Recently discovered asteroid 2023 FW ₁₃ has caused some confusion among asteroid watchers. It turns out that although it is in an orbit that is not only in 1:1 resonance with the Earth, but also in an eccentric orbit enough to pass half of Mars and half of Venus, it actually follows a revolving path around the Earth. .

There is no official definition for such objects, sometimes called semi-moons or semi-satellites. They move around the world, but usually no more than a few decades. Perhaps the most famous of these objects, known as Kamoʻoalewa, was discovered in 2016 and is believed to be the smallest, closest and most stable semi-satellite known. According to calculations made by Paul Chodas (Jet Propulsion Laboratory), its orbit has been in stable resonance with Earth for nearly a century and will remain so for many centuries to come.

But this newly discovered asteroid would easily dwarf that record if previous orbital calculations were correct. Some estimates suggest that it has been orbiting the Earth since at least 100 BC and will likely continue to do so until 3700 AD. If true, 2023 FW ₁₃ It will be the most stable semi-satellite of the entire Earth.

The asteroid was first observed on March 28 by the PanSTARRS observatory atop Haleakala in Maui, Hawaii. After other observations by the Canada-France Hawaii Telescope at Mauna Kea and from observatories at Kitt Peak and Mount Lemmon, the discovery was officially announced on April 1. “When I saw this announcement, the Earth-like semi-major axis seemed suspicious to me,” says French astronomer and journalist Adrien Coffinet. Coffinet was the first to determine the nature of the semi-moon’s orbit, after running orbital parameters through a simulator that predicted the past and future.

The simulations led others to find several observations before the discovery, starting in 2012. This made it possible to refine the orbit and confirm that it is not just a semi-satellite, it has been so for thousands of years. “It appears to be Earth’s longest known semi-satellite to date,” says Coffine. Sky and Telescopeand its orbit is expected to remain in this configuration for about 4,000 years.