Early this weekend, a new robotic explorer built by NASA is set to launch deep into the Solar System, with its sights set on exploring a large swarm of distant asteroids that flock around Jupiter. The uncrewed spacecraft, dubbed Lucy, is the first of its kind to target these mostly unexplored space rocks, in the hopes that studying them will tell us slightly more about how our celestial home came into being.
The asteroids that Lucy is visiting are known as the Jupiter Trojans, or simply, the Trojans, named after the Trojan War by the astronomer who first calculated their distinct orbits. They orbit the Sun along the same path as Jupiter, with one large cluster charging ahead of the massive planet and another cluster trailing behind.
No spacecraft has ever visited this population of space objects before. But scientists have been eager to study the Trojans up close, as they think these asteroids may provide some clues as to how our Solar System materialized. Just like other asteroids in our cosmic neighborhood, the Trojans are thought to have changed very little since they first formed roughly 4.5 billion years ago, during the dawn of the planets. In that way, they serve as baby pictures of the Solar System. Even more enticing is the theory that these Trojans didn’t form in Jupiter’s orbit but came from somewhere else. If that’s true, then they could tell us a lot about how the earliest objects and burgeoning planets in our Solar System crossed paths in those chaotic first years.
“We’ve studied a lot of asteroids and comets, and they all paint certain parts of the picture,” Bill Bottke, a member of Lucy’s science team at the Southwest Research Institute (SwRI), tells The Verge. “But the Trojans, we think, paint another part of the picture — but we’re not sure what part yet.”
The Trojan Source
The mission is named after the famous fossilized skeleton “Lucy,” a pre-human ancestor discovered in Ethiopia in 1974 that was found 40 percent intact. That Lucy provided crucial insight into the history of human evolution, and the mission team behind NASA’s Lucy hopes their spacecraft will do the same, but this time for our Solar System.
The original Lucy’s name comes from The Beatles’ song “Lucy in the Sky with Diamonds,” which blasted throughout the Ethiopian dig site when excavators found the fossil. Now, NASA is actually sending Lucy into the sky, but the “diamonds” the spacecraft will visit are like nothing we’ve ever seen before. These Trojans, numbering in the thousands, are clustered into two groups around Jupiter. One group, dubbed the “Greeks,” is in a pretty stable spot 60 degrees ahead of Jupiter in its orbit; the second group, the “Trojans,” is 60 degrees behind Jupiter. At an astronomical scale, those regions aren’t very big. So how did a large group of space rocks wind up there?
One idea is that the Trojans are just the leftover rocks that didn’t make it into Jupiter when the planet was forming. At our Solar System’s birth, a giant disc of gas and dust swirled around our Sun, and eventually, materials within that disc started clumping together, forming the planets. The Trojans are perhaps the nearby pieces that missed out on becoming Jupiter and instead got captured by the massive planet’s gravity in the locations they are now.
Another idea, which Bottke and others suggested in a study a few years ago, is that the Trojans actually came from somewhere else — possibly from a group of space rocks that once existed beyond Neptune. Then, the planets played musical chairs and shuffled the Trojans to their current spots. Plenty of scientists believe it’s possible that the giant planets — Jupiter, Saturn, Uranus, and Neptune — didn’t form in the orbits they hold right now. They think that they formed closer to the Sun, but when the gaseous disc that formed the planets went away, the entire Solar System went unstable and the planets shifted. Neptune wound up passing through a giant cloud of objects that existed at the Solar System’s edge, sending rocks all over the place — possibly a bunch to Jupiter.
If that’s the case, then the Trojans are the “first cousins,” as Bottke puts it, of objects in the Kuiper Belt, a huge cloud of icy space rocks at the outer edges of the Solar System that include Pluto and other dwarf planets. We’ve been to a few Kuiper Belt objects before. Studying the Trojans close up could help determine if the objects are similar, and that tells us if the planets actually did move around back in those early days.
Plus, scientists just want to know what kinds of space rocks the Trojans are, compared to the other objects that permeate our Solar System. “It’s really ‘What are they?’” says Bottke. “Are they similar to comets? Are they different from comets? Are they similar to the asteroids we’ve seen? Are they highly different? You can make a case for almost all the possibilities. So we need to find out.”
