Our solar system is a busy place. We all know about the Sun and the eight major planets, from Mercury all the way out to Neptune. We also know about the main asteroid belt, which is a huge ring of millions of space rocks orbiting the Sun between Mars and Jupiter. For a long time, we thought this was the main story. But it turns out, there are other, even stranger, collections of asteroids hiding in plain sight.
These are not your usual asteroids. They don’t orbit in their own “belt.” Instead, they travel with a planet, like a giant escort. The most famous and largest group of these objects belongs to Jupiter, the biggest planet in our solar system. These are known as the Trojan asteroids. They are two enormous swarms of ancient space rocks that share Jupiter’s orbit. One group travels in front of Jupiter, and the other group trails along behind it.
These asteroids are trapped in a perfect gravitational balance, and they have been stuck there for billions of years. They are like a hidden “fossil record” from the very beginning of our solar system. Scientists are incredibly excited about them because they hold clues to how planets like Jupiter were formed. But how can they share an orbit with such a giant planet without ever crashing into it or being flung away into deep space?
What Are These “Lagrange Points” That Trojans Live In?
The secret to the Trojans’ survival is all about gravity. You might think that Jupiter’s massive gravity would either pull these asteroids in or push them away. But space is a complex place, and gravity can do some surprising things. The Trojans are “stuck” in special areas called Lagrange points. These are specific spots in space where the gravitational pulls from two large bodies, in this case the Sun and Jupiter, balance each other out perfectly.
Think of it like finding a “sweet spot” while balancing. If you’re too far to one side, you fall. If you’re too far to the other, you fall. But if you’re right in the middle, you are stable. In the 1700s, a mathematician named Joseph-Louis Lagrange figured out that in any system with two big objects (like the Sun and a planet), there are five of these special points. He solved what is called the “three-body problem,” figuring out where a third, tiny object (like an asteroid) could orbit safely without being disturbed.
Three of these points, called L1, L2, and L3, are “unstable.” An object placed there is like a pencil balanced perfectly on its point. The slightest nudge will make it drift away. But two of the points, L4 and L5, are “stable.” An object placed there is like a marble at the bottom of a wide bowl. Even if you nudge it, it will just roll back to the center. These stable points, L4 and L5, form a perfect equilateral triangle with the Sun and Jupiter.
The L4 point is located 60 degrees ahead of Jupiter in its orbit around the Sun. The L5 point is 60 degrees behind Jupiter. The Trojan asteroids are gathered in these two “gravity wells,” like leaves collected in a small whirlpool. They are forever locked in a dance with Jupiter, leading and following the giant planet as it circles the Sun. They are not in one tiny spot; the L4 and L5 regions are huge, and the asteroids orbit around these points in wide, looping, kidney-bean-shaped paths.
How Did the Trojan Asteroids Get Their Strange Names?
When the first of these asteroids was discovered in 1906, astronomers realized they had found something very special. They decided these unique asteroids deserved a unique naming theme. They chose to name them after the heroes of the Trojan War, the famous conflict from the ancient Greek epic poem, The Iliad. This is why we call them “Trojan” asteroids.
The astronomers even took the naming theme a step further. They decided to divide the asteroids into two “camps,” just like the two armies in the story. The group of asteroids in the L4 point, the one ahead of Jupiter, is called the “Greek camp.” Asteroids in this swarm are named after the Greek heroes who fought in the war, such as 588 Achilles, 1143 Odysseus, and 911 Agamemnon.
The group in the L5 point, the one trailing Jupiter, is called the “Trojan camp.” These asteroids are named after the heroes who defended the city of Troy, such as 884 Priam and 1172 Aeneas. This naming convention creates a wonderful mental image of two armies of asteroids forever chasing each other around the Sun, with the giant planet Jupiter sitting in the middle.
There is also a fun little twist to this story. Before this naming rule was firmly in place, two asteroids were named “incorrectly.” As a result, each camp has an enemy “spy” in its ranks. The Greek camp (L4) contains 624 Hektor, who was a Trojan hero. And the Trojan camp (L5) contains 617 Patroclus, who was a Greek hero. This small mistake adds to the wonderful and human story of how we explore and name the objects in our universe.
How Many Trojan Asteroids Are There?
