Outgassing in Space Systems

Introduction

Outgassing is the release of absorbed, adsorbed, dissolved or trapped gases and volatile compounds from a material. It becomes particularly important in space because vacuum conditions allow released molecules to leave material surfaces and travel through the surrounding environment with few collisions.

Those molecules do not simply disappear. They can migrate through a spacecraft and condense on colder surfaces. Optical payloads, detectors, star trackers, thermal-control surfaces, radiators, mechanisms and scientific instruments can therefore be affected by contamination originating elsewhere in the spacecraft.

The risk is often gradual rather than immediately catastrophic. A small deposit can reduce optical transmission, alter surface absorptance or emittance, increase stray light, affect detector sensitivity or change the behaviour of a precision mechanism. The source may be an adhesive, lubricant, polymer, coating, cable material, cleaning residue or manufacturing contaminant that appeared stable under normal atmospheric conditions.

Within an ECSS programme, outgassing is addressed through materials and process selection, contamination budgeting, cleanliness control, material screening, design analysis, bakeout, thermal vacuum exposure and contamination monitoring. The objective is not merely to achieve a vacuum level. It is to demonstrate that the materials, hardware, facility and verification evidence are suitable for the mission and its contamination-sensitive surfaces.

Early assessment matters. Replacing an unsuitable material during design is relatively straightforward. Discovering the same problem after integration can create major cost, schedule and verification consequences.

Outgassing is the release of gaseous or volatile species from a material into its surrounding environment. The released species can originate from:

• Moisture absorbed from the atmosphere

• Molecules adsorbed onto material surfaces

• Residual solvents

• Plasticisers

• Unreacted constituents

• Lubricant fractions

• Cleaning residues

• Adhesive or coating products

• Products created by material degradation

• Compounds diffusing from within a material

Outgassing should not be confused with a leak. A leak introduces gas through an unintended opening or imperfect seal. Outgassing originates from the materials, surfaces and processes present within the vacuum environment.

Outgassing itself has no beneficial verification purpose. It is a physical behaviour that must be understood and controlled.

The engineering objectives are to:

• Identify potential contaminant sources

• Characterise the amount and nature of released material

• Predict where released molecules could travel

• Assess whether deposition could affect critical surfaces

• Reduce volatile content before flight

• Demonstrate compliance with project contamination requirements

• Generate traceable evidence supporting flight readiness

Programme phase	Typical outgassing and contamination activities
Design	Materials selection, contamination budgeting, view-factor assessment and protection of sensitive surfaces
Manufacturing	Process control, curing control, cleaning, approved materials and traceability
Assembly, Integration and Test	Controlled handling, purging, storage, witness samples, bakeout and chamber compatibility review
Verification	Material screening, analysis, TVAC exposure, contamination monitoring and documentation
Flight readiness	Verification closure, cleanliness declaration, unresolved-risk review and final contamination controls

Several mechanisms contribute to the release of material under vacuum.

Molecules attached to a surface can gain enough energy to leave that surface. Water is a common example because it is readily adsorbed onto many materials during storage and handling in normal atmospheric conditions.

Volatile molecules within a material can migrate through its bulk towards the surface. Diffusion-driven outgassing can continue long after easily released surface contaminants have been removed.

A material with sufficient vapour pressure can enter the gas phase. Liquids evaporate, while some solids can sublime directly into vapour. Reduced surrounding pressure increases the net loss of volatile material because fewer molecules return to the surface.

Heat, radiation, incomplete curing or chemical instability can break material constituents into smaller volatile species. These products may outgas even if the original material initially appeared acceptable.

Temperature strongly influences desorption, diffusion and vapour pressure. Increasing temperature generally accelerates molecular movement and increases the rate at which volatile species are released.

This principle is used during vacuum bakeout. Hardware is heated under controlled conditions so that volatile material is removed on the ground rather than during flight.

However, bakeout is not a repair method for fundamentally unsuitable materials. It can reduce the volatile inventory, but it does not automatically make a prohibited, unstable or mission-incompatible material acceptable.

