The Moon’s heavily cratered surface tells a powerful story of cosmic violence, serving as an archive of impacts that span billions of years. Unlike Earth, our nearest celestial neighbor lacks a substantial atmosphere to vaporize incoming space rocks and has no active geological processes like plate tectonics to erase impact scars. This means that nearly every collision, from the largest asteroid to the smallest speck of dust, leaves a permanent mark on the lunar crust. The resulting landscape records both the ancient solar system and the more recent, unintended consequences of human space exploration.
The Natural Impactors
The vast, circular scars covering the Moon are primarily the result of three types of natural objects: asteroids, comets, and meteoroids. The most intense period of lunar bombardment occurred roughly 4.1 to 3.8 billion years ago during the Late Heavy Bombardment (LHB). This era is responsible for forming the majority of the large impact basins visible today, though the exact timing and duration of the LHB remain a topic of scientific investigation. Analysis of lunar rock samples returned by the Apollo missions supported the theory of a spike in impact activity during this time.
Even today, the Moon is constantly exposed to a rain of smaller debris. Without the protection of an atmosphere, meteoroids—small fragments of asteroids or comets—strike the surface directly at high velocities. These impacts continuously pulverize the surface rock into a fine, powdery layer called regolith. The smallest of these particles, micrometeorites, vaporize upon impact, contributing to the tenuous layer of gas surrounding the Moon, known as the exosphere.
Recent Human-Made Impacts
In addition to natural space rocks, the Moon has become a receptacle for human-made debris, with recent events highlighting space junk as a new type of lunar impactor. These artificial collisions are significant to scientists because the composition and trajectory of the impacting object are often known, providing a valuable comparison to natural events. Intentional impacts have been used for scientific research, such as the 2009 Lunar Crater Observation and Sensing Satellite (LCROSS) mission. This mission deliberately crashed its spent Centaur rocket stage into a permanently shadowed crater near the lunar south pole to kick up a plume of debris.
The LCROSS impact allowed instruments to analyze the ejected material, successfully confirming the presence of water ice within the lunar soil. More recently, an unintentional event occurred on March 4, 2022, when a derelict rocket booster slammed into the far side of the Moon near Hertzsprung crater. Astronomers tracked the object, which was later confirmed to be the third stage of a Chinese Long March 3C rocket from a 2014 mission.
This particular piece of space junk created a distinctive double crater upon impact. The twin-crater feature was unusual and suggested that the rocket stage had a large mass concentrated at each end, possibly indicating an undisclosed internal payload. This event marked the first time a piece of space junk was confirmed to have accidentally collided with the lunar surface.
Tracking Current Lunar Impact Events
Scientists actively monitor the Moon to detect the flashes of light produced by ongoing, smaller impact events. This monitoring is primarily conducted using ground-based telescopes pointed at the dark portion of the Moon, an observational method employed by programs like NASA’s Lunar Impact Monitoring Project. When a meteoroid strikes, its kinetic energy instantly converts into heat and a brief, brilliant flash of visible light that can be recorded from Earth.
The use of two or more telescopes simultaneously observing the same area is a common technique to rule out false detections caused by cosmic rays striking a single camera sensor. This method has allowed researchers to observe hundreds of detectable impacts each year, helping to establish the current rate and size of meteoroids hitting the Moon. For larger impact events, orbital assets like the Lunar Reconnaissance Orbiter (LRO) are tasked with flying over the predicted impact site to photograph the fresh crater left behind, validating the ground-based observations.
Legacy Apollo mission seismometers also provided early data on lunar seismic activity, including moonquakes and impact tremors. Future lunar missions are expected to deploy new, more sensitive seismometers to better quantify the impact rate and the energy released by these collisions. Understanding the frequency and intensity of current impacts is an important factor for planning the safety and structural integrity of future human habitats on the Moon.