NIAC ("NASA Innovative Advanced Concepts") has approved the following research application for 2015 funding:


Cryogenic Selective Surfaces

Robert Youngquist (NASA Kennedy Space Center)


Selective surfaces have wavelength dependent emissivity/absorption. These surfaces can be designed to reflect solar radiation, while maximizing infrared emittance, yielding a cooling effect even in sunlight. On earth cooling to -50 °C below ambient has been achieved, but in space, outside of the atmosphere, theory using ideal materials has predicted a maximum cooling to 40 K! If this result holds up for real world materials and conditions, then superconducting systems and cryogenic storage can be achieved in space without active cooling. Such a result would enable long term cryogenic storage in deep space and the use of large scale superconducting systems for such applications as galactic cosmic radiation (GCR) shielding and large scale energy storage. We propose, during this Phase I effort, to theoretically model the performance of real world selective surfaces to see if superconducting temperatures can be passively achieved in a deep space environment at 1 A.U. from the sun.

A quick search gave me a sample material from Solec designed for the exactly opposite scenario.

What are the candidate selective surface coatings (low on solar spectrum absorptivity, high on IR emissivity)?

What are their technology readiness levels*?

Have any coatings been tested in vacuum for outgassing and its impact on performance?

* A white paper describing NASA TRLs (pdf)

  • $\begingroup$ It seems they want a material either the same or the opposite of the stuff behind these shutters discussed by @OrganicMarble. I'm still trying to interpret if "high ratio of" really means the first one is large and the second small. $\endgroup$ – uhoh Jun 10 '16 at 7:44
  • $\begingroup$ Based on this, I would suspect none. I wouldn't peg any of them above a TRL 3. Most I would put at TRL 2. $\endgroup$ – called2voyage Oct 28 '16 at 16:33
  • $\begingroup$ @Robert, you say that "... theory using ideal materials has predicted a maximum cooling to 40 K!". Actually, you can do better than that--at least behind a sun shield. The three near infrared (NIR) instruments on the James Webb Space Telescope (JWST) are passively cooled to 39K. The total surface area of the cold patches is about 100 square feet. I don't know the final surface material. You might want to ask. See: jwst.nasa.gov/faq.html#temps $\endgroup$ – Vince 49 Nov 28 '16 at 2:58
  • $\begingroup$ @Vince49: That's using a sun shield, not really selective surfaces. $\endgroup$ – Nathan Tuggy Nov 28 '16 at 5:51
  • $\begingroup$ @Nathan, 'good point. $\endgroup$ – Vince 49 Nov 28 '16 at 7:43

A common low A/high E material is silver teflon. I don't know it's official TRL, but it's been around for quite a while. The first link below is from 1973. There are some issues with EOL (End of Life) performance. Unfortunately, I don't know the details of the degradation. You can find a good bit of information by searching the web for "silver Teflon emissivity". The links below are a start.

Silver-Teflon: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740012426.pdf

Thermal control surfaces, including Silver Teflon: http://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-what-materials-are-used-for-thermal-control.html

Thermo-Optical Properties: http://webserver.dmt.upm.es/~isidoro/dat1/Thermooptical.pdf

  • $\begingroup$ I'd love to see a wavelength-dependent emissivity plot $\epsilon(\lambda)$ of this material or any thermal control material. The $\alpha_{solar} / \epsilon_{solar}$ values leave a lot to the imagination. This is a really helpful answer though!! $\endgroup$ – uhoh Nov 28 '16 at 2:34

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