# Distance to feel warmth from the sun

Hi SD.

I am a musician, not a science guy, so I have no clue about the answer to this question. I have been thinking about it all week. Please enlighten me.

We here on earth feel the sun’s heat…often pleasantly tolerable, sometimes way too uncomfortable, but never hot enough to kill us all. Once we leave the atmosphere and go into space, it gets cold again.

My question is this. Say you have just left the earth’s atmosphere and are able to survive in space. You can instantly travel to any point in space between here and the sun, and enjoy the view, without dying. Say you have amazing sunglasses that are guaranteed to keep you from going blind. Again, assume you can hang out without the slightest discomfort. At some point the coldness of space changes to the face-melting heat of the sun. What point is that? All else aside, how close to the sun do you have to be to feel a comfy temperature of 70 degrees? Solely from a temperature perspective, how close can you get and feel a nice cozy heat? From that vantage point, how would the sun look? Would it occupy your entire field of vision?

Thanks,
Dave

Sorry I do not have the specific answer. Someone will have very detailed information. But the question has a lot of variables. The Earth has several things that mediate the warmth you feel from the Sun. It also has things that protect from dangerous energy from the Sun and space.

A very basic calculation can be Watts per square meter of energy at a particular distance from the Sun. But that also depends on what level of that energy you are absorbing or reflecting. Black or white space suit? But your question is framed in a way that maybe you can just be there with no spacesuit. So it is difficult to calculate the feeling of warmth separated from all the Earthly mediation of sunlight energy. Conduction, convection etc.

It is difficult to give an accurate answer for a human floating in space without a suit of some sort. But a square meter of some material with a temperature sensor can likely be calculated quite well.

At the same time.

OK, the question ignores the fact that the atmosphere and hydrosphere of the Earth make the temperature bearable. Air and water will absorb heat from the Sun and move it around. This ameliorates the extreme temperatures the Earth would otherwise experience.

Also the roation of the Earth does as well. The Moon is the same distance from the Sun but doesn’t have air or water and also has a much slower rotation. So the daylight part of the Moon, after 13 days baking in the Sun, gets up to about 127 C and the nightime side, after radiating its heat away for 13 days, gets down to -173 C. (Those are Earth days, of course; one Lunar rotation = 27 Earth days.) If the Earth had no air or seas, it (or at least the non-polar regions) would not get to such extreme temperatures, but still would be much hotter and colder than it is now. The polar regions would get both hotter and colder than the Moon, since they have 6-month long days and nights.

At any rate, someone out in space without air and water around them will experience hot and cold just like a planet without them will. If the person is not rotating, the Sun-lit part of them is just going to keep heating up and the other part will keep getting colder. That’s why the person could both melt and freeze at the same time.

I don’t have the expertise to answer the OP myself, but I have a follow up question which may help highlight some conceptual errors, and possibly provide a definitive answer (that is, the answer to my question may be an answer to the OP’s as well).

Anyways, my question is, in the vacuum of space, in total darkness, and assuming you had a pressure suit but no cooling system, would you overheat from your body’s own internal processes (we are a heat engine, after all) and the inability to cool down by the evaporation of sweat to atmosphere, or would losses from thermal radiation be sufficient to allow you to maintain a livable core temperature?

My gut tells me that, even in total darkness, you’d actually die of heat stroke sealed up in a skin-tight pressure suit and surrounded by vacuum with no cooling system, but then I know that Apollo 13 saw frigid temperatures during their return to Earth. I’m just not sure how much of that had to do with sublimation of ice on the hull (if there was any) and the greater surface area of the spacecraft, relative to a suit, allowing for greater losses due to thermal radiation.

Just a nitpick: we don’t feel any heat at all from the sun - vacuum, after all, is a pretty good insulator. It’s the sun’s light that heats us up. The sun’s actual temperature is irrelevant.

I’m pretty sure it’s not irrelevant: the radiation profile depends on the temperature. After all, we don’t feel any heat from the moon, which is of similar apparent size!

Space is not cold. Objects in space near the Earth get heated up by about the same amount as objects on Earth (not quite as much, due to the greenhouse effect, but comparable). In fact, overheating is a common problem on spacecraft.

Radiation is a form of heat. If you sit directly in front of a roaring bonfire on a cold night and feel like your face is cooking, that’s the light from the fire. Would you say you weren’t feeling the heat of the bonfire? Or are you distinguishing between heat in the form of visible light and heat in the form of “heat radiation”, a somewhat distracting term for infrared radiation?

