The secret to protecting next-gen spacecraft might be eggshells

保护下一代航天器的秘诀可能在于蛋壳

In 2007, a piece of space debris punched a bullet-like hole through the radiator panel of the US space shuttle Endeavor. The shuttle program ended in 2011, but the space debris problem has only intensified as we launch more and more satellite constellations, telescopes, and spacecraft into orbit. That’s why Chinese scientists have devised a new aluminum material inspired by eggshells that they believe could offer enhanced protection against debris fragments, according to a new paper published in the Journal of Applied Physics.

2007年，一块太空碎片在美国“奋进号”航天飞机的散热板上撞出了一个子弹般的孔洞。航天飞机项目于2011年结束，但随着我们向轨道发射越来越多的卫星星座、望远镜和航天器，太空碎片问题反而愈演愈烈。据发表在《应用物理学杂志》上的一篇新论文显示，中国科学家受蛋壳启发，研发出一种新型铝材料，他们认为这种材料能为航天器提供更强的防护，以抵御碎片撞击。

Eggshells have long fascinated scientists because of their mechanical properties. For instance, it’s well known that cracking an egg requires applying just enough force to the center to achieve a clean break without completely shattering the shell. In 2012, MIT mechanical engineer Pedro Reis co-authored a paper demonstrating the link between an egg’s ovoid geometry and its rigidity, a major factor when predicting how much force an object can endure before cracking. (As I wrote for Slate at the time, rigidity is related to, but distinct from, strength. If one eggshell has tiny cracks and the other doesn’t, both shells have different strengths—the cracked one will break more easily—but the same rigidity.)

蛋壳因其独特的力学性能长期以来一直令科学家着迷。例如，众所周知，要敲开一个鸡蛋，只需在中心施加恰到好处的力，就能实现干净利落的破裂，而不会让蛋壳完全粉碎。2012年，麻省理工学院机械工程师佩德罗·雷斯（Pedro Reis）合著了一篇论文，论证了鸡蛋的卵形几何结构与其刚度之间的联系，这是预测物体在破裂前能承受多大力的一个重要因素。（正如我当时为《Slate》撰文所写，刚度与强度有关，但两者并不相同。如果一个蛋壳有细微裂纹而另一个没有，两者的强度不同——有裂纹的更容易破碎——但它们的刚度是相同的。）

Reis started studying eggshells after participating in a popular physics demonstration: walking on cartons of eggs without breaking them. The key, he learned, was to align the eggs with their narrow tip (the most crack-resistant part) pointing up, and then carefully place one’s feet to distribute one’s weight over the entire surface area. This ensures that no single egg is overloaded. While it takes around 5.5 pounds of force to crack an egg, that depends on the direction in which the force is applied, as well as its distribution over the shell’s surface.

雷斯是在参加了一个流行的物理演示后开始研究蛋壳的：在鸡蛋盒上行走而不踩碎它们。他了解到，关键在于将鸡蛋的尖端（最抗压的部分）朝上排列，然后小心地放置双脚，将体重分散到整个表面积上。这确保了没有任何一个鸡蛋会超负荷。虽然敲碎一个鸡蛋大约需要5.5磅的力，但这取决于力的施加方向以及力在蛋壳表面的分布情况。

In the 1950s and 1960s, the aerospace industry found eggshells’ resistance to shattering to be a useful model when conducting failure analysis studies of the metal shells used to build airplanes. That crack resistance is because of their structure, which is similar to tooth enamel or seashells. An eggshell is mostly comprised of calcium carbonate crystals embedded in a protein matrix, further bolstered by a thin collagen inner membrane. It’s been described as “nature’s perfect packaging” for that reason.

