What happens when a SpaceX rocket hits the Moon?

The collision of a SpaceX Falcon 9 rocket's upper stage with the Moon, expected in early March 2022, will create a new crater and scatter lunar material. This event, the first of its kind involving a human-made object of this size, raises questions about the long-term effects of space debris and the potential for future, unintended lunar impacts.

The specific object is the third stage of a Falcon 9 rocket that was launched in February 2015 for a mission to send NASA's Deep Space Climate Observatory (DSCOVR) into orbit. After completing its primary task, the rocket stage was left in a chaotic orbit, gradually influenced by gravitational pulls from Earth and the Moon. Over the years, its trajectory has shifted, leading to its current path towards a lunar impact. Scientists have been tracking this object for some time, and its inevitable collision course with the Moon was predicted.

The Background: A Growing Problem of Space Debris

The increasing amount of space debris orbiting Earth and beyond poses a significant challenge to space exploration and operations. This particular Falcon 9 upper stage, weighing approximately four tons, is not the first piece of human-made debris to reach the Moon. However, it is the largest and most massive object to date that is expected to impact the lunar surface unintentionally. Previous objects, such as the Ranger 4 probe in 1962, intentionally impacted the Moon, but this SpaceX rocket stage is a derelict piece of hardware.

The issue of space debris has been a growing concern since the dawn of the space age. Thousands of defunct satellites, spent rocket stages, and fragments from collisions or explosions now litter Earth's orbit. While most of this debris remains in Earth orbit, some objects, like this Falcon 9 stage, can be perturbed into trajectories that could lead them to impact other celestial bodies. This incident highlights the need for better tracking and mitigation strategies for space junk, especially as more missions venture further into the solar system.

The Mechanism: How a Rocket Stage Reaches the Moon

The Falcon 9 rocket's upper stage followed a complex orbital path after its initial mission. Launched in February 2015, its journey took it beyond Earth orbit. The gravitational forces of Earth and the Moon, along with solar radiation pressure, continuously altered its trajectory over approximately seven years. Unlike objects in stable Earth orbits, which can be de-orbited or managed, this stage entered a highly elliptical orbit that eventually intersected with the Moon's path.

Scientists at institutions like the Harvard-Smithsonian Center for Astrophysics have been using sophisticated orbital mechanics models to predict the impact. The object's trajectory is not perfectly predictable due to the myriad of subtle forces acting upon it, but the general path towards lunar impact has been clear for some time. The exact time and location of the impact are subject to slight variations, but the collision itself is considered a certainty.

Who is Affected and How, Concretely?

For the general public, the immediate effects will be minimal. There will be no observable changes on Earth, and the Moon's surface is vast enough that the impact will not be visible to the naked eye or even most telescopes from Earth. However, the primary impact will be on scientific understanding and future lunar activities. A new, relatively small crater, estimated to be about 20 meters (66 feet) in diameter, will be formed on the lunar surface.

This event provides a unique, albeit unintentional, opportunity for scientific study. NASA and other space agencies will be able to observe the impact's aftermath, analyzing the ejected lunar material to learn more about the Moon's composition and geology. It also serves as a stark reminder of the growing problem of space debris and its potential consequences for celestial bodies. For space agencies and private companies like SpaceX, it underscores the need for responsible space object disposal and the development of international guidelines to prevent such occurrences in the future.

What Happens Next, and What Would Have to Be True?

Following the impact, scientists will use lunar reconnaissance orbiters, such as NASA's Lunar Reconnaissance Orbiter (LRO), to study the newly formed crater and the ejected debris. This data will contribute to our understanding of lunar geology and impact processes. The incident is also expected to spur further discussions and potentially lead to the development of stricter international regulations regarding the disposal of space hardware, particularly for missions venturing beyond Earth orbit.

For future lunar impacts to be prevented, several things would have to be true. Firstly, there would need to be a greater global commitment to tracking and managing all space objects, regardless of their location. Secondly, international agreements and protocols for de-orbiting or safely disposing of rocket stages and satellites would need to be established and enforced. Thirdly, technological advancements in propulsion and orbital maneuvering would be required to allow spacecraft to perform controlled de-orbit burns or move to designated 'graveyards' in space, rather than being left in unstable orbits. Without these measures, similar unintentional impacts on the Moon or other celestial bodies remain a possibility.