Imagine a spacecraft landing on an asteroid to collect dust and small rocks. Its collection system has been carefully designed and tested, yet the material behaves differently than expected. Instead of flowing smoothly into a container, the tiny particles stick together and refuse to move.
This is not just a theoretical problem. Scientists are discovering that gravity plays a much more complicated role in the movement of rocky material than previously understood. In the low-gravity environments of asteroids, the Moon, and other celestial bodies, tiny attractive forces between particles can become powerful enough to stop granular material from flowing.
A research team led by Ian Madden has investigated this problem using laboratory experiments and computer simulations. Their findings could help engineers design more reliable systems for collecting asteroid samples, handling lunar soil, extracting resources in space, and preparing for future missions to Mars.
The Hidden Challenge of Handling Soil in Space
Rocky celestial bodies, including asteroids, moons, and planets, contain loose material known as regolith. This material consists of dust, small rocks, mineral fragments, and other particles formed through billions of years of impacts and geological processes.
On Earth, granular materials are everywhere. Sand flows through containers, grains move through industrial machines, and soil shifts under the influence of gravity. Scientists have spent decades studying how these materials behave.
However, the rules that work on Earth may not accurately predict what happens in space.
The biggest difference is gravity. On Earth, gravity continuously pulls particles downward, helping them move through openings and containers. On an asteroid, where gravity can be extremely weak, the weight of each particle becomes much smaller.
At the same time, particles can still attract one another through forces such as surface adhesion and electrostatic interactions. When gravity weakens, these forces can become much more important compared with the particles' weight.
As a result, material that flows easily on Earth may behave very differently in space.
The OSIRIS-REx Mission Revealed Why Predictions Matter
One important example comes from NASA's OSIRIS-REx mission, which visited the asteroid Bennu in October 2020.
The spacecraft collected material from Bennu using a specialized system called the Touch-and-Go Sample Acquisition Mechanism, or TAGSAM. On September 24, 2023, the spacecraft returned the sample capsule to Earth, delivering it to the Utah desert.
The mission returned approximately 121.6 grams of asteroid material, making it the largest asteroid sample ever brought back to Earth at that time.
Although the successful collection was a major achievement, the amount of material collected was difficult to predict precisely. Estimates had ranged from around 60 grams to as much as 120 grams.
Collecting more material than expected was a welcome outcome. Nevertheless, the difference between predictions and actual results highlighted a major engineering challenge: granular materials can behave unpredictably under the weak gravitational conditions found on asteroids.
Even when a collection mechanism passes tests designed to reproduce the expected operating conditions, the complex interactions between particles can produce unexpected results.
This makes it essential to understand how regolith moves, sticks together, and becomes trapped in different gravity environments.
Why Tiny Forces Can Become a Big Problem
To investigate the issue, the researchers focused on the movement of granular material through a hopper. A hopper is a container with a narrow opening at the bottom that allows material to flow out under gravity.
Hoppers are widely used in industries that handle grains, powders, sand, and other bulk materials. Engineers often use established mathematical relationships, including the Beverloo law, to estimate how quickly granular material will flow through an opening.
These models are useful on Earth, where gravity is relatively strong. However, they do not fully capture what happens when gravity becomes extremely weak and attractive forces between particles begin to dominate.
Consider a simple example.
On Earth, a handful of sand poured into a funnel usually moves downward because gravity pulls the grains through the opening. If the same material is placed in an environment with much weaker gravity, the grains exert far less weight on one another.
The attractive forces between neighboring particles do not necessarily decrease in the same way.
Consequently, the particles may form clusters, resist movement, or become trapped near the opening. This behavior is known as jamming. When the particles block the opening and prevent further movement, the resulting blockage is called clogging.
For a spacecraft attempting to collect or transfer lunar soil, such a blockage could interrupt an operation that depends on the reliable movement of material.
