The Moon's battered surface tells a story of countless collisions with large rocks over its long history. Scientists believe that these impacts should have sent debris flying into space, and they're eager to track it down. However, despite predictions of an abundance of these Lunar-origin Asteroids (LOAs), very few have been discovered so far, leaving astronomers puzzled. A recent study by Yixuan Wu and colleagues from Tsinghua University sheds light on this mystery and suggests a way to find more LOAs with the help of the Vera Rubin Observatory.
The discovery of a "temporary Moon" at the end of 2024, an asteroid named 2024 PT5, sparked media interest as it appeared to be of lunar origin. Another LOA, Kamo'oalewa, is even the target of a future Chinese mission to collect asteroid samples. But according to Wu's calculations, there should be a staggering 500,000 more LOAs of similar size hiding near Earth's orbit. This is a significant number, considering it's only about 1% of the total Near Earth Asteroids (NEAs) in that size range.
The most intriguing aspect of the study is how it proposes to differentiate LOAs from other NEAs without the need for costly spectral data collection. The key lies in their velocity and direction. LOAs typically have a velocity relative to Earth of around 12.8 km/s, significantly slower than the average velocity of 17.5 km/s for other NEAs. However, this is not a foolproof method, as even at speeds as low as 2.4 km/s, there's only a 30% chance an asteroid is an LOA. Still, it's a much higher probability than for any random asteroid.
Another characteristic of LOAs is their approach direction. They tend to come from the sunward or anti-sunward direction, avoiding the leading and trailing edges of Earth's orbital path. This unique behavior provides a clue to their origin.
The researchers developed a model to study how LOAs are formed and what happens to them over time. They simulated the Moon being hit by asteroids and tracked the particles ejected into space for 100 million years. Two simulations were run: one assuming an average number of impacts over time, and another focused on the impact that created the Giordano Bruno crater, which formed around 4 million years ago. The model also included the Yarkovsky effect, a tiny force exerted on asteroids by reflected sunlight, which can significantly influence their orbital mechanics over millions of years.
Most of the ejecta from these impact events did not survive the 100 million-year timeline. About 25% fell to Earth within the first 100,000 years, becoming lunar meteorites. After the full simulation time, only 1.6% of the ejecta remained in near-Earth space, with the rest landing on Earth, back on the Moon, or being flung into the wider solar system. Even with this low survival rate, it's still enough to account for the estimated 500,000 LOAs.
The challenge now is to find these elusive LOAs. Current surveys like Pan-STARRS and ATLAS are not well-suited for detecting these fast-moving, low-magnitude objects. However, the upcoming Vera Rubin observatory in Chile is expected to make a significant improvement, potentially finding around 6 LOAs per year - a substantial increase over existing surveys.
Studying LOAs can provide valuable insights into the impact history of our closest celestial neighbor, the Moon. It may also help us understand the potential impact of similar rocks on our own planet. As we continue our hunt for these hidden lunar debris, we open a new chapter in our understanding of the solar system and our place within it.
