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Outgassing is the phenomenon which refers to the release of a gas that was dissolved, trapped, frozen or absorbed in some material. In vacuum the phenomenon of outgassing can happen because the gases which are trapped in material under atmosphere pressure can vent out. Materials that are not normally considered absorbent can release enough light-weight molecules to interfere with industrial or scientific vacuum processes. Moisture, sealants, lubricants, and adhesives are the most common sources, but even metals and glasses can release gases from cracks or impurities.

These gases can cloud various sensors such as CCD cameras. Outgassing is a challenge to creating and maintaining clean high-vacuum environments. In spacecraft, outgassing can condense onto optical elements, thermal radiators, or solar cells, sensors and obscure them.

Even the metals can release the trapped gases and can develop cracks in the structure at the very fine level which may cause harm by reducing the structural integrity at micron level, since they become the potential cause of weak structure upon impact.

Therefore spacecraft is baked before launch to remove this effect. Cleaning surfaces or baking individual components or the entire assembly before use can drive off volatiles. The Thermal Vacuum test is used to approve the materials which are proposed for use in spacecraft and associated equipment and for optical devices. The aim of this test is to determine the amounts of outgassing products. In all of these tests usually the material is exposed to high enough room temperature in a sealed environment and water or moisture contents are observed after a number of hours.

Low levels of outgassing are usually associated with high-quality materials as well as well-controlled manufacturing processes. Since moisture contents can severely effect the performance of the spacecraft electronics systems therefore highest level of care is made to select and use the material having lowest level of outgassing, different circuit-board materials from Rogers Corporation, for example, have been tested according to NASA test procedure SP-R-0022A for TML, CVCM, and WVR and found well suited for spacecraft applications.