Innovative Solutions for Space Debris Protection
In 2007, a piece of space debris pierced the radiator panel of the US space shuttle Endeavour, with a diameter comparable to that of a bullet. While the shuttle programme concluded in 2011, the issue of space debris has only intensified as we launch an increasing number of satellite constellations, telescopes, and spacecraft into orbit. In response, Chinese scientists have developed an innovative aluminium material inspired by egg shells, which, according to recent research published in the Journal of Applied Physics, could provide enhanced protection against fragments of space debris.
Scientists have long been fascinated by the mechanical properties of egg shells. For instance, breaking an egg requires the application of the right amount of force at its centre to achieve a clean break without shattering the shell. In 2012, MIT mechanical engineer Pedro Reis co-authored a paper demonstrating the correlation between the egg’s ovoid geometry and its rigidity—a crucial factor in predicting how much force an object can withstand before breaking.
Reis became interested in egg shells after participating in a popular physics demonstration involving walking on egg cartons without breaking them. The secret lay in positioning the eggs with the narrower end facing upwards, which is the most resistant part of the shell. By carefully placing their feet to distribute weight evenly, participants ensured that no single egg was overwhelmed. While it typically takes about 2.5 kg of force to break an egg, this figure varies depending on the direction and distribution of the applied force.
The Composition and Strength of Egg Shells
During the 1950s and 1960s, the aerospace industry recognised that the breakage resistance of egg shells provided a useful model for conducting failure analysis on the metal casings used in aircraft construction. This remarkable resistance is attributed to their structure, which resembles that of dental enamel or seashells. Primarily composed of calcium carbonate crystals embedded in a protein matrix, egg shells are further reinforced by a fine inner membrane of collagen. Hence, they have been aptly described as “nature’s perfect packaging.”
Another well-known demonstration is the classic egg drop challenge, where students must devise the best method to protect an egg from breaking when dropped from a significant height. Traditionally, it has been believed that dropping the egg vertically is the optimal strategy, aligning with Reis’s findings from 2012.
However, at the beginning of this year, Tal Cohen, a colleague of Reis at MIT, decided to investigate this assumption further. The team discovered that in scenarios where impact absorption is critical, such as during a fall, eggs positioned on their sides performed better. When subjected to the same force, they deformed more and absorbed more energy. Experiments confirmed that when dropped from the same height, side-positioned eggs broke less frequently than those placed vertically.
Applying Egg Shell Science to Space Exploration
The physics behind egg shells is intricate, and scientists are continually uncovering new insights and innovative applications for their findings, including protection against space debris. A NASA report from 2021 estimated that there are approximately 34,000 fragments of space debris larger than 10 cm in low Earth orbit, around 900,000 pieces measuring between 1 cm and 10 cm, and about 128 million fragments smaller than 1 cm. Even the tiniest particles can inflict damage on spacecraft due to the high velocities they can achieve.
To safeguard spacecraft from debris impacts, various types of Whipple shields are employed, consisting of a thin outer layer with a gap between it and the spacecraft’s wall. The outer layer is designed to fragment incoming debris particles, dispersing the impact energy over a larger surface area. Over the decades, numerous versions of this shield have been developed, including designs with multiple outer layers or fillings between rigid layers.
Advancements in Impact Resistance Technology
To enhance the impact resistance of Whipple shields, researchers conceptualised a design reminiscent of the technique for walking on egg cartons, utilising the favourable geometry and biomechanical properties of egg shells. The team 3D printed three distinct aluminium designs: plain aluminium plates, water-filled aluminium spheres interspersed between plates, and egg shell-like structures filled with water. They conducted multiple simulations measuring hypervelocity impacts to identify the most resilient design and performed experiments using light gas guns on their 3D-printed matrices.
The water-filled egg shell structures proved to be the most effective, enduring high loads and reducing projectile velocities by nearly 65%. In contrast, the exclusive use of aluminium plates resulted in only a 51% reduction in speed. The presence of water plays a critical role in dissipating impact energy, as it prevents shock waves from propagating by moving within the shell.
The authors also tested various configurations of egg shell structures and found that the most effective arrangement involved positioning the eggs vertically with the narrow end in contact with the upper aluminium plate. Further experiments are required to refine the design and enhance the energy absorption capabilities of the material, but these initial results are promising.
Yuxin Wang, a co-author of the study and a researcher at Dalian University of Technology in China, remarked, “A single egg shell easily breaks under localised force, but the protective mechanism of the matrix of shells is entirely different.” Wang further explained, “The joint deformation of these structures allows for better distribution of impact force and energy dissipation, significantly increasing the resistance of the plates. Researchers hope this work will inspire the development of new protective structures based on nature.”