Scientists Test Lunar Soil Simulant Under Different Gravity Conditions
To investigate these effects, Ian Madden and his team studied a lunar regolith simulant called JSC-1A. A simulant is a material designed to reproduce important properties of real planetary soil for experiments on Earth.
The researchers examined how this material moved through a hopper under different gravitational conditions.
First, they conducted laboratory experiments under Earth's normal gravity. They also performed experiments under reduced gravity using the GraviTower Bremen Pro, a facility at the Center of Applied Space Technology and Microgravity in Bremen, Germany.
The team then developed computer simulations using a technique known as the discrete element method, or DEM.
Instead of treating the material as one continuous substance, DEM models track individual particles and calculate how they interact with neighboring particles, walls, and other surfaces. Researchers can adjust the simulated gravitational acceleration and examine how the material responds.
This approach allowed the team to explore conditions that are difficult to reproduce experimentally, including the extremely weak gravity found on asteroids.
The researchers compared their simulations with laboratory observations under Earth gravity. The qualitative agreement supported the use of the simulation framework to investigate the underlying particle interactions.
The Granular Bond Number Helps Explain the Behavior
A central concept in the research is the granular Bond number.
In simple terms, this number compares the attractive forces between particles with the force of gravity acting on them.
When gravity is strong relative to particle attraction, the material's weight can help particles move past one another. When gravity becomes weaker, particle attraction can become much more influential.
This comparison helps scientists understand why the same material may flow smoothly under one gravitational condition but become resistant to movement under another.
However, calculating this relationship is not always straightforward. Real regolith contains a mixture of minerals and particles of different sizes. Their surface properties and chemical compositions can influence how strongly they interact.
Even relatively small changes in particle properties or environmental conditions can therefore affect the material's overall behavior.
The researchers used their experimental and computational framework to examine how this balance changes as gravity decreases from Earth's level toward lunar and asteroid conditions.
Their results showed that granular flow is highly sensitive to the interaction between gravitational acceleration and cohesive forces.
What This Means for Future Space Exploration
The findings have important implications for upcoming space missions.
1. Better asteroid sample collection
Future spacecraft could use improved models to predict how asteroid dust and rocky fragments will behave during collection. This could help engineers develop mechanisms that are less vulnerable to unexpected material movement or blockages.
2. More reliable lunar construction
As space agencies plan to build infrastructure on the Moon, they will need to move and process large quantities of lunar soil. Understanding when regolith flows, sticks together, or jams could improve the design of excavation equipment and material transport systems.
3. Producing useful resources in space
In-situ resource utilization, or ISRU, involves using materials available on another celestial body rather than transporting everything from Earth. Regolith may eventually provide useful raw materials for construction and other applications. Reliable handling systems will be essential for making these processes practical.
4. Safer planetary defense experiments
Scientists also study how asteroids respond to impacts and other interventions intended to change their motion. A better understanding of granular material could help improve simulations of how an asteroid's surface and loose particles respond to external forces.
A Small Discovery With Major Implications
The research highlights a simple but important lesson: materials do not necessarily behave the same way when gravity changes.
A pile of dust that flows easily on Earth may resist movement in space because the forces holding its particles together become more significant than their weight.
By combining laboratory experiments with computer simulations, Ian Madden and his team have developed a way to investigate these complex interactions across a wide range of gravitational conditions.
Further research will be needed to translate these findings into reliable engineering designs for real missions. Different planetary soils, particle sizes, surface properties, and operating conditions may produce different outcomes.
As humanity prepares for more ambitious exploration of the Moon, asteroids, and Mars, understanding something as seemingly ordinary as dust could prove essential.
After all, a spacecraft may travel millions of kilometres through space, but if it cannot reliably move the material beneath its feet, even the most advanced mission could face an unexpectedly difficult problem.
Reference: Madden, I.P., Muruganandam, S., Missaoui, A. et al. Behaviors of lunar regolith simulants under varying gravitational conditions. npj Microgravity 11, 69 (2025). https://doi.org/10.1038/s41526-025-00501-z
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