Research has shown that the Earth trails an asteroid barely a kilometer across in its orbit about the Sun – only the second such body to have ever been spotted. It goes around the Sun on average two months ahead of the Earth, dancing around in front like an excited herald of our coming.
This object, known as 2020 XL₅, was first spotted in December 2020 using Pan-STARRS telescopes on the summit of Haleakala on the Hawaiian island of Maui. But the determination of its orbit required follow-up observations using the 4.1-meter SOAR (Southern Astrophysical Research) telescope in Chile.
Based on this data, a team led by planetary scientist Toni Santana-Ros of the University of Alicante in Spain has now announced that 2020 XL₅ is trapped for at least the next several thousand years in an orbit about one of the Sun-Earth “Lagrange points”. These are where the gravitational forces of the Earth and the Sun balance to create stable locations. It means the object keeps pace with the Earth as it goes around the Sun.
Lagrange points exist around other planets too, they are equilibrium points for any objects with small mass under the influence of any two much more massive bodies. There are three such points on the Sun-Earth line (L1, L2 and L3, see image below), first discovered mathematically by the Swiss mathematician Leonhard Euler. Spacecraft, such as James Webb Space Telecope (at L2) and DSCOVR (at L1), can be maintained there with only a small expenditure of fuel.
Two other points, L4 and L5, were discovered in 1772 by Euler’s student Joseph-Louis Lagrange. Here, a small-mass object making an equilateral triangle with Sun and Earth is in a stable equilibrium. These points are 60 degrees ahead of and 60 degrees behind the Earth, and because 60 degrees (see image above) is one-sixth of the Earth’s orbit this amounts to two months separation.
If a small-mass object is perturbed so as to move away from L4 or L5, the combined gravity of the Sun and Earth draws it back – bending its path into a stable orbit around the Lagrange point that looks kidney bean-shaped relative to Earth.
