The Earth is trailing an asteroid less than a kilometer in diameter around the Sun, according to research. This is only the second instance of such a body that has been discovered. It orbits the Sun two months faster than the Earth on average, and dances in front of us like an excited herald.
2020 XL5 was the first object to be spotted using Pan-STARRS telescopes at the summit of Haleakala, on Maui. However, it was necessary to confirm its orbit using follow-up observations made with the 4.1-meter SOAR telescope (Southern Astrophysical Research), Chile.
This data has led to a team of scientists from Spain’s University of Alicante, Toni Santana Ros, who announced that 2020 XL5 is now trapped in orbit around one of the Sun/Earth “Lagrange point” locations. These are the places where the Earth’s gravitational forces and the Sun combine to create stable positions. This means that the object moves in the same direction as the Earth, even though it is going around the Sun.
Other planets also have Lagrange points. They are equilibrium points for objects of small mass that are subject to the influence of two or more much larger bodies. Three such points are located along the Sun-Earth line (L1,L2 and L3). These points were first mathematically discovered by Leonhard Euler, a Swiss mathematician. Spacecraft such as the James Webb Space Telecope and DSCOVR can be kept there with a minimal fuel consumption.
Joseph-Louis Lagrange, Euler’s student, discovered two other points in 1772. This is a stable equilibrium, where a small-mass object forms an equilateral triangle that includes Sun and Earth. These points are 60° ahead of and 60° behind Earth. 60° (see image above), is one-sixth Earth’s orbit. This means that two months of separation can be achieved.
A small-mass object that is moved away from L4 and L5 by perturbation will be pulled back by the combined gravity of Earth and the Sun. This bends its path to a stable orbit around Lagrange point, which looks like a kidney bean relative to Earth.
XL5, but not fireball
2020 XL5 has been called a Trojan friend to Earth in analogy to Jupiter’s Trojan asteroids. Jupiter orbits nearly ten thousand asteroids, half ahead of Jupiter and half behind. Achilles, the first of these asteroids was discovered in 1906. It is named after a character who played a key role in the Siegge of Troy in Homer’s Iliad.
The Lagrange points that are associated with Earth’s orbit (sizes & distances not scale). Credit: NOIRLab/NSF/AURA/J. da Silva
Each one is named after a hero in the same story. Only the ones following Jupiter (clustered at Sun-Jupiter L5 positions) get Trojan names such as Hektor. Those ahead of Jupiter (at L4) get Greek names such as Achilles. They are collectively called Trojans, regardless of whether they are at L4 and L5.
A few Trojan asteroids were discovered to be associated with Mars (9), Uranus (1), and Neptune (23). 2020 XL5 is the second Trojan asteroids to be discovered on Earth. The first Trojan companion of Earth, 2010 TK7 was discovered in 2010. It measures only 300 meters in length, but 2020 XL5 is a lot larger at 1.2km.
Although there are likely many more Earth Trojans out there, they are difficult to find from Earth. They can only be seen at low altitudes in the sky if they are at L4 (like both 2020 XL5 and 2010 TK7), or at sunset if they are at L5 (where none have been found). Their orbits don’t hold up over millions of years so they cannot be remnants. They must have moved into their current location later.
The SOAR observations showed that 2020 XL5 is a carbon-rich asteroids (called C-type). It is an example of the material from which the Solar System was constructed. It would be interesting to examine Earth’s Trojan friends in greater detail as examples unaltered material.
Asteroid positions with Jupiter’s Trojans shown in green Credit: Mdf on English Wikipedia, Public Domain via Wikimedia Commons
Could we extract them, or do they have other uses? Santana-Ros points out that 2020 XL5 orbits above and below Earth’s orbital plane. To maneuver a spacecraft to a rendezvous (to orbit it or land on it), you would need significant velocity change. This would likely require too much fuel to make it practical. Same applies to 2010 TK7.
The study does however indicate that other Earth Trojans could be found in orbits less tilted. These might make useful bases for exploring the Solar System. Because of their slight gravity, it would be easier to launch them than from Earth or Moon. They might even be a source for resources we could mine.