In a sufficiently low-pressure environment, released molecules can travel in molecular flow. Their movement is then governed largely by:

• Source temperature

• Surface temperature

• Geometry

• Direct and reflected view factors

• Molecular sticking probability

• Residence time on encountered surfaces

• Re-evaporation from warmer surfaces

• The presence of cold or cryogenic surfaces

A warm internal panel can release material that later deposits on a colder optical element. The contamination source and affected surface may therefore be physically separated.

Cold surfaces are particularly vulnerable because molecules can remain on them for longer. Cryogenic detectors and optics may require substantially more restrictive controls than ordinary room-temperature hardware.

Outgassing screening data alone do not fully predict mission contamination. A useful assessment may also require material quantity, location, temperature history, surface geometry, mission duration and the sensitivity of the receiving surface.

Early materials assessment helps prevent contamination from being designed into the spacecraft. It also reduces the likelihood of late material substitution, repeated cleaning, additional bakeout or redesign.

ECSS contamination control requires a planned and documented approach. The current cleanliness and contamination-control standard includes requirements covering material screening, contamination-sensitive items, vacuum facilities, monitoring, documentation and verification.

The 2025 revision also clarifies project documentation milestones and introduces a cleanliness and contamination-control verification report and Cleanliness Declaration of Conformity at the relevant review stage.

Effective control produces objective evidence. Depending on the project, this may include:

• Approved material data

• Declared Materials Lists

• Screening certificates

• Bakeout records

• Chamber background data

• QCM measurements

• Witness-sample results

• Residual gas data

• TVAC records

• Contamination analysis

• Verification closure records

Contamination control protects hardware that may be impossible to clean, repair or recalibrate after launch, including:

• Cameras and telescopes

• Star trackers

• Infrared detectors

• Spectrometers

• Laser systems

• Solar arrays

• Radiators

• Thermal-control coatings

• Cryogenic surfaces

• Precision mechanisms

Characteristic	Sea-level environment	Space-representative vacuum
Ambient pressure	Approximately 101.3 kPa	Very low pressure
Gas convection	Present	Negligible
Molecular collisions	Frequent	Greatly reduced
Moisture behaviour	Can remain adsorbed or absorbed	Can desorb and migrate
Volatile loss	Often slower	Can increase substantially
Contaminant transport	Influenced by airflow	Dominated by molecular trajectories and surface interactions
Thermal transfer	Conduction, convection and radiation	Primarily conduction and radiation
Lubricants and polymers	May appear stable	Volatile fractions may be lost
Cold-surface deposition	Less dominant	Can become mission critical

Key lesson: A material that performs satisfactorily in a terrestrial environment is not automatically suitable for spacecraft use.

During early spacecraft development, engineers observed deposits, fogging and unexpected degradation on optical windows, detectors and thermal-control surfaces.

Investigations showed that apparently stable spacecraft materials could release volatile species after entering vacuum. Those molecules could then migrate and condense elsewhere, particularly on colder surfaces.

Adhesives, paints, wiring insulation, lubricants, polymers, process residues and cleaning products were identified as potential contributors. This led to the development of formal materials screening, cleanliness control, contamination budgets, controlled manufacturing processes, bakeout and contamination monitoring.

The result was a change in engineering approach. Cleanliness could no longer be treated simply as the visible removal of dust. Molecular contamination had to be considered throughout design, manufacturing, integration, testing and launch preparation.

ECSS-Q-ST-70-02 defines thermal vacuum outgassing screening for materials intended for spacecraft, launchers, payloads and associated equipment.

Typical screening parameters include:

• TML, Total Mass Loss: the percentage of original specimen mass lost during the defined exposure.

• RML, Recovered Mass Loss: mass loss after accounting for recovered water vapour.

• CVCM, Collected Volatile Condensable Material: the proportion of material collected on a controlled collector surface.

• WVR, Water Vapour Regained: moisture regained by the specimen during defined post-test conditioning.

These values support material selection but do not, by themselves, prove that an assembled spacecraft will remain contamination-free.

ECSS-Q-ST-70-01 addresses cleanliness and contamination control across the programme.

The 2025 revision requires outgassing screening to be performed in accordance with ECSS-Q-ST-70-02. It also states that outgassing data more than ten years old are not admissible for material selection and that screening is to be performed by a trusted test house capable of providing the appropriate conformity evidence.

