Spacecraft Thermal Control Coatings: Optical Properties, Degradation and Callouts
Spacecraft thermal control coatings set how much sunlight a surface absorbs (solar absorptance, α) and how much heat it radiates (emittance, ε). White paints, silvered or aluminized films and black paints or anodize are chosen for their α and ε, measured per ASTM E903 and E408, screened for outgassing per ASTM E595, and judged by how they degrade under ultraviolet light and atomic oxygen.
Updated
Absorptance, emittance and why the ratio matters
A surface in sunlight settles at a temperature set mostly by the ratio of solar absorptance to infrared emittance. A low α and high ε surface, such as a white paint or a silvered film, runs cold and is used on radiators. A high α and high ε black surface absorbs and radiates strongly and is used inside instruments and on baffles. A low ε surface, such as bare or vapor deposited metal, radiates little and is used where heat must be held in.
Those two numbers drive the thermal model, so the values you buy must be the values the analyst used. NASA Goddard's long running data compilation (Henninger, 1984) makes two points that still apply. Its published values are averages with a tolerance of about ±0.02, and because thickness, surface preparation, formulation and manufacturing all change α and ε, confirmation measurements should be made on the actual material before committing it to flight.
That is why thermal control coatings are specified by property and verified on witness coupons processed with the hardware, not just by product name.
How α and ε are measured
| Method | What it measures | Notes |
|---|---|---|
| ASTM E903 (E903-20) | Spectral absorptance, reflectance and transmittance with a spectrophotometer and integrating sphere, then solar weighted values | Applies to specular and diffuse surfaces and to coated materials. The solar spectrum used for weighting must be reported if it is not the standard reference |
| ASTM E408 (E408-13, reaffirmed) | Total normal emittance with portable or bench inspection meters | Method A measures energy reflected from the specimen, Method B measures energy emitted. Suited to fast, nondestructive production checks |
| Calorimetric emittance | Hemispherical emittance from transient heating and cooling in vacuum | Used at NASA Goddard as a reference method over about -70 to +70 °C |
The Goddard report describes its solar absorptance instrument covering 300 to 2,400 nm and its portable emissometer sensing in the 5 to 25 µm band, and notes that emissometer errors can be significant on spectrally selective materials unless corrections are applied. Normal emittance from an inspection meter is converted to hemispherical emittance for thermal analysis, so the cert should state which one is reported.
Put the method, the instrument type and the acceptance range on the PO, for example solar absorptance per ASTM E903, 0.15 ±0.02, and normal emittance per ASTM E408, 0.91 ±0.02, measured on witness coupons from the same lot and thickness.
Have a part like this?Send it in under a minute. A person replies within 1 business day.White and black thermal control paints
Inorganic white paints with silicate binders give the lowest absorptance and best stability, and black paints give high absorptance with high emittance. The manufacturer data below is for products with flight history and shows how strongly properties depend on thickness.
| Coating (manufacturer data) | Binder | Solar absorptance α | Thermal emittance ε | Thickness |
|---|---|---|---|---|
| AZ-93 white (AZ Technology version of Z-93P) | Silicate, inorganic | 0.15 ±0.02 at 5.0 mils or more | 0.91 ±0.02 | 5.0 ±1.5 mils over 85 percent of the area; full cure 7 days |
| AZW/LA-II ultra low α white | Silicate, inorganic | 0.09 ±0.02 at 10.0 mils or more | 0.91 ±0.02 | 7 to 13 mils; full cure 7 to 14 days |
| MLS-85-SB black | Silicone | 0.98 ±0.01 at 1.5 mils or more | 0.91 ±0.02 | Brush application may fall outside the published range |
Two things stand out. White paints only reach their low absorptance at full thickness, so a thin spot reads high in α and runs warm. And these silicate coatings are thick by aerospace finish standards, 5 to 13 mils, which must be allowed for at interfaces and fastener holes. AZ Technology also notes AZW/LA-II costs more than AZ-93, which is the usual trade for its lower α.
Inorganic white paints are also more demanding to apply than ordinary aircraft paint: they need controlled surface preparation, careful thickness build and long cure. The general primer and topcoat systems used on aircraft are covered separately on the paint and primer systems page.
Silvered and aluminized films, second surface mirrors and MLI
Second surface mirrors put a reflective metal layer behind a transparent film or glass. Sunlight passes through the top layer and reflects off the metal, while the top layer itself provides the infrared emittance. The result is very low α with useful ε, which is why silvered fluoropolymer film is a standard radiator surface.
