1. The desert that takes back what it is given

Frank Herbert’s Dune returns as the recalled work because the analogy is too precise to ignore. Arrakis is a planet that strips water from every exposed surface. Low Earth orbit is a vacuum that strips volatiles from every exposed material. The loss is slow, but it is relentless, and what leaves one surface must land on another.

This entry reads about outgassing and contamination in the vacuum of space.

2. What outgassing is

Outgassing is the release of volatile substances from a material when it is exposed to vacuum. The volatiles can be:

  • Adsorbed water and gases trapped during manufacturing or storage.
  • Unreacted monomers, solvents, or catalysts left in polymers and adhesives.
  • Plasticizers, stabilizers, and low-molecular-weight fragments.
  • Degradation products from UV, radiation, or thermal aging.

In orbit, these molecules travel in straight lines until they strike a surface cold enough to condense them. The result is contamination where it is least wanted.

3. How it is measured

The standard screening test is ASTM E595. A material sample is heated to 125°C at less than 10⁻⁵ torr for 24 hours. The test reports three numbers:

  • Total Mass Loss (TML): the percentage of original mass lost.
  • Collected Volatile Condensable Material (CVCM): the percentage that condenses on a collector plate at 25°C.
  • Water Vapor Regained (WVR): the mass regained when the sample is re-exposed to humid air.

The commonly used NASA screening limits are TML < 1.0% and CVCM < 0.1%. These are screening thresholds, not mission guarantees.

4. Where contamination does damage

Outgassed molecules are not equally harmful everywhere. The most sensitive targets are:

  • Optical surfaces: lenses, mirrors, and sensors can be dulled or darkened by molecular films.
  • Thermal control surfaces: contamination increases solar absorptance and reduces the effectiveness of radiators and coatings.
  • Solar cell cover glasses: a thin film reduces transmitted light and lowers power output.
  • Electrical contacts: condensates can insulate or corrode contacts over time.

As NASA’s degradation of spacecraft materials report notes, contamination sources include not only outgassing but also thruster products, vented gases, and debris from atomic-oxygen or UV interaction with materials.

5. What this changes

  • Outgassing is a material-selection problem, not only a cleanliness problem.
  • The standard screening test gives a first filter, not a final answer.
  • The next entry will read about how to mitigate outgassing and which material classes are safest.