteroids)
Lucy isn’t named after a scientist, astronaut, or Greek goddess — in fact, its namesake isn’t even human. Discovered in Ethiopia in 1974, the Australopithecus afarensis fossil nicknamed “Lucy” proved the bipedal nature of our human origins. And like that 3-million-year-old fossil, NASA’s Lucy spacecraft will hopefully reveal clues about our cosmic origins as it visits seven Trojans over the next decade.
So how exactly did the Trojan asteroids — named after both Greek and Trojan characters in Homer’s Trojan War epics — become bosom buddies with Jupiter? The Nice model suggests that billions of years ago, right after the formation of Jupiter, Saturn, Uranus, and Neptune, the planets migrated to their current orbits. However, Neptune had a particularly violent effect, which Lucy project scientist Keith Noll describes as a “snowplow.” By analyzing the orbits of many Kuiper Belt objects, scientists think Neptune essentially carved its way outwards and displaced many smaller bodies in the process.
“If you could think of it that way, it's plowing into this disk, scattering a lot of stuff everywhere. The stuff that gets retained, there’s clear patterns to it. And you can tell that this planetary migration happened,” Noll tells Inverse.
To figure out whether the Nice model is onto something, Lucy will investigate whether Trojans and Kuiper Belt objects are actually cosmic cousins. Observations from Earth suggest that Trojans are low-density and this could be explained if they are abundant in ice like Kuiper Belt bodies. Trojans also look different from one another and that variety could be a sign that they are made up of a range of material originally present in the primordial Solar System.
The discovery of Albion in 1992 broadened our perspective of the Solar System but expanding that frontier of knowledge only inspired new questions — and now we have the tools to answer those questions. The James Webb Space Telescope along with upcoming projects like the Vera C. Rubin Observatory will survey distant objects like never before, finally revealing the long-kept secrets of these icy TNOs. Coupled with Lucy’s decade-long mission, a Kuiper Belt and Trojan connection would help astronomers narrow down how our neighborhood formed, and whether our Solar System is just one among many similar stellar constructions or if our cosmic neighborhood is truly an extraordinary creation.