Touring the Trojans
NASA needs to get to the Trojans first, which is no easy feat. Lucy is taking one of the most complex trajectories ever conceived, spanning 4 billion miles over the course of 12 years. During that time, the probe will visit one asteroid in the Asteroid Belt and seven Trojan asteroids, while also swinging by Earth for some gravity boosts three different times. Because of this, Lucy will become the first spacecraft ever to return to Earth after journeying to the outer Solar System.
To figure out Lucy’s spaghetti-like path, NASA and SwRI teamed up with Lockheed Martin, which both built the spacecraft and came up with its trajectory. Originally, the mission team hoped to visit two different Trojan asteroids, with the goal of orbiting them and taking really high-resolution pictures. But it became clear that putting a spacecraft into orbit around a Trojan was going to cost way too much, exceeding the budget cap that NASA had placed on the mission.
Lockheed Martin had to get creative. “I said, ‘Okay how can I provide the same kind of science but do it for half the price?’” Brian Sutter, a mission architect at Lockheed Martin, tells The Verge, adding, “Rather than stopping at two asteroids, why don’t we do a flyby of a bunch of asteroids?”
The mission team gave Sutter a wishlist of 20 Trojans they’d like to visit and he figured out how to visit two of them pretty easily within the budget. But he was curious if they could get even more out of Lucy. So he sat down and created, of all things, an Excel spreadsheet. He put in Lucy’s proposed path and the known orbits of 750,000 other asteroids, to see if the spacecraft came close to any of them on its journey. “I had this cool little spreadsheet that I built,” Sutter says. “Actually somebody had seen it once and they called it the most ridiculously complicated Excel spreadsheet they’d ever seen.” Thanks to his Microsoft Office skills, he found more Trojans Lucy could swing by and a main-belt asteroid to boot.
That gives Lucy scientists a diverse range of objects to visit. First in 2025, Lucy will fly by a main belt asteroid named Donaldjohanson (in honor of the paleoanthropologist who discovered the Lucy fossil). That encounter will serve as a bit of a test run for what’s to come. In 2027, Lucy will visit two Trojans — Eurybates and Polymele — located in the swarm trailing Jupiter. Eurybates has a tiny moonlet called Queta, which scientists think was created during a past collision. The next year, Lucy will visit two more Trojans in the same swarm, Leucus and Orus. Then in 2033, Lucy will make its way to the swarm ahead of Jupiter, meeting up with two Trojans that orbit around one another, Patroclus and Menoetius. It’s an eclectic bunch. “We’re not only seeing a lot of these objects, but we’re sort of sampling how diverse they are in composition,” Bottke says.
If that wasn’t enough, Lucy may even have fuel leftover for an extended mission once it’s done. But the mission team will cross that bridge after the 12 years is up. Now, everyone is just focused on Lucy’s launch, which has been complicated by the COVID pandemic. Back when stay-at-home orders went into effect in March 2020, the mission team was five months out from piecing together Lucy and performing crucial pre-launch tests. “We really had to re-engineer the way we did [that process],” Donya Douglas-Bradshaw, the Lucy project manager at NASA, tells The Verge, “a lot of the activities that would have been done face to face were done remotely.”
The teams at NASA were able to keep the schedule on track, despite completely overhauling their protocols. Now, Lucy is set to launch on top of an Atlas V rocket, manufactured by the United Launch Alliance, this weekend out of Cape Canaveral, Florida. As of now, liftoff is scheduled for 5:34AM ET on Saturday, October 16th, but there’s plenty of time to send Lucy on its way. NASA has a launch window that spans 23 days beyond Saturday, with the option to launch within one hour each day during that period.
Of course, launch is just the beginning of a long road ahead. But for those who have been on the journey this far, it’s an emotional moment. “It feels a little surreal,” Douglas-Bradshaw says. “I mean last week was the last time that I will see the spacecraft, prior to, perhaps seeing it when it’s flying close to Earth. And, you know, it felt a little sad, because it’s truly a work of art.”