Earth could be about to get a new moon – but experts have been left baffled at what the mystery object actually is.
An object known as 2020 SO is heading towards Earth and from October it will be a "mini-moon", which could stay in orbit of the planet until May next year.
Another object, named 3753 Cruithne, has already been dubbed Earth's "second moon" – meaning 2020 SO would be our third.
Cruithne is in a normal elliptic orbit around the Sun.
Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of Earth.
Because of this, Cruithne and our planet appear to "follow" each other in their paths around the Sun and has led to the body being called "Earth's second moon".
Earth also regularly gets many small asteroids and meteors caught in its orbit, which astronomers have previously described as "mini-moons".
Technically, any natural object which is caught in a planet's gravitational pull is a "moon".
Astronomers say 2020 SO is a non-threatening object which is heading towards Earth and could get caught in the gravity of the planet for up to eight months, according to simulations from astronomers.
A video of the simulation shows the object 2020 SO making two close approaches to Earth while in orbit of our planet.
The object then looks like it will make an attempt to swing away from our planet, before getting sucked back in by the gravitational pull for a close approach on February 2, 2021.
However, this is only from initial observations and could easily change over the next few months.
Astronomer Tony Dunn said: "Asteroid 2020 SO may get captured by Earth from Oct 2020 to May 2021. Current nominal trajectory shows shows capture through L2, and escape through L1.
"Highly-chaotic path, so be prepared for lots of revisions as new observations come in."
However, experts have noticed something strange about the incoming mini moon.
The velocity of 2020 SO is much slower than any space rock, which has led to suggestions it could be something man-made.
The average space rock travels at a speed of anywhere between 11 kilometres per second and 72 kilometres per second.
The object 2020 SO has a speed of just 0.6 kilometres per second.
NASA's Paul Chodas of the Jet Propulsion Laboratory (JPL) has identified the object as possibly being a piece of old space junk.
More specifically, Mr Chodas said it is likely a piece of rocket from the Surveyor 2 spacecraft which was sent to the Moon all the way back in 1966.
Astronomer Kevin Heider said: "Asteroid 2020 SO is suspected of being the Surveyor 2 centaur rocket booster, launched on 20 September 1966.
"The Earth-like orbit and low relative velocity suggest a possible man-made object."
Earth's last mini-moon came earlier this year when a small meteor called 2020 CD3, which was about the size of a car, was captured by the planet's orbit.
The space rock stayed in orbit for around three months, before continuing its voyage across the solar system in March.