When you think of asteroids, you probably think of the main belt. But the Trojan swarms are just as impressive. The number of known Trojans has been growing rapidly as our telescopes get better. As of 2025, astronomers have officially found and cataloged more than 15,300 of these Jupiter Trojans. This is a huge number, far more than just a few stray rocks.
But this is only the “tip of the iceberg.” These are just the ones we’ve been able to see and track so far. The Trojans are very far away, about five times farther from the Sun than Earth is. They are also incredibly dark. Many of them are as dark as a lump of charcoal, reflecting very little sunlight. This makes them extremely difficult to spot from Earth.
Based on our best surveys, scientists estimate that there are about one million Jupiter Trojans larger than one kilometer (0.6 miles) wide. This is a number that rivals the main asteroid belt. If you were to count all the smaller ones, the total number would be in the millions, or even tens of millions. It is truly a “hidden” population of asteroids. The two swarms, the Greek camp and the Trojan camp, are thought to have roughly the same number of asteroids in them, forming two of the largest stable collections of small bodies in our solar system.
Why Are Scientists So Interested in These Asteroids?
The main reason scientists are so fascinated by the Trojan asteroids is that they are “time capsules” from the birth of our solar system. The planets we see today were formed from a giant disk of gas and dust about 4.5 billion years ago. The asteroids in the main belt are the “leftovers” from the inner solar system. But the Trojans, scientists believe, are not from around here. They are immigrants from a much more distant place.
One of the leading theories about how our solar system formed is called the “Nice model” (named after the city in France where it was developed). This theory suggests that the giant planets—Jupiter, Saturn, Uranus, and Neptune—were not formed where they are today. Early in their history, their orbits shifted around dramatically. They moved and migrated, a bit like a giant game of cosmic bowling.
During this chaotic time, Jupiter and Saturn’s immense gravity would have “grabbed” all kinds of smaller objects that were floating around in the cold, distant, outer solar system. This is the region where comets are born. Jupiter’s gravity would have pulled these objects inward and trapped them in the stable L4 and L5 Lagrange points. This means the Trojan asteroids are not made of the same stuff as the main belt asteroids. They are pristine, frozen “fossils” from the outer solar system.
Because they were trapped so long ago and have remained in these stable “parking spots” ever since, they haven’t changed much. They are the original building blocks that formed the outer planets. Studying them is like getting a sample of the raw material that built Uranus and Neptune, but without having to travel all the way out there. They hold the chemical secrets, the “DNA” of the outer solar system, which can tell us how our entire planetary neighborhood came to be.
What Are the Trojan Asteroids Made Of?
This is the billion-dollar question that a new space mission is trying to answer. Based on what we can see from Earth, we know they are very different from most asteroids. As mentioned, they are among the darkest objects in the solar system. Many of them have a distinct reddish color. This strange color isn’t paint; scientists think it’s the result of billions of years of sunlight and cosmic rays “zapping” the surface. This “space weathering” could have transformed simple ices and carbon-based molecules into complex organic compounds.
Because we think they came from the cold outer solar system, scientists believe the Trojans are made of a “fluffy” mix of rock, dust, and lots of ices. This isn’t just water ice, but also frozen gases like ammonia, methane, and carbon dioxide. On the surface, most of this ice has probably been baked away by the Sun over billions of years, leaving behind the dark, carbon-rich, rocky material and the red “gunk” we see. But deep inside these asteroids, the original, ancient ice may still be perfectly preserved.
Some of these asteroids have a very low density. For example, the Trojan asteroid 617 Patroclus (the “Greek spy” in the Trojan camp) is actually a binary system. It’s two asteroids orbiting each other. By studying their orbit, scientists found their density is less than that of water. This means they cannot be solid rock. They must be “rubble piles,” very porous and fluffy, with a lot of empty space inside, like a giant, loosely packed snowball made of ice and dust. This is very similar to the composition of comets.
How Are Trojans Different from Main Belt Asteroids?
It’s easy to get asteroids confused, but the Trojans and the main belt asteroids are two very different families. The easiest way to tell them apart is to remember their location, their origin, and what they are made of.
The first and biggest difference is location. The main asteroid belt is its own, independent ring, orbiting the Sun in the large, empty space between Mars and Jupiter. The Trojan asteroids, however, share Jupiter’s orbit. They are not in their own belt but are trapped in the L4 and L5 Lagrange points, following and leading the giant planet.
