Outgassing in space is the process where materials release trapped gases, vapors, and Volatile Organic Compounds (VOCs) when exposed to the vacuum environment. This phenomenon must be accounted for in the design and engineering of every piece of hardware launched beyond Earth’s atmosphere. The released gases can include common substances like water vapor, solvents, and plasticizers incorporated during manufacturing. Because the space environment lacks atmospheric pressure, these substances readily escape from the solid matrix of materials. This release of molecules begins immediately upon reaching orbit and is a primary consideration for mission planners.
The Mechanism of Outgassing in a Vacuum
The mechanism of outgassing is driven by the pressure differential between the material’s interior and the vacuum of space. Gases absorbed or dissolved within a solid matrix, such as a polymer, move toward the surrounding low-pressure environment. This molecular movement occurs through mechanisms including diffusion, desorption, and permeation. Diffusion involves the slow migration of trapped molecules from deep within the bulk material to the surface.
This process differs from simple evaporation because it involves substances chemically or physically bound within the solid. The outgassing rate is accelerated when materials are exposed to elevated temperatures, such as those caused by solar heating. Higher temperatures increase the vapor pressure of trapped substances, providing the energy needed for molecules to break free. Even seemingly inert materials, like certain metals and glasses, can release gases from microscopic cracks or impurities. Water vapor is often the dominant gas released initially, but heavier VOCs become the primary concern over time.
Common Sources and Materials in Spacecraft
Many materials used in satellites and probes are prone to outgassing. The primary sources are non-metallic, organic compounds that contain large amounts of Volatile Organic Compounds. These include polymers, such as plastics for structural components, and synthetic rubbers for seals and gaskets. Adhesives, paints, and thermal control coatings are also major contributors, often containing solvents or unreacted monomers. Specialized lubricants and wiring insulation must also be carefully selected.
To mitigate this problem, space agencies employ rigorous pre-flight testing on every material intended for flight. Standard procedures, such as the ASTM E595 test, screen materials by measuring the Total Mass Loss (TML) and the Collected Volatile Condensable Materials (CVCM) under vacuum and elevated temperature. To qualify as low-outgassing, TML must be less than 1.0 percent of the initial mass, and CVCM must be less than 0.1 percent.
Engineers often perform a thermal-vacuum bake-out procedure, heating components in a vacuum chamber to drive off volatile compounds before assembly. This controlled process reduces the material available to outgas once the hardware reaches space.
Impacts of Outgassing on Space Missions
The consequences of outgassing affect mission longevity and performance, primarily through molecular contamination. Gases released from warm spacecraft surfaces travel through the vacuum and condense as a thin film on any cold surface. This deposition is problematic on sensitive components, such as telescope mirrors, camera lenses, and detector windows, which are often cooled for maximum performance.
A thin layer of contaminant film on optics can scatter or absorb light, leading to a phenomenon known as fogging that reduces the clarity and throughput of an instrument. For example, several deep-space probes have experienced a gradual degradation in image quality due to a film coating the main camera’s sensor. Similarly, thermal radiators, which must efficiently shed heat to space, can become coated, increasing their solar absorptance and causing the spacecraft to overheat.
Operational Issues Beyond Contamination
The release of gases can also cause measurable operational issues. For sensitive scientific instruments, the continual loss of mass through outgassing can affect calibration and performance over a mission’s multi-year lifespan. The cloud of gas surrounding the spacecraft can interfere with measurements intended to analyze tenuous atmospheres, obscuring the natural signal. If the outgassing is non-uniform, the resulting recoil force from the stream of released gas can perturb the spacecraft’s orientation, requiring correction maneuvers.