在20世纪50年代和60年代，航空航天工业在对制造飞机的金属外壳进行失效分析研究时，发现蛋壳的抗碎裂性是一个有用的模型。这种抗裂性源于其结构，类似于牙釉质或贝壳。蛋壳主要由嵌入蛋白质基质中的碳酸钙晶体组成，并由一层薄薄的胶原蛋白内膜进一步加固。正因如此，它被称为“大自然的完美包装”。

Another popular demonstration is the classic egg-drop challenge, in which students must figure out how best to protect a single egg from breaking when dropped from a significant height. The longtime assumption has always been that dropping the egg vertically is best, which is consistent with Reis’s 2012 work. But earlier this year, Reis’s MIT colleague, Tal Cohen—whose department leads MIT’s annual egg drop challenge—decided to test that assumption. The team found that in situations where energy absorption is a critical factor, like the egg drop, horizontal eggs absorb more energy and can compress more under the same amount of force. Their follow-up experiments bore this out: horizontally positioned eggs cracked less frequently than vertical ones when dropped from the same height.

另一个流行的演示是经典的“鸡蛋坠落挑战”，学生们必须想出最好的方法来保护鸡蛋，使其从高处落下时不破碎。长期以来的假设一直是垂直坠落效果最好，这与雷斯2012年的研究一致。但今年早些时候，雷斯在麻省理工学院的同事塔尔·科恩（Tal Cohen）——其部门负责麻省理工学院年度鸡蛋坠落挑战——决定验证这一假设。研究小组发现，在能量吸收是关键因素的情况下（如鸡蛋坠落），水平放置的鸡蛋能吸收更多能量，并且在相同的力下能产生更大的压缩。他们的后续实验证实了这一点：在相同高度坠落时，水平放置的鸡蛋比垂直放置的鸡蛋破裂频率更低。

So eggshell physics is complicated, and scientists are still learning new things and finding novel applications for what they’ve learned—such as protection from space debris. A NASA report from 2021 estimated that there are around 34,000 pieces of debris larger than 10 centimeters in near-Earth orbit, about 900,000 pieces between 1 cm and 10 cm, and roughly 128 million pieces smaller than 1 cm. Even the smallest debris particles can damage spacecraft because of the high velocities they can achieve.

因此，蛋壳物理学非常复杂，科学家们仍在学习新知识并寻找其新颖的应用——例如抵御太空碎片。NASA 2021年的一份报告估计，近地轨道上有约34,000块大于10厘米的碎片，约90万块1厘米到10厘米之间的碎片，以及约1.28亿块小于1厘米的碎片。即使是最小的碎片颗粒，由于其能达到极高的速度，也可能损坏航天器。

Spacecraft are typically protected from debris impacts by some form of a Whipple shield, which has a thin outer bumper with a space between the bumper and the wall of the spacecraft. The bumper is designed to break up any incoming debris particles, spreading the impact energy over a larger area of the wall. There have been many iterations of the Whipple shield over the decades, including designs with multiple bumpers, or with a filling between the rigid layers (“stuffed” Whipple shields).

航天器通常通过某种形式的“惠普尔屏蔽”（Whipple shield）来抵御碎片撞击，这种屏蔽装置有一个薄薄的外部保险杠，保险杠与航天器壁之间留有空间。保险杠的设计旨在粉碎任何进入的碎片颗粒，将撞击能量分散到壁面的更大区域。几十年来，惠普尔屏蔽经历了多次迭代，包括多层保险杠设计，或在刚性层之间填充材料（“填充式”惠普尔屏蔽）。

The authors of this latest paper wanted to enhance the impact resistance of Whipple shields. They came up with a design similar in concept to the trick for walking across cartons of eggs, drawing on the favorable geometry and biomechanical properties of eggshells. The team 3D-printed three different aluminum designs: just aluminum plates, water-filled aluminum spheres sandwiched between aluminum plates, and eggshell structures filled with water. They ran multiple simulations measuring hypervelocity impacts to determine the most impact-resistant design and conducted light-gas gun experiments on their 3D-printed arrays. The water-filled eggshell-shaped arrays proved the most effective.

这篇最新论文的作者希望提高惠普尔屏蔽的抗冲击能力。他们借鉴了在鸡蛋盒上行走的技巧，利用蛋壳优良的几何形状和生物力学特性，构思出一种设计。研究小组3D打印了三种不同的铝制设计：纯铝板、夹在铝板之间的注水铝球，以及注水的蛋壳结构。他们进行了多次测量超高速撞击的模拟，以确定抗冲击性最强的设计，并对3D打印的阵列进行了轻气炮实验。结果证明，注水的蛋壳状阵列效果最好。