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Asteroids found approximately 60° ahead and 60° behind (at L4 and L5) a planet or moon in its orbit around the sun are called ‘Trojan asteroids.’
It feels like someone is in the news every day for discovering some XYZ planet a couple million light-years away. If not a planet, then it’s an asteroid that is gonna blast past Earth in a few decades. This isn’t a new phenomenon. In fact, back in February of 1906, Max Wolf discovered asteroids beyond the asteroid belt that were major news.
Asteroids are small rocky planets left over from the formation of our Solar System. We find most of these asteroids in the region between Mars and Jupiter in the ‘asteroid belt’. However, Max Wolf identified asteroids beyond this region. The asteroids he discovered both led and trailed behind Jupiter in its orbit around the sun at specific positions. These asteroids are now called ‘Trojan asteroids’.
Position of Trojan asteroids
Trojan asteroids lie approximately 60° ahead and/or 60° behind a larger celestial body in its orbit. These two points, along with three other points, are known as Lagrange points. At Lagrange points, the gravitational forces of two larger bodies (the sun and a planet or moon) equals the centripetal force on the third smaller body (asteroids). This balance of forces holds the smaller body at either of the two positions.
Even if the third smaller body gets kicked out of the planet’s orbit because of some perturbation, the gravitational forces of the two larger bodies will bring it back to the Lagrange point. At any other position than L4 or L5, the smaller body would go off on its own separate path.
Leonhard Euler determined the points L1, L2, and L3 that lie on the line passing through the centers of two large bodies. However, these points are unstable and no celestial body could stay there for long. Joseph-Louis Lagrange, a French mathematician and astronomer, found the other two Lagrange points, L4 and L5. These points form an equilateral triangle with two large bodies.
Lagrange stumbled onto the points while trying to solve the infamous three-body problem. The problem involves determining the path of motion of three bodies under the influence of their mutual gravitational forces. The problem remains unsolved, but in his award-winning paper to the ‘Académie des Sciences of Paris’, Lagrange predicted the existence of stable points in Jupiter’s orbit.
It was 1772 when he stated that any celestial body at points L4 and L5 would remain trapped there. He also predicted that celestial objects might indeed be found at these points in the future. More than a century later, when our telescopes became powerful enough to scan the outer planets, Max Wolf’s discovery of Trojan asteroids proved Lagrange’s prediction to be true!
The term ‘Trojan asteroids’ has primarily been used for the asteroids at the L4 and L5 positions of Jupiter. However, in recent times, asteroids have also been found at the L4 and L5 points of other planets. The term ‘Trojans’ has thus become more generalized, while ‘Trojan asteroids’ is still used to refer to the Trojans of Jupiter. Asteroids at L4 or L5 of other planets are termed ‘planet name’ + ‘trojans’ (e.g., Mars Trojans or Saturn Trojans).
Jupiter has the most discovered Trojans (more than 6,000 as of 2014) followed by the twenty-two Trojans of Neptune and nine Mars Trojans. Uranus has two, and we discovered a single Trojan asteroid at the L4 of Earth in 2010. Saturn itself doesn’t have any Trojans, but two moons of Saturn have trojans in their orbits.
Coming back to Jupiter, the two Trojan asteroid groups are also assigned specific names. The preceding cluster is called the ‘Greek camp’, while the trailing group of asteroids is named the ‘Trojans camp’. The names are adopted from the Trojan war of Greek mythology.
On a more structural front, most Trojan asteroids are D-type asteroids. These types of asteroids have very low albedo values (the amount of light reflected in comparison to the amount of light incident upon it), making them fairly difficult to discover. They are thought to be primarily composed of silicates and carbon, while their interiors may contain ice.
Trojan asteroids revolve in a rather complicated orbit around Jupiter. While most Trojans follow a tadpole orbit, simulations show that some asteroids may trace an even more complicated path around the Lagrangian points in the form of a horseshoe orbit.
Also, no one really knows from where and how these asteroids came into existence. One theory argues that the Trojans and Jupiter were formed in the same region of the Solar system and during their formation, the Trojans might have got stuck/entered Jupiter’s orbit. Another theory suggests that the Trojans might have been dragged along with Jupiter during the outward movement of the four giant planets.
Apart from this, we know very little about Trojan asteroids, but that might all be about to change.
As part of their Discovery Program, NASA announced Lucy in 2017, the first space mission to explore and study Trojans. The mission is named after a fossilized human skeleton that helped scientists uncover many critical mysteries of human evolution. Similar to this, Nasa believes that Lucy will aid us in uncovering mysteries of the formation and evolution of the solar system, among many other things. Lucy is scheduled to set flight in October of 2021.
Starting with a brief visit to one of the asteroids of the main asteroid belt, Lucy will then reach the asteroids at the L4 of Jupiter in 2027. Lucy will investigate 5 asteroids from the Greek camp before returning to Earth’s atmosphere. Then, with a boost from Earth’s gravity, Lucy will head over to one of the asteroids at the L5 of Jupiter. The whole mission will take 12 years to conclude and will cover 7 different asteroids.
Over these 12 years, Lucy will help us answer questions regarding the history and formation of the solar system, the movement of the four outer planets after their formation, the composition of Trojan Asteroids, and possibly even the origin of life on Earth!