A space debris shield of hollow metal eggs cut a projectile’s speed by 64.9 percent in simulations, well past the 51 percent that a flat plate managed.

Spacecraft have been armored against space debris the same basic way for almost 80 years. Out ahead of the hull stands a thin sheet of aluminum, with a gap behind it.

Before a scrap of orbital debris can reach anything that matters, it hits that sheet, breaks up, and sprays into a cloud of smaller fragments.

The approach works well against small debris, but larger fragments pose a much greater challenge.

So a team in China packed the gap with something strange: hollow, aluminum shells, each one shaped like an egg and filled with water.

In their simulations, a marble-sized ball 0.43 inches (11 millimeters) across lost 64.9 percent of its speed crossing that sandwich. Through a plain aluminum plate, it lost 51 percent.

The researchers built physical versions of the panels using 3D printing, but the high-speed impacts were simulated on a computer.

Orbit is full of fast space debris

Roughly 34,000 pieces of debris larger than 4 inches (10 centimeters) are circling in near-Earth orbit.

Another 900,000 or so measure between 0.4 and 4 inches (1 and 10 centimeters), and something like 128 million are smaller than that.

NASA’s Orbital Debris Program Office and the European Space Agency’s Space Debris Office both track that population, though the smallest fragments are the most difficult to spot.

Speed is what turns a small piece into a threat. Objects in low Earth orbit typically travel around 17,500 miles per hour.

When objects move toward one another, their relative collision speeds can be much higher.

A shield doesn’t have to stop something like that. It has to break it up and spread it out.

Thin plate takes the space debris first

The idea goes back to Fred Whipple, who proposed it in 1947. Engineers call the arrangement a Whipple shield, and it is still the working design for space debris protection.

That front plate is meant to be destroyed. Wrecking it is what turns one fast fragment into a wide, thin cloud.

It has a ceiling. A single aluminum plate protects well against debris under about a fifth of an inch (5 millimeters) across, and above that size the protection falls away.

Engineers have piled on fixes since. Two plates instead of one more than double the resistance, but they need a wider gap between them.

Aluminum honeycomb cores help as well, but they add weight and bulk.

Weight and bulk are the whole problem. Every pound of armor around a crew module or a satellite is a pound that has to be launched.

Eggshells collapse one after another

Before designing anything in metal, the team crushed two eggs in a testing machine. The two failed at about 82 pounds of force each (363 and 365 newtons), which is a lot for a shell that cracks on the edge of a bowl.

Yuxin Wang, who led the work at Dalian University of Technology, copied that shape in aluminum.

Each unit is a hollow metal egg about half an inch long (12 millimeters) with walls 0.04 inches thick (1 millimeter). Every one is full of water.

An egg on its own is still fragile, and inside an array that hardly matters.

“A single eggshell breaks easily under local force, but the protection mechanism of the eggshell array is completely different,” Wang said.

Under impact the shells crush in sequence rather than all at once. Each one deforms, absorbs part of the energy, and hands the rest to its neighbors.

Engineers have copied biological structures before, including in aircraft design, and this is that instinct directed at orbit.

The water matters too. Inside each shell it reflects the impact waves in many directions, and the curved wall pushes part of the load sideways instead of straight through toward the hull.

Narrow ends offer the best protection

An egg has a blunt end and a narrow end, so the team tried three arrangements. All narrow ends against the front plate, all blunt ends against it, and the two alternating.

Narrow ends won. In that layout the projectile came out the back at 5,890 miles per hour (2,633 meters per second). Through the plain plate it was still moving at 8,230 miles per hour (3,680 meters per second).

Blunt ends and the alternating layout landed in between. Water-filled aluminum spheres, run as a comparison, beat bare aluminum and lost to every egg arrangement.

The holes tell the same story. The egg panels tore open in ragged, petal-shaped perforations up to 1.8 times the width of the clean circle punched through the plain plate.

A bigger hole sounds worse, and here it means the energy went sideways instead of driving straight on through.

The panels were never gun-tested

Firing a marble-sized projectile at 16,800 miles per hour (7.5 kilometers per second) takes a light-gas gun, and those tests are costly. So the impacts were simulated instead.

The method was checked against two published gun experiments first. In a lead-on-lead test, the simulated debris cloud came within 3.5 percent of the measured one.

Predicted hole sizes in aluminum landed within 0.2 to 6.0 percent of the real ones.

What the model can’t do is follow a single crack. It judges whether material breaks and perforates. Tracking a crack from start to finish would need a finer model and the team hasn’t built one yet.

The researchers did build physical versions of the panels. The shells were 3D printed in aluminum powder, filled with water, and laser welded between the plates.

However, the panels have not yet undergone impact testing, so the comparison of the three layouts is based entirely on computer simulations.

The full study was published in the Journal of Applied Physics.

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