The Goddard compilation shows how film thickness sets emittance. For Teflon film with silver backing, it lists α of about 0.08 to 0.09 with ε rising from 0.68 at 2 mil to 0.81 at 5 mil and 0.88 at 10 mil. Thicker film radiates better but adds mass and changes handling. Aluminum backing gives somewhat higher α than silver. The same report shows aluminized Kapton emittance climbing steadily with film thickness, because the polymer, not the metal, does the radiating.
Multilayer insulation (MLI) uses the opposite property. Blankets stack many layers of thin polymer film with vapor deposited aluminum on one or both sides, separated by low conductance spacers, so each low emittance layer radiates little to the next. The outer cover layer is chosen for the external environment, often a more durable film with its own α and ε requirement. MLI blankets need grounding to control charging and venting so they do not balloon during ascent, both of which belong in the blanket drawing.
Films and tapes are cleanliness sensitive. Fingerprints and outgassed films raise α, so handling, bagging and packaging rules belong on the PO. See precision cleaning for cleanliness levels.
Black anodize for optics and instruments
Black anodize is common on optical housings, baffles and instrument structure because it is thin, durable and dimensionally controlled. It is not one optical property, though. The Goddard compilation lists two black anodized aluminum samples at α 0.65 with ε 0.82 and α 0.86 with ε 0.86, and flags that coating thickness is critical for anodized samples.
That spread comes from alloy, anodize thickness, dye, seal and the processor's bath. For thermal or stray light performance, specify the optical properties you need and require witness coupons from the same alloy and lot, measured per ASTM E903 and E408, rather than relying on a color callout. If the part is in an optical path, ask for the reflectance band of interest, since a surface that looks black in visible light can reflect strongly in the infrared. Dyed coatings should also be screened for outgassing and checked for color stability under the mission's ultraviolet exposure.
Anodize specifications, types and thickness are covered on the anodizing page, and alloy selection on the aluminum alloys page.
Atomic oxygen, ultraviolet and charging
In orbit, coatings degrade mainly by ultraviolet darkening, which raises α, and by atomic oxygen erosion of organic binders and films in low Earth orbit. Emittance usually changes less than absorptance, so radiators warm over a mission as α creeps up. Thermal designs carry both beginning of life and end of life values.
Flight and ground test data from manufacturers shows what stable coatings can do. AZ Technology reports AZ-93 was exposed by NASA to an atomic oxygen fluence of 5.6 x 10^22 atoms/cm², charged particle radiation of 4.5 x 10^15 electrons/cm² and 701 equivalent sun hours of vacuum ultraviolet with less than 4 percent change in α and less than 1 percent change in ε. A precursor flown on the Long Duration Exposure Facility returned after 5.8 years with an overall α change of 0.01, and AZW/LA-II returned from the Materials International Space Station Experiment after 4 years with a change of 0.03.
Ground ultraviolet testing has a trap. The Goddard report notes that some materials partly recover from ultraviolet and particle damage when re-exposed to air, so accurate degradation data often requires measurement in situ, inside the vacuum chamber. Ask how any degradation data you are given was measured.
Spacecraft charging is the third concern. Insulating coatings can build charge and discharge, so missions in harsh charging environments use electrically conductive thermal control paints. AZ Technology, for example, describes its AZ-2000-IECW white and AZ-1000-ECB black paints as conductive versions intended for surface charge dissipation. Surface resistivity then becomes a third property to specify and test.
Outgassing and contamination
Thermal control surfaces are both sources and victims of contamination. Outgassed material from paints, adhesives and tapes condenses on optics and on cold radiators, where even thin films raise α. Screen every coating, primer and tape per ASTM E595: 24 hours at 125 °C in vacuum, with condensables collected on a 25 °C plate.
ASTM E595 does not set limits. It notes that 1.0 percent total mass loss and 0.10 percent collected volatile condensable material have historically been used as spacecraft screening levels, and NASA Goddard's outgassing database lists materials meeting those levels. Many programs add a vacuum bakeout of coated parts before integration. AZ Technology notes MLS-85-SB has very low outgassing compared with some of its other black coatings, which is the kind of distinction that matters when choosing a black for a cold optical cavity. For polymer materials generally, see engineering plastics.
What to call out on the drawing or PO
- The coating or film by product name and process specification, with the coated surfaces and masking shown on the drawing.