The standard applies different criteria according to:

• Material quantity

• Temperature

• Proximity to sensitive items

• Direct and indirect view

• Presence of cryogenic surfaces

• Mission-specific contamination sensitivity

For materials near room-temperature sensitive items, the permitted CVCM level becomes more restrictive as the quantity of material increases. More stringent limits may be necessary where colder or cryogenic surfaces are present.

ECSS-Q-ST-70-71 addresses materials, processes and associated data selection. It requires material choices to satisfy mission performance and environmental requirements.

The standard includes specific restrictions and assessments for polymers, rubbers, elastomers, plasticisers and materials exposed to vacuum. Material selection must therefore consider more than a generic supplier data sheet or terrestrial service history.

ECSS-E-ST-10-03 defines requirements for test planning, specification, execution and documentation.

It links environmental testing to the wider verification and AIT framework. Test conditions, configuration, monitoring, tolerances, functional states, pass/fail criteria and reporting requirements must be defined before execution.

ECSS requirements are not applied blindly. The contamination-control approach should be tailored according to:

• Mission orbit and lifetime

• Hardware model philosophy

• Optical sensitivity

• Detector wavelength

• Operating temperatures

• Cryogenic surfaces

• Material quantities

• Internal geometry

• Contamination budget

• Customer requirements

• Launch-provider requirements

• Available verification evidence

Tailoring must be documented and approved. It should not be used to remove a requirement simply because compliance is inconvenient.

Customer, prime-contractor and launch-provider requirements may exceed the baseline ECSS position.

Examples include:

• Lower permitted deposition rates

• Specific prohibited-material lists

• Additional kinetic outgassing data

• Dedicated bakeout profiles

• Witness samples

• QCM acceptance criteria

• Molecular contamination budgets

• Purge requirements

• Special packaging and transport controls

• Additional cleanliness declarations

The contractual requirement takes precedence where it is more demanding.

• Identify contamination-sensitive and contamination-critical hardware.

• Review customer and mission requirements.

• Establish the contamination budget.

• Define material-screening and verification needs.

• Allocate responsibilities within the contamination-control plan.

• Identify required reviews and approval points.

• Compile the Declared Materials List.

• Confirm material quantities, locations and operating temperatures.

• Review adhesives, coatings, lubricants, tapes, cables and printed materials.

• Confirm cleaning and curing processes.

• Assess direct and indirect view of sensitive surfaces.

• Define bakeout, witness, QCM and RGA requirements.

• Review chamber cleanliness and blank-test evidence.

• Perform approved cleaning and handling activities.

• Conduct required material screening through a suitable test house.

• Perform controlled bakeout where specified.

• Verify chamber background and configuration.

• Execute the approved vacuum or TVAC profile.

• Monitor pressure, temperature, hardware status and contamination indicators.

• Record deviations and anomalies.

• Compare achieved conditions with the specification.

• Review TML, RML, CVCM or kinetic data where applicable.

• Assess QCM, witness or RGA results.

• Review hardware inspection and functional results.

• Evaluate anomalies and contamination excursions.

• Confirm traceability to the tested configuration.

• Complete the test report.

• Close non-conformances and review actions.

• Update the verification control documentation.

• Issue required cleanliness and conformity evidence.

• Confirm readiness for further integration or flight.

Material screening provides standardised data about volatile mass loss and condensable products. The data supports selection decisions but must be current, applicable to the material formulation and traceable to the actual product used.

The material list should identify materials, suppliers, grades, quantities, locations, curing conditions and supporting evidence. Generic descriptions such as “epoxy”, “silicone” or “plastic” are normally inadequate.

Bakeout uses controlled temperature, time and vacuum to accelerate the removal of volatile material.

A valid bakeout process requires:

• Defined temperature limits

• Defined pressure

• Hardware compatibility

• Controlled heating and cooling rates

• A justified duration or stopping criterion

• Chamber background assessment

• Contamination monitoring where required

• Traceable records

The 2025 contamination-control standard includes detailed expectations for vacuum facilities, chamber cleanliness, bakeout background and contamination monitoring.

TVAC testing exposes assembled hardware to controlled pressure and temperature conditions. It can reveal outgassing and contamination behaviour that is not apparent during material-level assessment.

TVAC is not automatically an ECSS-Q-ST-70-02 screening test. Material screening and assembled-hardware TVAC serve different verification purposes.

