Second Earth Trojan will be stuck in orbit for 4000 years—should we pay a visit?

Earth Trojans may have complex orbits but are held in place by Earth’s and the Sun’s gravitational fields. Illustration: Nasa

Astronomers have now determined with certainty that the 2020 XL5 asteroid discovered on 12 December 2020 is a second Earth Trojan orbiting at Lagrange point 4 (L4).

These findings were published in a new article in Nature Communications.

Trojans are asteroids that follow a planet in its orbit around the Sun—about 60 degrees ahead or behind the planet along its orbit.

The article also states that it is most likely an asteroid that has been ejected from the asteroid belt between Mars and Jupiter due the effect of Jupiter’s gravitational field. However, closer observations are required to determine this with greater certainty.

Based on observations and a number of calculations, the researchers conclude that the asteroid was captured at L4 500 years ago and that it will remain there for approximately another 4000 years.

The first of Earth’s L4 Trojans is the 2010 TK7 asteroid, which was described in a Nature article in 2011.

Trojans were first found near Jupiter

The history of Trojans began when German astrophotographer Max Wolf found an asteroid with a very unusual orbit near Jupiter in 1906.

The five Lagrange points. Illustration: Nasa

Swedish astronomer Carl Charlier soon found out that the asteroid was located at one of the five points where a small body could be held by gravitational fields of two larger objects—in this case the Sun and Jupiter.

These points were predicted by Joseph-Louis Lagrange in 1772, but had until then been a mathematical curiosity.

Several such asteroids were soon found near Jupiter. They were all named after heroes from the Trojan War described in The Iliad, and they afterwards became simply “Trojans”.

For those particularly interested in their origin, it should be added that Wolf may not have been the first to find a Trojan, as can be read in most sources—a Trojan may have already been found in 1904. However, this is less significant in the context of the new discovery.

Stable and unstable Lagrange points

As mentioned, there are five Lagrange points.

Many are probably familiar with the fact that L1, which lies on a direct line between the Sun and Earth, and L2, which lies on the extension of this line, are popular points to place space telescopes in order to observe the universe or the Sun. The James Webb Space Telescope has just been placed at L2.

But neither L1 nor L2 are stable points.

A small effect on an object at L1 or L2 in addition to the Sun’s and Earth’s gravitational fields—for example from Venus, Mars, or Jupiter—would be amplified and could quickly send the celestial body or satellite astray. Satellites stay in place at these points thanks to regular small corrections.

The same is true for L3, which is located on the other side of the Sun relative to Earth.

Therefore, asteroids are not found at L1, L2, or L3.

Small objects at L4 or L5, on the other hand, could be in stable orbits at one of these points in the sense that a small push seeking to send the object astray would be offset by an effect that pulls the object back in again.

For L4 and L5 to be stable points, the mass ratio of the heavy to light object must be greater than 25.

Trojans at L4 and L5 may have relatively complex orbits, as illustrated at the top of this article.

However, the stability does not necessarily last forever. A sufficiently violent impact can send the object off course.

Earth Trojans are hard to spot

In addition to Jupiter, Trojans have been found near Venus, Mars, Uranus, and Neptune.

So far, only one Trojan was found near Earth before 2020 XL5, and that happened as late as 2011—not because Earth Trojans are expected to be very rare, but because they are very difficult to spot.

Many of them would be in Earth’s shadow and therefore appear very dim.

Shortly after 2020 XL5 was discovered, astronomers were already aware that it was a Trojan candidate, but numerous further observations were required to confirm the assumption.

Its size is now estimated at 1.18 km, and the size of 2010 TK7 is estimated at 0.3 km.

Should we send a mission?

The obvious next step is to consider the possibility of sending a space mission to one or the other of the now two known Trojans.

The research group behind the article in Nature Communications, with Toni Santana-Ros from the Institute of Cosmos Sciences of the University of Barcelona at the helm, has also given this some consideration.

However, they concluded that neither of the two asteroids is particularly suitable for landing a probe due to their special orbit at L4. On the other hand, they believe that a fly-by mission to 2020 XL5 would be an option.

Finally, they highlight the possibility that further surveys of the L4 and L5 regions may lead to the discovery of other Trojans with orbits that may allow probes or even scientific hardware to land on such asteroids, so we could take advantage of their peculiar position in the universe in relation to Earth.