The second difference is origin. Most main belt asteroids are “natives” of the inner solar system. They formed in the warmer region near Mars and Jupiter. The Trojans are “immigrants.” They are believed to have formed in the cold, dark, outer solar system, far beyond Neptune. They were then captured by Jupiter during the solar system’s early, chaotic childhood.
The third difference is composition. Because they formed in different places, they are made of different things. Main belt asteroids are mostly rock (silicates) and metal (like iron and nickel). Trojans are more like comets. They are made of a mix of rock, dust, and lots of ices, and are rich in carbon compounds. This is why they are so dark and reddish, while main belt asteroids are typically grayer.
Here is a simple table to show the main differences:
| Feature | Main Belt Asteroids | Jupiter’s Trojan Asteroids |
| Location | In their own belt between Mars and Jupiter | Share Jupiter’s orbit (at L4 & L5) |
| Origin | Formed in the warmer, inner solar system | Formed in the cold, outer solar system |
| Composition | Mostly rock and metal | Icy, carbon-rich, and “fluffy” |
| Color | Varies, but often gray | Very dark and reddish |
What Is NASA’s Lucy Mission?
For all these amazing theories, we need proof. We can only learn so much by looking at tiny, dark dots of light from telescopes on Earth. To truly understand the Trojans, we have to go see them up close. That is the exact job of a groundbreaking NASA mission called Lucy.
The Lucy mission launched in October 2021 on a 12-year-long journey to become the first spacecraft ever to visit these mysterious asteroids. The mission was named after the famous “Lucy” fossil, an early human ancestor whose skeleton revealed secrets about human evolution. In the same way, the Lucy spacecraft is on a mission to study the “fossils” of our solar system’s formation.
Lucy’s path through space is incredibly complex. It’s not flying straight to Jupiter. Instead, it is using Earth’s gravity in a series of “slingshot” maneuvers to build up speed and bend its path. This allows it to visit a record-breaking number of different asteroids in one single mission. Before it even gets to the Trojans, it is practicing on a few main belt asteroids.
In November 2023, Lucy had its first asteroid encounter. It flew by a small main belt asteroid named (152830) Dinkinesh. This flyby was a huge success and revealed a wonderful surprise: Dinkinesh is not one asteroid, but two! It has a tiny moon orbiting it, which has now been named Selam. This was a perfect test of Lucy’s tracking and imaging systems.
More recently, in April 2025, the Lucy spacecraft completed its second flyby. It passed another main belt asteroid, (52246) Donaldjohanson, named after the scientist who discovered the “Lucy” fossil. The data from this encounter is helping scientists prepare for the main event.
The real excitement begins in 2027. That is when Lucy will finally arrive at the L4 “Greek camp” in front of Jupiter. Between 2027 and 2028, it will fly by five different Trojan asteroids: Eurybates (which has its own small moon), Polymele, Leucus, and Orus. It will study their surfaces, measure their temperatures, and try to figure out what they are made of.
But the mission doesn’t stop there. After visiting the Greek camp, Lucy will swing all the way back past Earth for another gravity assist. It will then travel across the solar system to the other side of Jupiter, arriving at the L5 “Trojan camp” in 2033. There, it will visit its final target: the amazing binary asteroid system 617 Patroclus and its partner, Menoetius. This will be the grand finale of its historic tour.
Do Other Planets Have Trojan Asteroids?
Jupiter is the king of the Trojans, with by far the largest and most famous collection. But the L4 and L5 Lagrange points are a natural feature of every planet’s orbit. So, it’s not surprising that other planets have their own, smaller swarms of Trojan companions.
Mars has its own family of Trojans. As of 2025, we have confirmed at least four asteroids that share Mars’s orbit, trapped at its L4 and L5 points. Neptune, the distant ice giant, also has a healthy collection. We have found several dozen Neptune Trojans, and there are likely many, many more hiding in the darkness of the outer solar system.
What about our own planet? For a long time, scientists wondered if Earth had any Trojans. They are very hard to find because, from our point of view, they are located in the daytime sky, close to the Sun. But in 2011, we finally found one: 2010 TK7. And in 2021, a second Earth Trojan was confirmed, named 2020 XL5. These rocks are “ours,” sharing our planet’s journey around the Sun.
