Earth has acquired a group of cosmic stragglers that follow our planet in lockstep around the sun, taking the exact same time to complete an orbit as we do. These "co-orbitals"—small asteroids locked in a 1:1 orbital resonance with Earth—have puzzled astronomers for years with a deceptively simple question: where do they come from?
For decades, researchers assumed these space rocks wandered in from the main asteroid belt between Mars and Jupiter. But spectral analysis threw a wrench in that theory, revealing that some co-orbitals look suspiciously like the space-weathered lunar silicates that coat the moon's surface. This sparked an ongoing debate: are these objects ancient asteroid belt drifters, or are they fragments blasted from the moon by massive impacts?
A new study published in Icarus by researchers Elisa Alessi and Robert Jedicke offers strong evidence that the asteroid belt is the source—though soon, a spacecraft will settle the question definitively.
The most famous co-orbital is (469219) Kamo'oalewa, a space rock between 24 and 107 meters in diameter whose spectrum strikingly resembles the moon. Some researchers proposed it was ejected during the impact that carved the Giordano Bruno crater—a 22-kilometer-wide feature created somewhere between 1 and 10 million years ago. But launching a 50-meter rock into a stable quasi-satellite orbit around Earth requires an incomprehensibly massive amount of energy. According to Alessi and Jedicke's calculations, such an event should occur only once every 20 billion years—nearly double the current age of the universe. Their models put the probability that Kamo'oalewa has a lunar origin at just 21 percent.
To test their hypothesis, the researchers ran supercomputer simulations tracking 12,000 synthetic particles launched from the lunar surface at varying speeds and angles over millions of years. The result was striking: only about 70 objects larger than 10 meters would stabilize into the various types of co-orbital orbits—quasi-satellites, horseshoes, and "tadpole" orbits.
When the team modeled how objects drift from the main asteroid belt into near-Earth space using a simulation called NEOMOD3, they found a very different picture. Under similar conditions, the asteroid belt would supply roughly 1,600 co-orbitals. This means any given co-orbital larger than 10 meters has only a 4.3 percent probability of lunar origin.
The current sample of 57 known co-orbitals in that size range is too small to confirm these statistics, but scientists won't have to wait long for answers. Tianwen-2, launched in May 2025, is making its final approach to Kamo'oalewa, where it plans to collect 1 kilogram of surface samples and return them to Earth for detailed analysis. Either outcome will reshape our understanding: if Kamo'oalewa proves to be an asteroid belt stray, scientists must explain its puzzling lunar-like spectrum; if it's truly lunar silicate, the moon's impact mechanics and crater dynamics may need complete revision. The answer is coming soon.