🔭 astrophysics

Assessment of the 5 August 2026 Falcon 9 Upper-Stage Lunar Impact: Energetics, Crater Scaling, and Comparison with LRO Observations

This study characterizes the 2026 impact of a SpaceX Falcon 9 upper stage on the Moon, demonstrating that while standard scaling laws overpredicted the resulting crater size, empirical analogs accounting for the rocket's hollow, elongated geometry accurately matched the 18-meter diameter crater subsequently observed by the Lunar Reconnaissance Orbiter.

Original authors: Mavia Anjum

Published 2026-08-25

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Mavia Anjum

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

For decades, the Moon has served as a silent witness to human activity, accumulating a growing collection of discarded spacecraft and spent rocket stages. While most of these objects simply drift in orbit, some eventually fall to the lunar surface, creating fresh craters. Unlike natural meteorites, which strike the Moon with unknown masses and speeds, these human-made impacts are controlled experiments. Scientists know exactly what the object is, how heavy it is, and how fast it is moving. This unique knowledge turns every artificial impact into a precise test of how energy transforms into a hole in the ground. By studying these events, researchers can refine the rules that govern crater formation, helping them understand not only the Moon's surface but also the physics of high-speed collisions in general.

On August 5, 2026, a specific piece of space debris provided a perfect opportunity for such a test. A discarded upper stage from a SpaceX Falcon 9 rocket, left in a chaotic orbit after a mission in early 2025, struck the Moon near the Einstein crater. The object was a hollow, thin-walled aluminum tank, roughly 12.6 meters long and 3.66 meters wide, weighing about 4,000 kilograms. It hit the surface at a speed of 2.43 kilometers per second. Before the impact, ground-based telescopes tracked the spinning, elongated shape of the rocket stage, allowing scientists to predict where it would land and what kind of crater it might make. The goal was to see if the standard mathematical rules used to predict crater sizes could accurately describe what happens when a hollow, lightweight tube hits the ground, or if the object's strange shape would throw those rules off.

To answer this, researchers first calculated the sheer force of the collision. The rocket stage carried a kinetic energy equivalent to roughly 2.8 tonnes of TNT. With this energy figure in hand, they applied two different methods to estimate the size of the resulting crater. The first method treated the rocket as a solid, compact ball of metal, using standard scaling laws that assume the object is a dense, uniform sphere. This approach predicted a massive crater, somewhere between 46 and 57 meters wide. The second method took a more practical approach, comparing the Falcon 9 impact to previous, measured impacts of similar rocket bodies. This empirical method suggested a much smaller crater, likely between 20 and 30 meters wide, with a best guess of 25 meters.

The true test came a week later, between August 11 and 12, when the Lunar Reconnaissance Orbiter (LRO) photographed the impact site. The images revealed a crater approximately 18 meters in diameter and less than 3 meters deep. The reality was significantly smaller than the first prediction. The standard "solid ball" model had overestimated the crater size by a factor of nearly three, suggesting that treating a hollow rocket stage as a solid object is a fundamental error. Even the more careful comparison method, which initially predicted a 25-meter crater, was still too large by about 39 percent.

The key to understanding the discrepancy lay in the specific shape of the impactor. The researchers realized that the hollow, elongated geometry of the rocket stage was the dominant factor. Because the mass was concentrated at the engine end and the rest of the structure was empty, the object did not transfer its energy to the lunar surface as efficiently as a solid object would. When the scientists adjusted their comparison method to look at a previous impact that produced a double crater, they found that if they considered only one of the two lobes—matching the single-crater nature of this event—the predicted size dropped to exactly 18 meters. This matched the observed crater perfectly.

The study concluded that for hollow, tube-like objects, the standard rules of crater scaling fail because they cannot account for how the object's shape affects the impact. The most reliable way to predict the size of such a crater is not to treat the object as a generic sphere, but to find a similar past impact and use that as a direct reference. The 18-meter scar left by the Falcon 9 stage serves as a clear reminder that in the physics of space debris, the shape of the object matters just as much as its speed or weight.

