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If you were told that the Moon we see in our night sky isn't Earth's only one, you'd probably be a bit surprised. But some people have started to call the strange object that seems to orbit our planet Earth's 'second moon' - and now scientists may have discovered where it came from. In fact, there are many moon-like objects around us in space, but only a handful of the over 200,000 'near-Earth asteroids' (NEAs) have similar orbits to Earth. One of these, catchily named 469219 Kamo'oalewa, orbits the Sun but moves in sync with our orbit so appears to orbit the Earth. This makes it our 'quasi-moon' or, to some, an 'Apollo asteroid'.
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Potentially as big as the Statue of Liberty, Kamo'oalewa is between 40 and 100m (131-328m) across and rotates fast: every 28 minutes. Generally, NEAs are space rocks that scientists think have come from the Main Asteroid Belt between Mars and Jupiter. But new research reveals Kamo'oalewa may be more moon than asteroid. Published in journal Nature Astronomy, the new study used existing analysis of Kamo'oalewa's spectrum of light. This analysis revealed that it had silicates (a type of mineral) that are more common in lunar samples. In other words, our 'second moon' probably came from our first. The research team from Tsinghua University, China, used computer model simulations to test this theory. They found that Kamo'oalewa's physical shape and orbit fit with it being a fragment chipped off in a massive impact that caused a crater on the far side of the Moon. "Look at our Moon by eye or through binoculars, and you can see that is has had a violent past - each of those craters you see has blasted chunks of rock into space," astronomer Dr Darren Baskill, who was not involved in the study, told BBC Science Focus. "Most of those fragments rain back down onto the Moon due to its gravity, but some escape the Moon's hold and drift through space. Such violent impacts have caused some fragments from the Moon (and even Mars) to land on Earth. The next time you see a shooting star, just think - it could have originated from the Moon!" What's more, the fact that Kamo'oalewa still exists is part of the case that it came from a young crater - and therefore a very recent collision. The scientists think that if it were older it likely would have been slowly pulled away by other planets' gravity or demolished in a space collision. This crater probably formed within the last few million years and is probably 10-20km (6.2-12.4 miles) big. Specifically, they think it might be a crater named Giordano Bruno, and they also think they can narrow down which asteroid caused the impact. Everything we know about Kamo'oalewa so far has come from ground-based observations. But in 2025, the quasi-moon will be the target of China's asteroid sampling mission Tianwen-2, which might confirm the results. Still, Kamo'oalewa is not technically Earth's second moon. "While the term quasi-moon helps us appreciate the origin of these small fragments, they have no effect on us and are completely different to other moons in our solar system," said Baskill. "The Earth could have millions of quasi-moons, if we include all the small fragments that have been ejected into space by such collisions." Dr Darren Baskill is an outreach officer and lecturer in the department of physics and astronomy at the University of Sussex. He previously lectured at the Royal Observatory Greenwich, where he also initiated the annual Astronomy Photographer of the Year competition. Read more:About our expert
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