The Moon is not the only natural object travelling through space alongside Earth. Several small asteroids travel around the Sun in near lockstep with our planet. And just like Earth, these space rocks also take a year to complete a full orbit. Today, we know of eight such “quasi-moons” or quasi-satellites.
One of them may, in fact, be a fragment of the Moon itself.
This intriguing quasi-satellite is known as 469219 Kamo'oalewa or “oscillating celestial object” in Hawaiian, a nod to its apparent path across the sky. Measuring between 30 and 60 metres across, it follows a remarkably stable Earth-like orbit around the Sun.
But scientists are keen to explore the asteroid in detail for other reasons than its unusual orbit. A Chinese spacecraft is about to fetch a sample so we can learn more.
Heavily debated origin
Based on the way it absorbs and reflects light, Kamo'oalewa was thought to be a chunk of lunar material that a meteor impact ejected from the Moon’s surface long ago.
However, more recent analysis suggests it may simply be a common type of stony asteroid, a so-called LL chondrite, whose surface has been severely weathered by the harsh space environment.
To shed light on this confounding mystery, in May 2025 the China National Space Administration launched a scientific probe, Tianwen-2, an ambitious mission to study the asteroid at close range and return samples to Earth. Its mission is technically demanding. Kamo'oalewa rotates once every 28 minutes, making sample collection challenging.
The spacecraft is now getting close to its destination. Observations from a ground station in Bochum, Germany suggest the probe briefly fired its propulsive engines to make some small manoeuvres, followed by a main burn on 7 June. It’s now believed to be in orbit around the asteroid, with science operations expected to commence in the first week of July.
Tianwen-2 will attempt several approaches to collect samples for detailed laboratory analysis on Earth. It will hover over the object’s surface as it sucks up loose dust, briefly touch down to collect a more sizeable rock sample, and attempt to fire robotic extensions into the subsurface rock layer.
So what exactly is a quasi-satellite, why has Kamoʻoalewa puzzled astronomers, and why are samples collected by Tianwen-2 likely to succeed where even our most powerful telescopes have fallen short?
Not quite a moon
Quasi-satellites never become true moons. The Moon orbits Earth because it’s gravitationally bound to it, but quasi-satellites remain gravitationally bound to the Sun.
However, from our vantage point they often appear to orbit our planet, sometimes for centuries. Kamoʻoalewa’s orbit around the Sun is expected to remain in lockstep with Earth’s for a remarkably long time.
Although it appears to accompany Earth, the asteroid never comes particularly close to us. Long-term orbit analyses have allowed astronomers to conclude that Kamoʻoalewa entered its current quasi-satellite orbit some 100 years ago.
At its closest approach, on December 27 1923, Kamoʻoalewa was 12.44 million kilometres from Earth; by late May 2369, it will have reached a distance from us of twice the Sun–Earth separation.
Valuable samples
The reason we don’t know for sure where Kamoʻoalewa came from is because it’s so difficult to observe distant objects from Earth.
Astronomers are good at figuring out what an object in space might be made of thanks to a fundamental aspect of physics – the wavelengths of light (or spectra) reflected from object surfaces carry information about the chemical elements present.
Early spectra of Kamoʻoalewa looked a lot like those of materials from the Moon. More recent observations have challenged that interpretation, suggesting instead that prolonged space weathering – just existing in and being exposed to the harsh environment of outer space – may have tricked the observers.
From a distance, we can only study an object’s surface. And space weathering can change the appearance of that surface very dramatically. So when we get a sample from below the surface of an asteroid, analysing that can be invaluable in determining its true nature and origin.
Previous missions, such as Japan’s Hayabusa-2 and NASA’s OSIRIS-REx, demonstrated the value of close-range observations and sample returns. Laboratory analysis can reveal mineral compositions, isotopic signatures and other clues that are impossible to measure from Earth.
Both missions revealed unexpected details about the asteroids they targeted that we couldn’t have inferred from Earth-bound observations.
Next steps
If current expectations are on target, Tianwen-2 will begin its most important scientific observations on or around July 4 2026. Images, measurements and sampling attempts may quickly provide new clues about Kamoʻoalewa’s origin.
The returned samples, however, will provide the ultimate test. Once analysed in the lab, they should allow scientists to determine whether the asteroid is indeed a fragment of the Moon or, instead, an ordinary asteroid from the Solar System’s main asteroid-belt family, transformed by millions of years of weathering in the harsh space environment.
Either conclusion would be scientifically valuable. Ruling out a popular hypothesis can be just as important as confirming it.
Kamoʻoalewa may soon lose some of its mystery, but in doing so it could teach us something new about the evolutionary history of the Earth–Moon system and the dynamic environment of the early inner Solar System.
This article was originally published on The Conversation. Read the original article.
