The moon will get a companion this week as an asteroid around the length of a double-decker bus enters Earth's orbit.
The temporary guest, called 2024 PT5, should arrive on 29 September and is expected to befriend the regular moon for around two months.
"A mini-moon is a small object orbiting another solar system body," said Daniel Brown, an associate professor in astronomy at Nottingham Trent University.
It can either be made of rock, which is an asteroid or a mixture of rock, dust and ice, which is a comet.
"In either case, we have to think about these not orbiting around us forever but for a short amount of time," said Dr Brown.
2024 PT5 was first discovered in August last year, the day before it approached Earth at a distance of 353,200 miles (568,500km).
It belongs to the Arjuna asteroid belt, a group of space rocks in the solar system that revolve around the Sun.
Dr Brown said the mini-moon will orbit Earth for 57 days before eventually swinging out of the planet's gravitational pull.
"There is nothing explosive or dramatic happening to it for it to leave, it simply has too much energy to hang around us for too long," he said.
The mini-moon will be back
Although a temporary visitor, Earth will not see the last of this mini-moon.
Scientists predict it will re-enter our orbit in 2055.
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This is not the first time the Earth has had a mini-moon.
Four years ago, scientists identified another mini-moon called 2020 CD3, which orbited the Earth for more than a year.
Read more: New mini-moon discovered orbiting Earth - but it's only temporary
Dr Brown said mini-moons are fairly common but "what tends to be less common is being able to observe them, as they are small, very faint and very fast-moving".
This year's mini-moon will not be visible to the naked eye and can only be seen through telescopes.
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Tap here2024 PT5 is a near-Earth object - a group of space rocks that pass by Earth regularly.
Astronomers track these objects in case they are a threat to Earth in the future but experts said it is unlikely 2024 PT5 will pose such a risk.
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