Technical Summary: Assessment of the 5 August 2026 Falcon 9 Upper-Stage Lunar Impact

Problem Statement
The study addresses the challenge of predicting the dimensions of craters formed by artificial space debris impacting the Moon. While natural meteoroid impacts are common, the mass, velocity, and geometry of human-made objects are often known with greater precision, offering controlled "cratering experiments." However, standard impact-scaling laws often assume compact, solid bodies, which may not accurately represent hollow, elongated rocket stages. This paper evaluates the accuracy of theoretical scaling relationships and empirical analogs by characterizing the impact of a discarded SpaceX Falcon 9 upper stage on August 5, 2026, and comparing pre-impact estimates against the crater subsequently measured by the Lunar Reconnaissance Orbiter (LRO).

Methodology
The research utilized a multi-faceted approach to characterize the impactor and estimate crater dimensions:

Impactor Characterization: The impactor was identified as the second stage of a Falcon 9 (v1.2 Block 5) rocket, an elongated, rotating, hollow aluminum-lithium cylinder (approx. 12.6 m length, 3.66 m diameter) with a nominal dry mass of 4,000 kg. The impact velocity was adopted as 2.43 km s⁻¹ at an angle of approximately 34° from the vertical.
Energetics Calculation: Kinetic energy and linear momentum were calculated using standard equations. The kinetic energy was converted to TNT equivalents for comparative analysis.
Theoretical Scaling (π-Group): The study applied the compact-body π-group scaling relationships (Collins et al. 2005; Holsapple 1993) to estimate the transient and final rim-to-rim crater diameters. Two bounding cases were modeled: a solid aluminum sphere of equal mass and a low-density body distributed over the full stage diameter.
Empirical Analog Scaling: Crater diameters were estimated by scaling from measured artificial impacts, specifically the Apollo S-IVB stages and the March 2022 rocket-body impact (which produced a double crater). The Falcon 9 impact energy was compared to these analogs using a power-law relationship.
Observational Validation: Estimates were validated against high-resolution images acquired by the LRO Narrow Angle Camera (NAC) between August 11 and 12, 2026, which provided the first direct measurements of the new crater's diameter and depth.

Key Results

Impact Energetics: For a mass of 4,000 kg, the impactor possessed a kinetic energy of $1.18 \times 10^{10}$ J (approx. 2.8 tonnes of TNT) and a linear momentum of $9.7 \times 10^6$ kg m s⁻¹.
Theoretical Scaling Discrepancy: The compact-body π-group scaling significantly overpredicted the crater size. Depending on the assumed density model, the predicted final diameter ranged from 46 m to 57 m. This represents an overprediction by a factor of 2.6 to 3.2 compared to the observed 18 m crater.
Empirical Analog Performance:
- Using the combined dimensions of the 2022 double crater (28 m) or Apollo S-IVB craters yielded estimates of 26–27 m, overpredicting the observed size by 44–50%.
- However, when the 2022 event was treated as a single lobe (18 m) rather than a combined double crater, the empirical scaling yielded an estimated diameter of approximately 18 m.
Observed Crater: The LRO measured the final crater to be approximately 18 m in diameter and less than 3 m deep. The crater exhibited a V-shaped ejecta pattern consistent with an oblique impact.
Geometry Factor: The study determined that the hollow and elongated geometry of the rocket stage, rather than a single equivalent bulk density, is the principal factor governing the reduced crater size. The hollow structure couples kinetic energy to the lunar regolith less efficiently than a compact body of equal mass.

Significance and Claims
The paper claims to provide the first direct comparison of impact-scaling methods against a measured artificial lunar crater where the impactor's identity and trajectory were known. The primary contribution is the demonstration that standard compact-body scaling laws are inappropriate for hollow rocket stages, leading to substantial overpredictions of crater size.

The study concludes that mass-matched empirical analogs are the more reliable predictor for such impacts, provided the analog's morphology (e.g., single vs. double crater) matches the impactor's configuration. By treating the nearest-mass analog as a single crater, the estimate closely reproduced the observed dimensions. This work highlights that the internal mass distribution and geometry of artificial impactors are critical variables in cratering physics, offering a refined framework for interpreting future artificial lunar impacts and space debris hazards.

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