# Does sunlight warm an astronaut's face during a spacewalk?

I understand astronauts' helmets have dichroic optical filters to protect from many different frequencies of EM radiation. Do the filters prevent the astronaut from feeling warmth on their skin when sunlight is reaching their face?

The impression I get from these photographs is the sunlight would be hitting their face. I am wrong about my assumptions of what I think I see in these photos?

• Each of the photos have the sun above the astronaut not directly shining on the face. I would imagine it is as hot if not hotter on the skin. – Muze the good Troll. Apr 17 '19 at 4:14
• @ Muze As I understand it, if you can see the reflection of the sun on the visor, then the frequencies that pass through the visor will most likely be reaching the astronaut's face. – Bob516 Apr 19 '19 at 0:06

## 2 Answers

Does sunlight warm an astronaut's face during a spacewalk?

Perhaps a little, but not much.

Most About 40% of the power in the Sun's spectrum is in visible wavelengths (not a coincidence, in several ways) so if the astronauts can see, then the Sun can see them too! There's no such thing as a one-way mirror.

Looking at the photos in the question, you can't see the astronauts' faces but instead see a fairly bright reflection. The visor is essentially an overgrown pair of mirror sunglasses, so reflective that only a small amount of light makes it through.

Exposed directly to sunlight in space with a solar constant of 1361 W/m^2, a 20x16 cm^2 face would receive 40 watts of heat, and that's a lot! It would only be a little less with a face plate transparent to all wavelengths on a suit, and a combination of air conditioned air would remove some of it and the extensive blood flow to the face would remove some, but it would probably feel a bit warm if the astronaut turned from facing space to facing the Sun.

However, assuming the faceplate reflected or absorbed much of the invisible infrared light, this would be less. As shown below, in space the ATM0 spectrum integrates to roughly 1350 W/m^2 but the part in the visible range is only about 530 Watts/m^2. So a 20x16 cm^2 face would only receive about 17 watts of heat induced by visible light.

Then, assuming the mirror attenuates say 80 or 90% of the light, then we'd be around 3.4 to 1.7 Watts, and that's probably well below the warming from the natural thermal infrared radiation that the face plate or anything will already radiation towards the astronaut's face.

So yes, there will be a non-zero amount of heating, but with the reflective visors the astronauts use, it will be small and not very noticeable.

Python plot with https://pastebin.com/T3dY016N

• I am puzzled by the statement that most of the power in the Sun's spectrum is in visible wavelengths. I was under the impression that the UV part of the spectrum (for one example) has more power than visible light. – Bob516 Apr 21 '19 at 3:16
• @Bob516 actually, in space, above the atmosphere, it's a little less than half of the power that's in the visible part of the spectrum. On the Earth's surface, there's a lot of absorption of IR which doesn't happen in space. commons.wikimedia.org/wiki/File:Solar_spectrum_en.svg I'll add some more to this answer about it, thanks for bringing it up! As far as UV, the total power (W/m^2 in the UV is small, but each photon has more energy than visible or infrared. – uhoh Apr 21 '19 at 3:29
• @Bob516 I've made an edit. – uhoh Apr 21 '19 at 4:57

Very rarely do astronauts face the Sun. They have visors to help the block the sun more than the gold visors. As technology progresses visors will be improved to regulate the light to about the same as getting Sun on Earth until then it is still very bright and hot. Notice how the helmet below has a sun visors similar to a car off the center eye shade.

Each of the photos you provided has the sun above the astronaut.

• The images you included in your answer are of the Lunar Extravehicular Visor Assembly. Based on the images I added, it seems the astronauts in LEO don't have such visors. – Bob516 Apr 17 '19 at 3:41
• Those visors slide back @bob516 - you can see the visor "handles" in the images in your question. (although I don't think they have the hinged bit) – JCRM Apr 17 '19 at 7:34
• partial side eyeshade deployment visible here – JCRM Apr 17 '19 at 7:42
• This doesn't answer the question. – GdD Apr 17 '19 at 8:50
• @OrganicMarble, Sunlight warms everything that it touches. If any sunlight at all is able to reach the astronaut's face, then the skin will absorb some of that light and be warmed. All of the commentary about visors is relevant because the amount of warmth is directly proportional to the amount of light that the visor lets in. – Solomon Slow Apr 17 '19 at 15:35