An RGA identifies gaseous species within a vacuum chamber. It can support:

• Leak investigation

• Vacuum-quality assessment

• Identification of water or hydrocarbons

• Observation of changing gas composition

• Investigation of potential contamination sources

An RGA does not directly measure all condensable contamination deposited on a critical surface.

A QCM or TQCM detects very small changes in deposited mass by monitoring the resonant frequency of a quartz crystal.

Its usefulness depends on:

• Location

• View factor

• Crystal temperature

• Sensitivity

• Background level

• Data interpretation

• Representativeness of the protected surface

ECSS-Q-ST-70-01 states that a mass spectrometer alone is insufficient for monitoring condensable contaminants. QCM and witness methods have specific roles in contamination-sensitive testing.

Witness samples are positioned to collect representative molecular or particulate contamination. They can then be examined using an agreed analytical method.

Their position, material, temperature and exposure duration must represent the surface or risk being assessed.

A material can satisfy a generic mass-loss criterion but still release a compound that is damaging to a particular detector, coating or optical wavelength.

Late identification of an unsuitable adhesive, coating or lubricant can require disassembly, material replacement, recuring, cleaning, bakeout and repeat verification.

Contamination is often inexpensive to prevent but expensive to remove. The greatest costs arise when the issue is discovered after integration.

Using old, untraceable or non-representative material data can create false confidence. A passing material certificate does not automatically cover a changed formulation, cure process or supplier.

Weak material descriptions, missing batch traceability, incomplete cleaning records and undocumented configuration changes can prevent verification closure.

The chamber, fixture, cabling, feedthroughs and support equipment can become contamination sources. A clean test item placed into an unsuitable chamber does not produce valid contamination evidence.

RGA, QCM and witness data require context. A numerical result without agreed limits, location, temperature and background information may not support a defensible conclusion.

Do not defer contamination control until TVAC booking. By then, materials may already be embedded within inaccessible assemblies.

Record the manufacturer, product, grade, batch, cure state and supporting outgassing evidence.

A small amount of material far from a sensitive surface may represent less risk than a larger quantity with direct molecular view of a cold optic.

Use covers, purging, temporary protection and controlled exposure where appropriate.

Incomplete curing can increase volatile content. The actual cure process must match the qualified condition.

Where possible, remove volatiles before sensitive hardware is integrated. Later bakeout may be limited by the least temperature-tolerant component.

Review chamber background, previous use, cleaning records, pump configuration, fixture materials and contamination-monitoring arrangements.

Do not wait until data are available before deciding what constitutes a pass.

The tested hardware, materials, fixture, cabling, software and operating state must be identified and traceable.

Unresolved contamination actions can become significantly more difficult after the next integration stage.

Typical documentation includes:

• Cleanliness Requirements Specification

• Cleanliness and Contamination Control Plan

• Cleanliness and Contamination Control Verification Report

• Declared Materials List

• Declared Process List

• Contamination budget

• Molecular contamination analysis

• Verification Plan

• AIT Plan

• TVAC Test Specification

• Bakeout Procedure

• Test Procedure

• As-run Procedure

• Test Report

• Verification Control Document

• Cleanliness Declaration of Conformity

• Non-Conformance Reports

• Requests for Waiver

• Requests for Deviation

The ECSS testing framework distinguishes between the test specification, which defines what must be achieved, and the test procedure, which gives the step-by-step method for execution. The completed procedure becomes part of the as-run evidence.

All applicable material, cleanliness, contamination and test requirements are addressed by approved evidence.

Predicted and measured contamination remains within the allocated limits for each sensitive surface.

Materials are supported by current, traceable and applicable data.

QCM, witness or analytical results remain within agreed thresholds where monitoring is required.

The hardware remains physically and functionally acceptable following bakeout or vacuum exposure.

Test results, anomalies, waivers and non-conformances are formally reviewed and closed.

The tested configuration and supporting material evidence are linked to the flight configuration.

There are no unresolved contamination risks that could compromise mission performance.

Resonate Testing can review the supplied hardware description, materials information, mission environment, customer specification and programme constraints before environmental testing is committed.

