Trojan Asteroids-(Rock Soldiers of Planets)

“Trojan Asteroids” -written by Sayali.

We all feel excited about sci-fi or any science – oriented films but with this excitement there is also a fear that comes along. Especially, the movies like Deep Impact and Armageddon give us goosebumps when we look at the asteroid hitting the earth, nature’s worst possible catastrophe. 

Now, of course after watching such movies or even thinking about it, we assume that our Earth is highly vulnerable to asteroids and the main culprit is the asteroid belt. But what if I say that there are even many asteroids which are orbiting along with the other planets (both inner and outer planets) and even share their orbits with the planets. Our Earth also shares its orbit with one of those asteroids that we are scared of. Of course, our satellites and astronauts are vulnerable to such celestial rocks.
Unfortunately, as well as fortunately, this is true that asteroids are not only found in the asteroid belt or Kuiper belt, but they are found all over the Solar System. Such asteroids are given a special name by planetary scientists, as “Trojan asteroids”. Named after the characters from the Iliad, Homer’s epic poem about the Trojan war. Unfortunate, of course, because of the obvious reasons, but it’s fortunate as well because these celestial bodies are believed to be remnants of the early Solar System and could serve as a medium to learn about the origin and evolution of our Solar System.

What are Trojan asteroids?

Trojan asteroids are believed to be the remains of the rocks which came together and formed the planets in the early periods of the solar system, which got trapped by the gravitational effect of those planets, in the Lagrangian points, which are special equilibrium positions in the planets orbit. At Lagrange points, the gravitational force of the two large bodies (in this case, Sun and the planet; however, those large bodies can be any body whose mass ratio exceeds 24.96) equals the centripetal force required for any small body to move with them. In the figure below, we can see the Earth and the Sun aligned in a straight line vertically. As per this system, the two large bodies which are the Sun and the Earth, in this case, their gravitational pull has created few spots for the small bodies, labeled as L1, L2, L3, L4 & L5 . These are called Lagrangian points for this system. Out of these five, the points labeled as L1, L2, L3 are the unstable points and L4 & L5 are the stable ones where the Trojan asteroids are trapped. These points as you can see from the figure are located 600 ahead and behind the earth in its orbit.

LaGrange points

Such Trojan asteroids can not only be trapped by the Sun-Earth system but also by other systems like Sun-Jupiter, Sun-Uranus, Sun-Mars, Earth-Moon, Saturn-Tethys (one of the Saturn’s moons) in an identical manner as in the Sun-Earth system.

The given image above is an example of Saturn having Trojan moons which include Tethys, Telesto and Calypso.

Jupiter -Trojans (also called as Jupiter’s co-orbital asteroids):

However, out of all these systems, the Sun-Jupiter system is the largest and special one as most of the Trojan asteroids are found in Jupiter’s orbit. Because astronomers found 2 big swarms of asteroids orbiting along with Jupiter in its orbit: one orbiting 60 degrees ahead, and the other 60 degrees behind Jupiter.

Till now we know of around 6300 Jupiter Trojans.

There are various ideas on how planetesimals could have been captured as Trojans in the early phases of Jupiter’s growth, based on different physical processes. The origin of Trojan asteroids however, out of all the ideas appears to be more consistent with early trapping of planetesimals during the formation of Jupiter.


The Earth trojans include 2010TK7, which was discovered by the Wide-field Infrared Survey Explorer (WISE) satellite in October 2010 and the other one 2020XL5 was discovered on 12th December 2020 by the Pan-STARRS1 survey telescope in Hawaii. 2020XL5 is over a kilometre wide and about three times larger than the 2010TK7, and is in a stable orbit approximately 60o behind the Earth. It is believed that 2020XL5 could have been ejected from the asteroid belt following an interaction with Jupiter. Astronomers predict that 2020XL5 will remain in orbit for at least 4000 years.

Similarly, there are 5 Trojan asteroids found orbiting along with Mars.


Scientists estimate that, since Trojans are dark surfaced and being far away are difficult to detect, there could be more than 6,00,000 Jupiter Trojans but we haven’t detected them yet. So, scientists planned for the Lucy mission (which is a NASA probe launched on 16th Oct 2021) will study these Jupiter Trojans as they can be the source of information about the history of the Solar System.


Leave a comment