# In layman's terms, how hot will the skin of a spacecraft become, 144 million miles from the Sun?

Ok, last week I asked how hot would a piece of metal become at a distance from Earth to Mars facing the Sun. Thank you for all the nice replies, but I do not have much math knowledge. So let me rephrase the question.

When an astronaut goes outside the space station his suit facing the sun gets hot and the suit is designed to cool those hot sections. I remember when they were trying out the new gloves they had to stop so often to show their hands to the camera and report any hot spots. So I know at 93 million miles from the Sun things get hot.

Using that as guide, would a space suit become just as hot at 144 million miles from the Sun or would it remain cool because of the distance.

• they'd get roughly as hot, but take about two and a half times as long to do so in the absence of active cooling. Or, the other way around, you'd need about half the cooling.
– user20636
Feb 13, 2020 at 13:25

To answer your question, it is important to specify if you mean the temperature inside the suit or on the outer surface!

The main purpose of the active coolant system is to conduct the thermal energy the astronaut (as every person) is "producing" out of the suit. (Comparable with wearing a thick jacket in summer: it is getting hot because the energy your body makes cannot "escape" to the environment). The internal cooling system is designed to keep a constant temperature inside the suit so it has to cool about 150 W the astronaut is producing.

The outer surface is designed to reflect most of the thermal radiation. (https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_Garment). Of course most does not mean all so there is thermal energy absorbed by the suit. This absorbed energy will heat the outer surface of the suit. BUT, by heating the outer surface more thermal energy will be radiated by the surface.

SO, there will be a temperature where the absorbed and radiated energy and the heat flow from/into outside reaches an equilibrium. So (first ignoring the heat flow into/from the inside) being 231 million km away from the sun means the equilibrium is at a lower temperature than 149 million km away from the sun.

As pointed out, there is still the topic heat flow into/from the suit. It is not possible to answer this question without knowing details .... and I really mean details ... about the suit, ... but, as an engineer I would assume this flow to be negligible small. Especially because the suit is insulated to prevent this flow.

Sorry, no better source available but "the engineers feeling"...

• Thanks for your input Tom.
– tajs
Feb 14, 2020 at 15:26