As to the OP. “Feeling the warmth of the Sun” depends on how much heat you are losing or receiving in other ways. On a very cold winter’s day it can feel like the Sun isn’t doing anything at all, a change in weather bringing in warmer air can change that drastically, despite the radiation from the Sun being the same.

So in space it would depend on how well your suit was working, what temperature it was keeping you at, as well as the distance from the Sun.

This is difficult to convey because thermal transfer in space is so different from earth at ground level. On earth our bodily comfort is affected by convective, conductive and radiative heat transfer. In space it is only radiative.

Space itself is really cold - about 2.7 degrees Kelvin or -454.75 Fahrenheit. That is the temperature of the space backdrop. Even on earth we see evidence of this. A metal object left outside on a clear windless night becomes very cold. Its heat has radiated away to space, despite atmospheric convection.

However space is also “hot” in that solar radiation is not attenuated by atmosphere and there is no convective cooling, not the slightest breeze. At earth’s distance from the sun, the solar irradiation above the atmosphere is about 1,300 watts per square meter. This is why thermal control in space is difficult.

You can easily bake on one side and freeze on another, and you will never find a distance from the sun that is “comfortable” without external thermal management apparel of some kind.

In spacecraft we see examples of this difficulty. Immediately upon reaching orbit the space shuttle had to open the payload bay doors, exposing large radiators. The heat carried to the radiators by circulating coolant was radiated to the cold backdrop of space. Failure to open the doors was an emergency since the backup “flash evaporation” system could only cool the vehicle a few hours: https://spaceflight.nasa.gov/gallery/images/shuttle/sts-113/hires/iss005e21546.jpg

The opposite case is Apollo 13 where after powering down the vehicle, the astronauts nearly froze - despite the vehicle being externally irradiated by about 1.3 KW per square meter of solar energy. The Apollo capsule was actually quite reflective to aid in thermal control under normal situations. With the vehicle powered down this probably prevented solar irradiation from making the cabin more comfortable: http://www.nasa.gov/sites/default/files/images/136229main_image_feature_429_ys_full.jpg

On Apollo 13 we see the difference between normal internal heat generation vs a powered down situation and how this makes thermal control difficult in space.

With humans it is similar. A resting human produces about 100 watts of heat, but under vigorous exercise this may increase to 1,200 watts. This wasn’t fully appreciated early in the space program and astronaut Gene Cernan almost died from overheating during an EVA on Gemini 9. Based on those lessons, the later Apollo and shuttle astronauts wore liquid thermal cooling garments during EVAs: http://airandspace.si.edu/webimages/highres/WEB10864-2008h.jpg

Mars is a good benchmark because the average equatorial temperature is 68 degrees mid-day although, because of the thin atmosphere, the temperature plunges far below zero at night. I’d say that’s pretty much as far from the sun as you can get in regards to the parameters you specified.

Tell that to Princess Leia!

I thought the 68F was too high so I checked here. Not only does it mention that temp but it also says “The Spirit rover recorded a maximum daytime air temperature in the shade of 35 °C (308 K; 95 °F), …”

Whoa.

But it’s a dry heat

Or Bernie Taupin!

Or Khan.

Sorry, physics was never my strongest subject. I was under the impression that heat was a property of matter, and that in the absence of matter, there could be no heat. Radiation - of any type - could be converted into heat (and heat into radiation) but radiation was not heat in and of itself. I understand it’s a bit more complicated than that.

You’re pretty much right and it’s actually not much more complicated than that. Heat is the motion of atoms and molecules; the faster they move, the hotter it is. What you get from a fire and the Sun is infrared radiation along with the visible light. IR light is absorbed by your skin which converts it to heat and you feel that. Of course, fires and the Sun also heat up the air and you feel that heat too. The only difference is that fires can also heat the air through conduction, while the Sun only does it via radiation.

Strictly speaking, heat is the transfer of thermal energy. When you go out and get a tan, you can fairly say that the Sun is losing heat through radiation, and you’re gaining heat from radiation, and that a (very small) fraction of the heat the Sun lost is the heat you gained. But you can’t say that anything (not you, not the Sun, not the space between) “has” heat.

And “thermal energy” is just an adjective-noun phrase. Thermal energy is energy that is thermal. Or, to expand that, energy that has a thermal distribution. Both space (containing electromagnetic radiation) and matter (such as a person or a star) can be said to have thermal energy.