Eggshell-shaped aluminum arrays cut spacecraft impact velocity by 65%

Near-Earth orbit holds nearly 1 million pieces of debris larger than 1 centimeter, according to the Journal of Applied Physics release describing the new study. A separate NASA estimate cited by Ars Technica puts the number of fragments between 1 and 10 centimeters at around 900,000, with roughly 34,000 pieces larger than 10 centimeters and about 128 million smaller than 1 centimeter. Even the smallest fragments can damage a spacecraft because of the extremely high speeds at which they travel. Ars Technica notes that in 2007 a piece of space debris punched a bullet-like hole through the radiator panel of the space shuttle Endeavour, an example of the kind of damage this debris field can cause.

Researchers at Dalian University of Technology in China set out to design a better shield against this kind of impact. Their approach, described in a paper in the Journal of Applied Physics, borrows its geometry from something familiar: the eggshell.

An eggshell seems like an odd model for strength, since a single egg breaks easily under a small, localized push. Study author Yuxin Wang said the mechanism changes when eggshells are arranged into a group. "The cooperative deformation of these eggshell units transforms the local impact load into distributed energy dissipation across the metastructure," Wang said, meaning that instead of one shell taking the full blow, the impact energy spreads out and each shell collapses in sequence rather than shattering all at once.

The team built its shielding as a sandwich: an array of small aluminum eggshells, filled with water, placed between two flat aluminum plates. The eggshells sit blunt side down, and each one is filled with water rather than left hollow. Under a high-speed impact, that water sloshes inside the shell and interferes with the shockwaves trying to pass through it, which further reduces and dissipates the impact energy.

To find the best design, the researchers compared three 3D-printed and simulated setups: plain aluminum plates with nothing between them, aluminum plates with water-filled aluminum spheres sandwiched inside, and aluminum plates with the water-filled eggshell array. They also tested different orientations of the eggs within the array. The configuration that worked best had the eggs standing upright, with the narrow tip touching the top plate — the same orientation, Ars Technica notes, that people use when walking across cartons of eggs without breaking them, since it distributes weight so no single egg bears the full load.

The water-filled eggshell array outperformed the alternatives in the tests. It reduced the velocity of an impact projectile by nearly 65%, compared with a 51% reduction for plain aluminum plates alone. The researchers ran both simulations and light-gas gun experiments — a laboratory method for firing projectiles at very high speed — on their 3D-printed arrays to reach this result.

This kind of layered shielding is a variation on what is known as a Whipple shield, a design already used to protect spacecraft, which places a thin outer layer some distance in front of the spacecraft wall so that incoming debris breaks apart before it can reach the hull. Some existing versions of this design, called "stuffed" Whipple shields, already include a filling between the rigid layers. The eggshell array is a new attempt to improve on that layered concept using a shape adapted from biology.

The researchers describe their result as an early step rather than a finished shield. Before the material could be used for lightweight shielding on spacecraft, its geometry and its water filling need further optimization and further impact testing, according to the study. The team says it is currently refining the thickness of the aluminum eggshells, their aspect ratio, and their layout within the array to improve how much energy the material can absorb.

Wang described the work as evidence that biology-inspired, lightweight structures are a promising direction for protecting spacecraft against very fast impacts, and said the team hopes the research draws more attention to bio-inspired protective structures generally. The claim is limited to what the tests so far have shown, since the design has not yet been optimized or tested beyond the current 3D-printed and simulated setups.