It sounds like something out of a science-fiction movie. For the past couple of weeks, Earth's orbit has been home to a "mini-moon" the size of a city bus. The celestial object is more than just a guest staying over for a quick visit. It's here to stay, or at least until Thanksgiving week. But it's not a moon. In fact, it's an asteroid named 2024 PT5. It entered Earth's orbit on Sept. 29 and will be taking up residency in our skies until Nov. 25 before returning to the solar system. 

Scientists call such phenomena mini-moons. The asteroid was found by a group called ATLAS, which stands for Asteroid Terrestrial-impact Last Alert System, in August. The researchers who found it published a nonpeer-reviewed study about the asteroid.

Some past mini-moons, such as this one from 2020, have been discovered to be random pieces of space junk. The 2020 mini-moon was eventually identified as a rocket booster from the 1966 Surveyor 2 Centaur launch. But ATLAS reports this new mini-moon is likely to be a real asteroid, which is simply defined as a small, rocky object that orbits the sun.

Amateur astronomer Tony Dunn posted a simulation to X of what the asteroid's path will look like. 2024 PT5 has already been near Earth since July.

During the event, the asteroid's geocentric energy will become negative and remain that way for 56.6 days. In Dunn's simulation, the orbit is shown as the red line, and it only circles about 25% of the Earth.

The asteroid won't complete a full orbit of Earth, so some astronomers refer to it as a temporarily captured flyby. Mini-moons that do complete a total orbit of Earth are referred to as temporarily captured orbiters.

It's unlikely you'll see the mini-moon. NASA says the absolute magnitude of 2024 PT5 is 27.593. That means it's quite dim and won't be visible even if you have a telescope. For reference, the dimmest magnitude that is visible to the naked eye at night is around 6.5 and a 12-inch telescope can see objects with a magnitude of around 16 or 17. That means amateur astronomers are going to have to sit this one out, since you'd need a much larger telescope to see 2024 PT5.

Mini-moons aren't particularly rare. They occur almost every year. Earth had a mini-moon in 2022 thanks to the peculiar flight path of the 2022 YG asteroid. Another one arrived in 2020 courtesy of the 2020 CD3 asteroid. Some of them are even bright enough to see with amateur astronomy instruments. 

Many asteroids come back over and over again for repeat visits as well. The 2022 NX1 asteroid became a mini-moon in 1981 and 2022. It's scheduled to make its return in 2051. One of the best examples is 2006 RH120, which orbited Earth for a whole year between July 2006 and July 2007. The phenomenon is so consistent that some researchers say that Earth always has a mini-moon lurking around somewhere.