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This 2013 JPL presentation Solar System Escape Architecture for Revolutionary Science March 2013 Conference NIAC Phase 1: 2012-2013 by PI: Jeff Nosanov Co-Is: Dr. Daniel Grebow, Dr. Brian Trease, John West, Dr. Henry Garrett found in this comment shows a proposed solar sail craft capable of reaching the heliopause.

The trajectory includes a relatively close flyby of the Sun to maximize impulse from sunlight.

I noticed that the annotations for the image of the craft on page 20 show the "front" or Sun-facing side of the sail would be coated with aluminum for maximum reflectivity, and the back side would be coated with chromium for maximum emissivity.

Questions:

  1. What is the importance of high emissivity in this application?
  2. Why would a shiny metal like chromium have a high emissivity value?

enter image description here

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    $\begingroup$ May be it is not metallic chromium but chromium(IV) oxide which is brown to black. $\endgroup$
    – Uwe
    May 21, 2020 at 14:52
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    $\begingroup$ Shininess definitely is not a desired feature when it comes to increasing emissivity. "A good reflector is a bad emitter." This apparently is a very thin film of metallic chromium rather than chromium oxide. Thin metallic films have rather bizarre effects that can be highly frequency dependent. Here the goal is to maximize emissivity in the thermal infrared. $\endgroup$
    – David Hammen
    May 21, 2020 at 15:24
  • $\begingroup$ High emissivity means it isn't shiny. (though something with low reflectivity but a glossy, metallic appearance may still seem somewhat shiny). $\endgroup$
    – ikrase
    May 21, 2020 at 15:40
  • $\begingroup$ @ikrase I know what it means, but having first hand experience evaporating thin layers of chrome and being old enough to remember when cars had chrome bumpers and mirrors and door handles, I know that chrome is indeed shiny like most metals, at least in the visible range. $\endgroup$
    – uhoh
    May 21, 2020 at 22:40
  • $\begingroup$ @Uwe I have a hunch you are right, see comments $\endgroup$
    – uhoh
    May 21, 2020 at 22:41

1 Answer 1

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1) Is simple to answer - to get rid of heat. Aluminium has a reflectivity of about 90% in the range below 1000nm wavelength. At a perihelion of 0.2 AU the total power of the Sun is about 30 kW/m² of which maybe 2 kW/m² are absorbed by the sail. A large emissivity helps in getting rid of that.

2) Massive Chromium has an emissivity of about 0.3 in the IR range, which isn't too bad and one of the "un-shiniest" metals. Here we're talking about a very thin coating of chromium, likely a few dozen nanometer only. This is a lot thinner than the wavelength of infra-red radiation, so they behave differently to bulk materials. These thin layers of metals often have an emissivity that is a lot larger. I found values around 70% in some sources, although not in a detailed study:

[1] Solar Sailing: Technology, Dynamics and Mission Applications, Colin R. McInnes claims 0.64. [2] Space Sailing, Jerome L. Wright claims 0.63 - 0.73

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    $\begingroup$ Interesting answer. Can you provide some of those mentioned sources? $\endgroup$
    – Polygnome
    May 21, 2020 at 18:16
  • $\begingroup$ This definitely needs a supporting reference for the high thermal emissivity of a thin chromium layer. Also, wouldn't allowing it to oxidize a bit substantially increase the emissivity? $\endgroup$
    – uhoh
    May 21, 2020 at 22:11
  • $\begingroup$ Interestingly after checking the numbers in some random tables from search hits the values really do range from 0.05 to 0.3 and where indicated, its the higher temperatures that correspond to the higher values. I don't know if this means that emissivity is higher at the peak wavelengths for blackbody radiation at the elevated temperatures, or if it just oxidizes in air when heated. In semiconductor manufacturing photomask blanks commonly have an oxide layer on top of the chrome wet etch layer for antireflection photo-sciences.com/binary-photomask for optical pattern generation. $\endgroup$
    – uhoh
    May 21, 2020 at 22:27
  • $\begingroup$ random internet tables: 1, 2, 3 $\endgroup$
    – uhoh
    May 21, 2020 at 22:28
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    $\begingroup$ I've got a copy of the Spacecraft Thermal Control Handbook and there are two materials listed with chromium: Chromium, vapor deposited, on glass: alpha = 0.56, eps = 0.17 Chromium, vapor deposited, on 5 mil Kapton: alpha = 0.57, eps = 0.24 These values would have come from another source not listed and of course some variation is expected between this handbook value and what might actually be applied. $\endgroup$
    – aranedain
    May 22, 2020 at 15:10

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