When a crewed spacecraft plunges into Earth’s atmosphere at speeds exceeding 7.8 kilometers per second, its surface temperature can soar to over 1,600°C; yet when it remains in orbit, the temperature on its sun-facing side can plummet to around –150°C. Even more daunting is the fact that outer space is awash with high-energy particle radiation and ultraviolet radiation—energies powerful enough to cleave the molecular bonds of most conventional materials.

In the face of such an extreme environment—characterized by stark contrasts between scorching heat and freezing cold, coupled with intense radiation bombardment—spacecraft require a specialized “skin” to safeguard their delicate onboard instruments and precious crew members. Among the many candidate materials, a seemingly ordinary organic polymer—silicone—has emerged as an indispensable “guardian” in the aerospace field, thanks to its unique molecular structure and continuously advancing manufacturing processes.

I. Coping with High Temperatures: The Transformation from “Soft Rubber” to “Hard Ceramic”

High temperatures are the greatest adversary of organic materials. Most plastics and rubbers decompose, melt, or even ignite above 300°C. However, specialty organosilicon materials employ a unique strategy: turning the enemy into an ally.

Core Mechanism: Ceramicization Transformation

Scientists have incorporated specific “ceramic-forming fillers” (such as mica and wollastonite) into silicone resins. When the temperature exceeds 600°C, the organic side chains of the silicone begin to decompose; however, the silicon–oxygen backbone (Si–O–Si) remains intact and reacts chemically with the fillers, rapidly forming a dense, hard ceramic layer on the material’s surface.

This ceramic layer is like “temporary armor” for the spacecraft. It serves three purposes:

Thermal insulation: Ceramics are poor thermal conductors, effectively preventing external high temperatures from transferring inward.

Reflective heat flux: The dense, vitrified layer can reflect a portion of the thermal radiation, thereby slowing down the rate of heat penetration.

Preventing combustion: the ceramic layer isolates oxygen, thereby preventing further oxidative decomposition of the internal materials.

Practical Applications: This ceramifiable organosilicon material is playing a critical role in the engine compartments of reusable launch vehicles, ablative materials for reentry capsules, and thermal protection coatings on the surfaces of hypersonic vehicles. Experimental data show that, under high-temperature flame impingement exceeding 1,000°C, an organosilicon coating only a few millimeters thick can remain intact for several hundred seconds without being burned through, thereby providing invaluable protection time for spacecraft during atmospheric entry.

II. Withstanding High Pressure and Extreme Cold: Remaining “Flexibly Compliant” at -120°C

Spacecraft must not only withstand extreme high temperatures but also maintain their sealing and vibration-damping performance under cryogenic and high-pressure conditions. Components such as rocket engine fuel lines, valve seals, and hatch seals are subjected to two extreme challenges: extremely low temperatures and extremely high pressures.

Molecular-level flexibility

Common rubbers, such as natural rubber and nitrile rubber, become hard and brittle at around –50°C, losing their sealing capability. In contrast, silicone polymers have a silicon–oxygen backbone, and the methyl groups (–CH₃) attached to the silicon atoms can rotate freely, enabling the polymer chains to maintain excellent flexibility even at extremely low temperatures—down to –120°C.

The Secret to Dealing with High Pressure

In high-pressure environments—such as rocket fuel pumps, where pressures can reach tens of megapascals—seal materials must exhibit both excellent compressive strength and sufficient resilience. Organosilicon materials meet these requirements through the following mechanisms:

Crosslink density control: By precisely regulating the number of chemical crosslinks formed during vulcanization, the material achieves both sufficient strength to withstand high-pressure extrusion and excellent elastic deformation capability.

Low compression set: A specialized silicone formulation can keep the compression set below 10%, meaning that even after prolonged compression, the material will quickly return to its original shape once the load is removed, thereby ensuring reliable performance for repeated use.

Practical applications: Specialty silicone materials are extensively used in the hatch seals of the International Space Station and in the life-support system tubing seals of SpaceX’s Dragon spacecraft. These materials consistently perform their sealing function under the combined effects of the intense vibrations and high pressures during rocket launch, as well as the extreme cold of the space environment.

III. Addressing Strong Radiation: Constructing a Molecular “Bulletproof Vest”

Space is permeated by high-energy particles—protons, electrons, gamma rays, and more—originating from the Sun and the far reaches of the universe. These high-energy radiations act like countless invisible “bullets,” capable of penetrating ordinary polymeric materials, breaking molecular chains, and causing cracking, pulverization, and a loss of mechanical properties. This phenomenon is known as radiation-induced aging.

Innate Advantages of Silicones

Compared with polymers featuring carbon–carbon backbones, organosilicon polymers with silicon–oxygen backbones exhibit significantly higher bond energies (the Si–O bond energy is approximately 452 kJ/mol, whereas the C–C bond energy is about 348 kJ/mol). This means that breaking a silicon–oxygen bond requires substantially more energy, thereby endowing organosilicon materials with inherently superior radiation resistance relative to most organic polymers.

The “combined punch” reinforced the day after tomorrow

To address the cumulative radiation doses that spacecraft accumulate in orbit over periods ranging from several years to several decades, researchers have also employed a variety of mitigation strategies:

Incorporation of radiation-resistant additives: Introducing “radical scavengers” (such as hindered amine compounds and aromatic compounds) into the silicone matrix. These additives act like a “sponge,” actively absorbing and neutralizing harmful free radicals generated by radiation, thereby preventing them from further attacking the polymer backbone.

Nano-fillert enhancement: Nano-silica, carbon nanotubes, and other nanofillers are added. These nanoparticles can act as a “physical barrier,” impeding the penetration depth of radiation particles within the material, while their large specific surface area facilitates the formation of numerous interfaces with the silicone matrix, thereby dissipating radiation energy.

Phenyl Modification: Introducing phenyl groups into the organosilicon molecular chain. The aromatic ring structure exhibits a conjugation effect, enabling efficient absorption and dissipation of high-energy radiation, thereby significantly enhancing the material’s radiation resistance.

Actual data: A specially engineered phenyl silicone rubber retains more than 80% of its original mechanical properties even after exposure to γ-ray irradiation exceeding 10⁶ Gy (gray, the unit of absorbed radiation dose; 1 Gy = 1 joule per kilogram). In contrast, conventional silicone rubber may completely pulverize under the same dose.

IV. Integrated Application: Transitioning from “Supporting Role” to “Core”

It is precisely because silicone possesses a unique combination of exceptional properties—high-temperature resistance, low-temperature resistance, radiation resistance, and high-pressure resistance—that its applications in the aerospace sector are shifting from auxiliary materials to functional core materials.

Conclusion: The Promising “Space Guardian” of the Future

From its earliest incarnation as a “joint sealant” to its current role as a “multi-functional protective material,” the evolution of silicones in the aerospace sector is itself a testament to humanity’s relentless pursuit of technological advancement in the face of extreme environments. As missions such as reusable spacecraft, deep-space exploration, and long-term space station operations continue to expand, the demands on material performance will only grow ever more stringent.

Future organosilicon materials are advancing toward higher temperature resistance—withstanding instantaneous temperatures exceeding 2,000°C—enhanced radiation shielding, and longer service life. Perhaps in the near future, when humans land on Mars, organosilicon will still be a key component of the sealing materials that protect astronaut habitats. This material, composed of silicon, oxygen, and simple hydrocarbon groups, will continue to serve as a quiet yet indispensable “guardian” on humanity’s journey among the stars and across the cosmos.