The Earth’s Trojan

The Iliad is an epic poem by the Greek poet Homer, one of the greatest epic poets to have ever lived. It is one of many works of Greek literature that tells the tale of the ten-year siege of the city of Troy (now uncovered to be in modern day Turkey) by the Greeks some time in the Bronze Age.

The most well-known event of the Trojan War is, of course, the tale of the Trojan Horse. After a futile ten-year siege, the Greeks constructed an enormous wooden horse as a victory trophy for the Trojans, and pretended to sail away. Hiding a select force of Greek warriors inside the horse, they managed to infiltrate the defences and destroy the city, decisively ending the war.

But today I won’t be writing about the Trojan War, although I do recommend having a look into Greek mythology. The term ‘trojan horse’ has now come to also mean something that stealthily subdues an enemy, usually by infiltration. A very common example is a Trojan Horse computer program, a type of malware that can infiltrate computers by disguising itself as something else. They differ slightly from computer viruses, in that they don’t insert copies of themselves into programs or files.

But today I also won’t be writing about computer malware. Another meaning of the term trojan refers to a type of asteroid that occupies special positions around a planet, and shares its orbit. In a way they ‘infiltrate’ and imitate its orbit.

The majority of asteroids in our Solar System reside within the asteroid belt between Mars and Jupiter, and many more small extraterrestrial bodies lie much further away, past Neptune, in the Kuiper belt. The Kuiper belt is home to remnants from the formation of the Solar System. Many dwarf planets, including Pluto, are located in this region.

A few asteroids may be attracted by the gravity of a nearby planet, inviting them to orbit with it. Unfortunately the combination of the gravitational pulls of the Sun and the planets will usually cause the asteroid to soon float away from the planet again.

However, extensive calculations have pinpointed five areas relative to a planet where, if an asteroid were to occupy these, it could stay there. These are called Lagrangian points, and are labelled L₁ to L₅. These are the points where the combined gravitational pull of the Sun and the planet is exactly equal to the centripetal force required to move with them.

L₄ and L₅ (which lie 60° ahead and behind the planet in its orbit) are known as Trojan points, and are more stable than the other three. If an asteroid at L₄ or L₅ were to be displaced slightly from the point, gravitational forces can pull it back into place. L₁ to L₃, on the other hand, are rather unstable, and any minor displacement can result in the asteroid spiralling away. As a result it is rare to find bodies at those points.

Not all of the planets have trojans. There are many more known Jupiter trojans than any other body in the Solar System, with over 6000 found to date. Jupiter trojans are all named after characters in the Iliad, such as Achilles and Odysseus. The asteroids at L₄ are referred to as the ‘Greek camp’ whereas those at L₅ are referred to as the ‘Trojan camp’.

Venus, Mars, Uranus and Neptune trojans have been found, and even Ceres and Vesta, two of the largest objects in the asteroid belt, have temporary trojans. The Earth’s first trojan, 2010 TK₇, was discovered in 2010; with a diameter of about only 300 m, it leads the Earth 80 million km away at L₄.

Lagrangian points can be home not only to asteroids, but also to artificial satellites that have various applications, from producing a model of the Milky Way galaxy to studying cosmic microwave background radiation that originates from the birth of the Universe. In addition, NASA and ESA both have spacecraft that occupy L₁ and L₂.

The study of trojans has provided us with a stepping stone to discover more amazing things in the Universe, and who knows what else we can find in our Solar System?

Yanhao