Earth's moon will soon have some company — a “mini moon”.

The mini moon is actually an asteroid about the size of a school bus at 10 metres. When it whizzes by Earth on Sunday, it will be temporarily trapped by our planet's gravity and orbit the globe — but only for about two months.

The space rock — 2024 PT5 — was first spotted in August by astronomers at Complutense University of Madrid using a powerful telescope located in Sutherland, South Africa.

These short-lived mini moons are likely more common than we realise, said Richard Binzel, an astronomer at Massachusetts Institute of Technology. The last known one was detected in 2020.

"This happens with some frequency, but we rarely see them because they're very small and very hard to detect," he said. "Only recently has our survey capability reached the point of spotting them routinely."

The discovery by Carlos de la Fuente Marcos and Raúl de la Fuente Marcos was published by the American Astronomical Society.

This one won't be visible to the naked eye or through amateur telescopes, but it "can be observed with relatively large, research-grade telescopes," Carlos de la Fuente Marcos said in an email.

Binzel, who was not involved in the research, said it's not clear whether the space rock originated as an asteroid or as “a chunk of the moon that got blasted out”.

The mini moon will circle the globe for almost 57 days but won't complete a full orbit. On November 25, it will part ways with the Earth and continue its solo trajectory through the cosmos. It's expected to pass by again in 2055.