Earth to briefly gain second 'moon', scientists say

Get ready for a cosmic surprise this autumn - Earth is about to get a second moon, according to scientists.

A small asteroid is going to be captured by Earth’s gravitational pull and temporarily become a "mini-moon".

This space visitor will be around from September 29 for a couple of months before escaping from Earth's gravity again.

Sadly the second moon is going to be too small and dim to be seen, unless you have a professional telescope.

The asteroid was first spotted by NASA’s Asteroid Terrestrial-Impact Last Alert System (ATLAS) on 7 August.

Scientists worked out its trajectory in a study published in Research Notes of the American Astronomical Society.

The asteroid, which scientists refer to as 2024 PT5, hails from the Arjuna asteroid belt, which contains rocks that follow an orbit quite similar to Earth’s.

Occasionally, some of these asteroids get relatively close, getting as near as 2.8 million miles (4.5 million kilometers) from our planet.

According to the researchers involved in the study, if an asteroid like this is moving at a relatively slow speed of around 2,200mph (3,540km/h), Earth’s gravitational field can exert a strong influence, enough to trap it temporarily.

Which is exactly what’s about to happen - starting this weekend, this small asteroid will spend about two months orbiting Earth.

Dr Jennifer Millard, astronomer and host of the Awesome Astronomy podcast, told the BBC's Today programme that the asteroid would enter orbit on the 29th of September and then was predicted to leave on 25 November.

"It's not going to complete a full revolution of our planet, it's just going to kind of have its orbit altered, just twisted slightly by our own planet and then it'll continue on its merry way," she said.

The asteroid is approximately 10 meters long, which is tiny in comparison to Earth's moon, which has a diameter of approximately 3,474 kilometers.

Because it is small and made of dull rock it will not be visible to people on earth even if they use binoculars or a home telescope.

"Professional telescopes, they'll be able to pick it up. So you'll be able to look out for lots of wonderful pictures online of this little dot kind of moving past the stars at great speed," said Dr Millard.

Mini-moons have been spotted before, and it's thought many more are likely to have gone unnoticed.

Some even come back for repeat visits, the 2022 NX1 asteroid became a mini-moon in 1981 and again in 2022.

So don’t worry if you miss this one - scientists predict 2024 PT5 will also return to Earth’s orbit again in 2055.

"This story highlights just how busy our solar system is and how much there is out there that we haven't discovered, because this asteroid was only discovered this year.

"There are tens of thousands, if not hundreds of thousands, of objects out there that we haven't discovered and so I think this highlights the importance of us being able to continually monitor the night sky and find all of these objects," said Dr Millard.