- Required solar absorptance and emittance with tolerances, the measurement standards (ASTM E903, ASTM E408 or calorimetric), and whether emittance is normal or hemispherical.
- Dry film thickness range and measurement method, since α on white paints depends on thickness.
- Witness coupons of the same substrate, surface prep and lot, measured and kept with the cert package.
- Outgassing screening per ASTM E595 with the program's limits, and any bakeout before delivery.
- Surface resistivity where a conductive coating is required.
- Handling, cleanliness, packaging and storage requirements, including gloves, bagging and humidity limits during cure.
- For MLI: layer count and materials, grounding, venting and outer cover film properties.
More context on how these parts are built is on the spacecraft and satellites page. Related coatings: conformal coating, dry film lubricants and thermal spray coatings, all from the coatings hub.
Get thermal control coatings applied and measured
If you have radiators, panels, brackets or optical parts that need a white or black thermal control paint, black anodize with optical requirements, or film application, send the drawing and the optical property callout. We source the work from processors experienced with the product you specify, flow down thickness, witness coupon, α and ε measurement, outgassing and handling requirements, check the measurement data before parts ship, and a person replies within one business day. We sign a mutual NDA before files, and controlled designs follow the process on controlled programs.
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Questions
What is a good solar absorptance for a spacecraft radiator coating?
Radiators want low absorptance and high emittance. Published manufacturer values include AZ-93 white at 0.15 ±0.02 α with 0.91 ε, and AZW/LA-II at 0.09 ±0.02 α with 0.91 ε at full thickness. NASA Goddard data lists silver backed Teflon film near 0.08 to 0.09 α. The right value comes from the thermal model, including end of life degradation, not from the lowest number available.
How is emittance measured on thermal control coatings?
Production checks usually use a portable inspection meter per ASTM E408, which measures total normal emittance by reflected energy (Method A) or emitted energy (Method B). Reference labs may use calorimetric methods in vacuum to get hemispherical emittance. Solar absorptance is measured with an integrating sphere spectrophotometer per ASTM E903. State on the PO which method and which emittance, normal or hemispherical, you need.
Why does white thermal paint need to be so thick?
White pigments reach their low absorptance only when the film is thick enough to hide the substrate. AZ Technology specifies AZ-93 α of 0.15 at 5 mils or more and AZW/LA-II α of 0.09 at 10 mils or more. A thin area absorbs more sunlight and runs warmer. Specify a thickness range and verify it, and allow for the build at interfaces and holes.
Can I just call out black anodize for an optical baffle?
Not if optical or thermal performance matters. NASA Goddard data shows black anodized aluminum samples ranging from 0.65 to 0.86 in solar absorptance, with thickness flagged as critical. Specify the absorptance, emittance or reflectance you need, require witness coupons from the same alloy and lot, and have them measured per ASTM E903 and E408. Screen dyes and seals for outgassing as well.
How do coatings degrade in orbit?
Ultraviolet exposure darkens many coatings and raises absorptance, atomic oxygen in low Earth orbit erodes organic binders and films, and contamination from outgassing deposits on surfaces. Stable inorganic paints change little: AZ Technology reports less than 4 percent α change for AZ-93 after NASA atomic oxygen, particle and ultraviolet exposure. Designs carry beginning and end of life values to cover the drift.
What outgassing limits apply to thermal control coatings?
Screening is done per ASTM E595, 24 hours at 125 °C in vacuum. The standard sets no limits, but notes 1.0 percent total mass loss and 0.10 percent collected volatile condensable material have historically been used for spacecraft materials, and NASA Goddard's outgassing database applies those levels. Programs near sensitive optics often impose tighter limits or a bakeout, so check the contamination control plan.
Related
Sources
- NASA Goddard: Solar Absorptance and Thermal Emittance of Some Common Spacecraft Thermal-Control Coatings (Henninger, 1984)
- AZ Technology: AZ-93 white thermal control paint
- AZ Technology: AZW/LA-II ultra low absorptance white paint
- AZ Technology: MLS-85-SB black thermal control paint
- AZ Technology: AZ-2000-IECW conductive white paint
- AZ Technology: AZ-1000-ECB conductive black paint
- ASTM E903-20: solar absorptance, reflectance and transmittance using integrating spheres
- ASTM E408: total normal emittance by inspection meter techniques
- ASTM E595-15(2021): outgassing in a vacuum environment
- NASA Goddard outgassing database user guide
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