This phenomenon even works for moons orbiting planets. The giant planet Saturn has two moons, Tethys and Dione, that have their own tiny “Trojan moons.” Tethys has two small moons, Telesto and Calypso, in its L4 and L5 points. Dione also has two, Helene and Polydeuces. This shows that this amazing gravitational “trap” is a common feature all across the solar system, at many different scales.
Conclusion
The Trojan asteroids are far more than just a few bits of space rock. They are a vast, hidden population of objects that are as numerous as the asteroids in the main belt. They are “trapped” in two special zones by a perfect balance of gravity from the Sun and Jupiter. These two swarms, the Greek camp and the Trojan camp, are locked in an endless chase, leading and following the giant planet.
More importantly, these dark, reddish, and “fluffy” asteroids are not from our neighborhood. They are ancient, frozen fossils from the outer solar system, captured billions of years ago. They are the leftovers from the very beginning of our solar system, holding the original recipe for how the giant planets were made.
With NASA’s Lucy mission now well on its way, we are on the verge of finally unlocking their secrets. We are moving from theory to fact, from seeing faint dots to viewing entire worlds up close. As we continue to explore these dark, ancient worlds, what other long-lost secrets about our own origins will we uncover?
FAQs – People Also Ask
Why are Jupiter’s Trojans in two groups?
The Trojan asteroids are in two groups because they are trapped in two special, stable “gravity wells” in Jupiter’s orbit. These are called the L4 and L5 Lagrange points. The L4 point is 60 degrees ahead of Jupiter, and the L5 point is 60 degrees behind it, forming a stable triangle with the Sun and Jupiter.
Is the Lucy mission going to land on a Trojan asteroid?
No, the Lucy mission will not land on any of the asteroids. It is a “flyby” mission, which means it will fly very fast and very close to its targets. As it speeds past, its cameras and science instruments will collect data, take pictures, and study the asteroids before it continues on to its next target.
How big is the largest Trojan asteroid?
The largest Jupiter Trojan is 624 Hektor, which is in the “Greek camp” (even though Hektor was a Trojan hero). It is not round like a planet but is very long and stretched out, almost like two asteroids stuck together. It measures about 225 kilometers (or 140 miles) across its longest side.
Could a Trojan asteroid ever hit Jupiter?
It is extremely unlikely. The L4 and L5 points are “stable” regions, meaning the asteroids are trapped there. Even though they are in Jupiter’s orbit, they are very far from the planet itself. Their orbits are balanced by the combined gravity of the Sun and Jupiter, keeping them at a safe distance.
Why are the Trojan asteroids so dark and red?
Scientists believe the Trojans are covered in complex organic molecules. These asteroids likely formed in the very cold outer solar system and are rich in ices and carbon compounds. Over billions of years, sunlight and radiation from space have “cooked” these compounds on the surface, creating a dark, reddish, tar-like substance.
What is the difference between an asteroid and a comet?
The line can be blurry, but generally, asteroids are made of rock and metal and formed in the warmer, inner solar system. Comets are made of ice, dust, and rock (“dirty snowballs”) and formed in the cold, outer solar system. Jupiter’s Trojans are fascinating because they are in an asteroid’s location but appear to have a comet’s composition.
Does Earth’s Trojan asteroid 2020 XL5 pose any danger?
No, 2020 XL5 is not a danger to Earth. It is a “Trojan” asteroid, which means its orbit is stable and tied to Earth’s L4 point. It will stay in this general region, orbiting the Sun ahead of our planet, for at least the next several thousand years and will not come close enough to pose a threat.
What did the Lucy mission find at asteroid Dinkinesh in 2023?
When the Lucy spacecraft flew by the main belt asteroid Dinkinesh, it made a surprise discovery. It found that Dinkinesh is not alone; it has its own tiny moon. This small satellite, which is a “contact binary” (two objects touching), has been officially named Selam.
What is the next target for the Lucy mission in 2025?
After its 2023 flyby of Dinkinesh, the Lucy mission’s next target was another main belt asteroid called (52246) Donaldjohanson. The spacecraft successfully flew past this asteroid in April 2025. This was its final flyby before it heads out to the main Trojan swarms, which it will begin to reach in 2027.
Why did they name the Trojan asteroids after a war?
When the first few were discovered in the early 1900s, astronomers wanted a heroic and classic theme for these unique objects. They chose the heroes from Homer’s The Iliad, which tells the story of the Trojan War. This led to the two groups being named the “Greek camp” and the “Trojan camp.”