Even though some objects in space do not exactly travel with us, they accompany us for a while, reminding us that we are not alone. Such objects are called Earth’s semi-satellites, and although they are not full moons of our planet, their orbit around the Sun means that they also orbit the Earth. And now Earth seems to have a new semi-satellite.
The object, called 2023 FW, is not much larger than a four- or five-story building, so it is relatively small. From the point of view of our planet, it travels around us with an average distance of 26.9 million kilometers, but it is still not considered a natural satellite. The reason this object and other satellite-like objects have such a peculiar orbit is that they are in complete resonance with the Earth. This means that they make a full rotation around the Sun at the same time as our planet.
But these objects lie far beyond the gravitational influence of the Earth, which extends up to 1.5 million kilometers and is called the Hill Sphere (or Roche Sphere). The movement of semi-satellites around the Earth is coincidental, and it is not our planet’s gravitational pull that keeps them in this orbit. These semi-satellites do not constantly accompany our planet. The most stable known examples are 469219 Kamoʻoalewa, discovered in 2016. Said to be part of the Moon, this object has been orbiting the Earth for at least a century and will continue to do so for centuries to come.
A preliminary analysis of the 2023 FW conducted by French astronomer and journalist Adrien Coffinet suggests that Kamoʻoalewa’s record could be surpassed by this new object. This object may have been accompanying us since at least 100 BC, and it looks like it will continue with us for a long time to come.
“It appears to be the longest semi-satellite Earth known to date,” Coffinet said in a conversation with Sky and Telescope.
Although the 2023 FW is a Near Earth Object, it does not pose any risk to us. It’s keeping its distance from our planet, and if that changes, we’ll know about it decades before it poses any threat to our planet.
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