Would a body freeze in LEO?

[pavelcherepan]

So, suppose an unlucky astronaut went out of the airlock of ISS naked for whatever reason. Let's say it was because of "space dementia"

Obviously, they are going to pass out in a few seconds and die within a couple minutes, but what would happen with the body? It would spend slightly more than half of the time in light compared to the dark, because of Earth's atmosphere bending the light. Based on what I could find, black body radiation of human body would be around 100 W, whereas the solar radiation would give in excess of 1500 W. Only part of those 1500 W would be in IR, but still the amount of infrared should be sufficient to more than compensate for the radiative heat loss.

One thing I'm concerned is whether it would have enough time to freeze over while going through Earth shadow for ~40 minutes. 

https://en.wikipedia.org/wiki/Black-body_radiation#Human-body_emission

Edited December 27, 2017 by pavelcherepan

[EdEarl]

There are many factors that affect the result. A person covered in a shiny space suit would collect and emit less heat than one covered in black. Eventually the body would be near the difference between maximum hot and cold, because ones body could not loose all heat during the dark cycle. Although, if a body is is exposed and all its water evaporates, the total mass might be low enough that it would alternate between frozen and baking.

[swansont]

Radiative heat loss is ~100 W because the temperature is 37 degrees C. Once you're dead that will no longer be the case.

[John Cuthber]

I guess the average temperature of a body in space would be near to that of the average temperature of the Earth- which is a body in space.

However people don't have much radioactive decay heat so they would get colder.

 

Also the Earth's usually only in the shadow of the Earth, but a body would be screened from the sun half the time.

Both those suggest that the body in LAO would be rather cooler than the Earth, and that suggests it would freeze.

I'm fairly sure that a shiny or black suit would affect the range of temperature from day to night, but not the average.

Edited December 27, 2017 by John Cuthber

[EdEarl]

For sure average temperature. Since one side of the person is always dark and the other sunlit, except in Earth's shadow, it matters whether the orbit is polar or equatorial. The one spending most time in Earth's shadow will be colder on average.

[MigL]

No suit involved; he is going out naked.

Biggest factor would be the evaporation of all moisture from permeable membranes like your skin, eyes, etc.
Your body cools by skin evaporation, and hospitals 'freeze' skin and layers below by spraying with low boiling organics.

Water readily 'boils' in a vacuum, so yes, the body's outer layers would freeze and start 'cracking', leading to more moisture loss, until the whole body would freeze/dehydrate.

[mistermack]

It's  a complicated scenario. If the suit lost power, it would soon be venting gases. That would cause the body to spin. So the heating and cooling would average out around the body. The object would simply reach an equilibrium temperature. 

If you have a situation where heating is 1500 watts and cooling is 100 watts, the temperature will rise very quickly. Then, the 100 watts radiative cooling figure climbs rapidly, and eventually reaches equality with the heating figure. The surface of the Moon is about 100 degrees c during the day. Even though the subsurface is very cold, and conducts heat away.

Most of the water would escape as vapour very quickly, so the dry matter that was left wouldn't freeze, it would just get cold and hot quickly as it went in and out of the Earth's shadow. 

[pavelcherepan]

9 hours ago, mistermack said:

Then, the 100 watts radiative cooling figure climbs rapidly, and eventually reaches equality with the heating figure.

Yep, so what would that equilibrium point be? Intuition tells me it's higher than body temperature, but my intuition is too often wrong.

14 hours ago, MigL said:

Water readily 'boils' in a vacuum, so yes, the body's outer layers would freeze and start 'cracking', leading to more moisture loss, until the whole body would freeze/dehydrate.

Ok, but after that, will it remain frozen? Consider now negligible radiative heat loss, most of the water already gone, and the body spends slightly more than half the time in the sun.

Edited December 29, 2017 by pavelcherepan

[Sensei]

At lower pressure (or lack of it) water boils at lower (e.g. room) temperature.

 

Buy vacuum pump ($500+), piece of meat, and try making experiment like above one.. Make video, and you will have return from investment straight away..

 

 

[mistermack]

2 hours ago, pavelcherepan said:

Yep, so what would that equilibrium point be? Intuition tells me it's higher than body temperature, but my intuition is too often wrong.

Ok, but after that, will it remain frozen? Consider now negligible radiative heat loss, most of the water already gone, and the body spends slightly more than half the time in the sun.

 

There are indications you can take as clues. In wikipedia, it is claimed that without greenhouse gases, the surface temperature of the Earth would be about −18 °C. 

But against that, the surface temperature of Halley's comet was measured at nearly 80  °C when roughly our distance from the Sun. And the International Space Station has a powerful system using ammonia circulating through external radiators to dump heat, without which it would seriously overheat.

So it seems that the Earth is much better at dumping heat than the space station. Which makes one wonder, why don't they coat the space station in darker material to make it lose heat faster, and not rely on radiators? Maybe it would get too cold when in the Earth's shadow. 

[Sensei]

13 hours ago, mistermack said:

Which makes one wonder, why don't they coat the space station in darker material to make it lose heat faster, and not rely on radiators?

Dark material absorbs visible light spectrum...

White material reflects visible light spectrum.

Radiators increase area which is emitting photons in the all directions.

 

[mistermack]

On 30/12/2017 at 2:59 AM, Sensei said:

Dark material absorbs visible light spectrum...

White material reflects visible light spectrum.

Radiators increase area which is emitting photons in the all directions.

 

Yes, I should have made it clear, I meant coat the dark side in dark non-glossy coating, and the sunny side in a highly reflective surface.

Ideally, to aim at a body that is neutral, radiating exactly what it absorbs without any need for active management, failing to about 20 deg C.

[Moontanman]

On 12/28/2017 at 2:31 PM, MigL said:

No suit involved; he is going out naked.

Biggest factor would be the evaporation of all moisture from permeable membranes like your skin, eyes, etc.
Your body cools by skin evaporation, and hospitals 'freeze' skin and layers below by spraying with low boiling organics.

Water readily 'boils' in a vacuum, so yes, the body's outer layers would freeze and start 'cracking', leading to more moisture loss, until the whole body would freeze/dehydrate.

Hmmm ummm, freeze fried astronaut... 

[Sensei]

2 hours ago, mistermack said:

Yes, I should have made it clear, I meant coat the dark side in dark non-glossy coating, and the sunny side in a highly reflective surface.

Ideally, to aim at a body that is neutral, radiating exactly what it absorbs without any need for active management, failing to about 20 deg C.

If spacecraft would have one side always pointing at the Sun, the other one to cosmos, temperatures wouldn't spread uniformly.

One side would be always overheated, and the other one would be always overcooled.

 

[pavelcherepan]

On 12/29/2017 at 6:08 PM, Sensei said:

At lower pressure (or lack of it) water boils at lower (e.g. room) temperature.

Sure, definitely all the moisture on the surface layers of the skin will quickly boil off. This was in fact experienced by an unfortunate astronaut trainee, who got locked in a vacuum chamber and depressurized to almost the vacuum conditions. He distinctly recalled (before passing out) that saliva was boiling in his mouth.

On the other hand, abdominal cavity and skull, for example, are very much air tight and could potentially maintain some pressure inside. 

 

P.S. Happy New Year everyone!

 

[mistermack]

On 03/01/2018 at 3:05 AM, Sensei said:

If spacecraft would have one side always pointing at the Sun, the other one to cosmos, temperatures wouldn't spread uniformly.

One side would be always overheated, and the other one would be always overcooled.

 

That could be a factor, but with suitable insulation design and convection inside the cabin, I doubt if it would be much of a problem. A few solar powered fans would sort it out, if it was.