Hunting for Lunar Debris Hiding Near Earth

China - Ekhbary News Agency

Hunting for Lunar Debris Hiding Near Earth

The Moon's pock-marked, cratered surface tells a story of a celestial body frequently bombarded by large space rocks. Scientists have long theorized that such impacts would inevitably eject debris into space, some of which might eventually be captured in orbits around Earth, becoming what are known as Lunar-Origin Asteroids (LOAs). Despite this theoretical abundance, the actual discovery of these LOAs has been surprisingly sparse, presenting a significant puzzle for astronomers. A new paper authored by Yixuan Wu and colleagues at Tsinghua University delves into the reasons behind this scarcity and explores how upcoming astronomical facilities, particularly the Vera Rubin Observatory, might revolutionize the search for these elusive objects.

The term 'rare' in scientific discovery often implies scarcity, but it doesn't mean non-existence. Recent media attention was captured by asteroid 2024 PT5, dubbed a 'temporary Moon,' which exhibits characteristics suggesting lunar origins. Another confirmed LOA, Kamo’oalewa, is already slated to be the target of a future Chinese asteroid sample return mission, underscoring the growing interest in these bodies. However, the scale of what might be out there is immense. Calculations presented in the new study estimate that potentially 500,000 LOAs, each approximately 5 meters in diameter, could be lurking in the cislunar space – the region between the Earth and the Moon.

It's important to contextualize this number. Even 500,000 LOAs would constitute only about 1% of the total population of Near-Earth Asteroids (NEAs) within that specific size range. The vast majority of NEAs originate from the asteroid belt, residing much farther out in the solar system. These asteroids are typically nudged into inner solar system trajectories through gravitational interactions or collisions within the belt. A key breakthrough highlighted in the Tsinghua University paper is the development of a method to distinguish LOAs from other NEAs without the need for costly and time-consuming spectral analysis of each individual object. The distinguishing factors lie in their velocity and direction of approach relative to Earth.

Researchers found that a typical LOA possesses a velocity relative to Earth of approximately 12.8 km/s. In contrast, other types of NEAs, particularly those originating from the asteroid belt, tend to have higher average velocities, around 17.5 km/s. While this velocity difference offers a valuable clue, it's not foolproof. The study notes that even at speeds as low as 2.4 km/s, there's still a significant 30% probability that an asteroid is of lunar origin, a probability more than 30 times greater than that for a randomly selected asteroid. Further differentiating LOAs is their orbital path; they tend to approach Earth from directions aligned with or opposed to the Sun's direction, effectively avoiding the 'leading' and 'trailing' edges of Earth's orbital path around the Sun.

These findings are the result of sophisticated computer modeling designed to simulate the formation and long-term orbital evolution of LOAs. The researchers meticulously modeled the Moon's impact history, tracking ejected particles over periods spanning up to 100 million years. Two distinct simulation scenarios were run: one assumed a steady, average rate of impacts over geological time, while the other focused intensely on the aftermath of a single, major impact event – the one that created the Giordano Bruno crater approximately 4 million years ago. Crucially, the models incorporated the Yarkovsky effect. This subtle thermal radiation force, caused by the anisotropic emission of heat from an asteroid's surface after absorbing sunlight, acts like an inefficient solar sail, gradually altering an asteroid's orbit over vast timescales.

The simulations yielded sobering insights into the survival rate of lunar ejecta. A significant portion, around 25%, fell back to Earth within the first 100,000 years, contributing to the known lunar meteorite collection. Over the full 100-million-year simulation period, only a small fraction, a mere 1.6%, of the initial ejecta remained in near-Earth space. The majority either landed back on Earth, returned to the Moon, or were flung into the wider solar system. Despite this low survival rate, the researchers argue that the remaining 1.6% is still sufficient to account for the estimated 500,000 LOAs hypothesized to exist.

The primary challenge, therefore, shifts from theoretical prediction to practical detection. Current sky survey instruments, such as Pan-STARRS and ATLAS, are not ideally suited for identifying these potentially faint and fast-moving LOAs. However, the forthcoming Vera Rubin Observatory, currently under construction in Chile, is expected to revolutionize this field. Its advanced capabilities are projected to detect approximately six LOAs per year, representing an order-of-magnitude improvement over existing survey efficiencies. While this is a substantial leap forward, it remains a relatively small number compared to the vast population of LOAs that likely populate our cosmic neighborhood.

Nevertheless, initiating the search is paramount. The study by Wu and colleagues provides a crucial framework for identifying these objects, marking a vital step in understanding the impact history of our closest celestial neighbor. Furthermore, by studying LOAs, scientists may gain deeper insights into the potential impact hazards posed by such bodies to our own planet.

Ekhbary News Agency