The team can help identify potential concerns involving:

• Adhesives

• Coatings

• Polymers

• Lubricants

• Harnessing

• Printed materials

• Battery-related materials

• Optical payloads

• Sensitive detectors

• Unknown cleaning or bakeout history

Resonate can support development of an agreed TVAC or bakeout campaign, including:

• Hardware configuration

• Mounting

• Vacuum level

• Temperature profile

• Dwell periods

• Functional monitoring

• Feedthrough requirements

• Contamination considerations

• Reporting expectations

• Hold points and pass/fail criteria

Support can include review or development of:

• Test specifications

• Test procedures

• Checklists

• Configuration records

• Data requirements

• Test reports

• Verification evidence

An independent engineering review can identify missing inputs, unsupported assumptions and contamination risks before hardware enters the chamber.

Resonate can support the environmental verification campaign and help assemble traceable evidence for customer review.

Scope limitation: Dedicated ECSS-Q-ST-70-02 material screening should only be described as a Resonate service if the required method, equipment, quality controls, accreditation scope and trusted test-house status have been formally approved. Monitoring options such as RGA and TQCM should also be confirmed as operational and included within the agreed test scope before commitment.

Your Test Facilitator. Not simply a test facility.

A leak allows gas to enter through an opening or defective seal. Outgassing is the release of volatile species from materials and surfaces within the vacuum environment.

No. The risk depends on the species released, quantity, release rate, location, temperature, receiving surface and mission sensitivity. Water may dominate early pump-down, while small quantities of an organic condensable material can be more damaging to an optic.

Adhesives, coatings, polymers, elastomers, tapes, lubricants, cable materials, printed parts, cleaning residues and incompletely cured materials commonly require review.

TML measures total specimen mass lost during the defined test. CVCM measures volatile material that condenses on a controlled collector. Both support screening decisions, but neither independently predicts total spacecraft contamination.

No. Material quantity, location, CVCM, operating temperature, formulation, cure condition and sensitivity of nearby hardware must also be considered.

Screening characterises material behaviour under a standardised method. Bakeout removes or reduces volatile material from a material or assembly using controlled heat and vacuum.

No. Bakeout can reduce volatile content but cannot correct every material incompatibility, degradation mechanism or prohibited-material issue.

No. TVAC testing assesses assembled hardware under defined vacuum and thermal conditions. ECSS-Q-ST-70-02 screening uses a specific material-level method and controlled measurements.

An RGA identifies gases and partial pressures within the vacuum chamber. It can support leak detection and outgassing investigation but does not directly quantify every contaminant deposited on a cold surface.

A QCM measures very small changes in deposited mass on its crystal. Its position, temperature and view factor determine how representative it is of the protected hardware.

Ideally during material selection and preliminary design. It should then be revisited following material, supplier, process, configuration or mission changes.

Provide:

• Applicable standards and customer specifications

• Hardware description

• Mission and operating environment

• Material or bill-of-materials information

• Adhesives, coatings and lubricants

• Previous cleaning or bakeout history

• Sensitive optics or detectors

• Temperature and vacuum requirements

• Available contamination budget

• Required verification evidence

• Qualification Testing Under ECSS

• Acceptance Testing Under ECSS

• Protoflight Testing

• Space Hardware Model Philosophy

• EQM vs PFM vs FM

• Thermal Vacuum Testing

• Vacuum Bakeout

• Thermal Cycling Under Vacuum

• Vibration Testing

• Shock and SRS Testing

• Acoustic Testing

• EMC Testing

• ECSS-Q-ST-70-02 Outgassing Screening

• TML, RML, CVCM and WVR Explained

• Spacecraft Contamination Control Plans

• Materials Selection Under ECSS

• Residual Gas Analysis

• QCM and TQCM Monitoring

• Declared Materials Lists

• Verification Planning

• Verification Matrices

• Verification Evidence

• Environmental Test Planning

• Space Qualification Process

• Test Readiness Reviews

• AIT Plans

• Test Specifications

• Test Procedures

• Test Reports

• NCRs, Deviations and Waivers

Share your material list, customer specification, hardware description or proposed TVAC profile with Resonate Testing.

We can help identify missing information, contamination-sensitive features and verification considerations before the test campaign begins.

Primary CTA: Submit Your Materials List
Secondary CTA: Book a Technical Review
Supporting CTA: Discuss a TVAC or Bakeout Requirement