The so-called “mini-moon” of the Earth which has been in the skies since last September begins its journey towards the sun today Monday and is estimated to have disappeared by 2055.

The school bus-sized asteroid known as 2024 PT5 may actually be a huge boulder broken from the moon after another space rock crashed into it centuries ago, astronomers say.

Currently, the “mini-moon” is located two million miles from Earth, about nine times farther than the moon and never got close enough to be trapped in the planet’s gravity.

But his farewell pass will bring him within 1.1 million. miles in January, before the sun’s gravitational pull carried it deeper into space.

The size of the mini-moon, at about 10 meters across, and its distance meant that it would never be visible to the naked eye, only with the use of powerful telescopes. NASA has been tracking the asteroid through its deep space network since it was first spotted by a South Africa-based telescope owned by the University of Hawaii in August.

It has been a “distant companion” to Earth since then, NASA said, and studies have shown it was not a man-made object.

“Given the similarity between the motion of asteroid 2024 PT5 and the motion of our planet, scientists at NASA’s Center for Near-Earth Object Studies suspect that the object could be a large piece of rock ejected from the lunar surface after asteroid impact a long time ago,” Josh Handal, a program analyst for the space agency’s Planetary Defense Coordination Office, wrote in a briefing.

“Rocket bodies from historical launches can also be found in such Earth-like orbits, but after analyzing the motion of this object, it was determined that 2024 PT5 is more likely to have a natural origin.”

It has been following a horseshoe-shaped path around Earth for the past two months and will increase speed exponentially once the sun’s gravitational pull takes full effect from today. Its speed during its close pass in January will be at least twice as fast as in September, astrophysicist Raul de la Fuente Marcos of the Complutense University of Madrid told The Associated Press.

NASA will monitor the asteroid for more than a week in January using the Goldstone Solar System Radar Antenna in California’s Mojave Desert.