Heat Loss in Space

Factual Questions
1

You’re on a spaceship travelling at a speed of 1AU/day from the outer edge of the solar system heading back to Earth. About 30 AU from the Sun, as you flyby Neptune, your Heater breaks and cannot be fixed. All the other systems work fine but there is no other way to generate heat. (No fires or macgyvering a new heater)

Assuming the spaceship is similar in construction methods and materials to the current Space Shuttle - but strong enough to handle the speed safely… Do you make it home without freezing? (29-31 Days, depending on where the Earth is when you get there)

My hypothesis is that you’d make it home just fine and heat loss requires a conductive medium - like hydrogen atoms, but there just aren’t enough to suck up the heat on the way home.

2

You do not need conduction/convection to lose heat. You radiate heat off all the time by emitting EMR. That’s how the Earth loses heat.

You also will gain heat by radiation from the Sun but I don’t know how much heat you’ll gain out by Neptune compared to the rate at which you’ll radiate it off. IIRC spacecraft sometimes rotate to even out the heat/gain loss over the surface.

3

Apollo 13 got very cold in just a few hours after they had to power down most of their systems to conserve energy. As CookingWithGas points out, even in the vacuum of space you can lose heat through radiation.

A lot depends on how much heat the heater was providing compared to the waste heat generated by the other shipboard systems. If it’s a small amount, you might be able to compensate by running other systems harder. On the other hand, if the bulk of your heat was coming from the heater, you’ll be a popsicle long before you reach Earth.

4

I thought that one of the problems with unmanned craft in space was that it was difficult to GET RID of the heat generated by instruments and electronics in space, not keeping things warm.

5

After you rotate away from the sun, your side of the Earth has a temperature drop of maybe 10-30[sup]o[/sup]F. That’s in just a few hours, and all the heat loss is due to radiation.

6

My sources on Apollo 13 tell me it got cold because they left the AC on without leaving on all the heat sources(people, computers) that the AC is designed to compensate for.

At Neptunes Orbit solar radiation won’t be sufficient to heat the ship. But at Jupiter’s orbit, turning the dark side to the sun may be. I’m looking for someone who knows.

Space suits, as I understand it, have a cooling unit and insulation, but no heater beyond the human body. But they are designed for operating at 1 AU.

I am aware of radiating heat, but am looking for someone with the knowledge to say how long it would take for an space shuttle style insulated ship to lose too much heat for human survival. Any rocket scientists here?

7

No. The cabin got down nearly to freezing and the cold was a major discomfort for the astronauts. After the explosion they powered down everything that wasn’t absolutely essential for survival to make sure they had enough battery power to make it back to Earth. Why, under those circumstances, would they have left the AC running?

8

What “source” are you relying on and how could you explain how an AC draw (itself heat generative) would have been overlooked. (Or more directly, stop making up shit in GQ)

9

IIRC the Space Shuttle has more issues getting rid of heat than it does trying to stay warm (which is why they fly around with the bay doors open). I know it seems odd but remember that of the three heat loss mechanism radiating heat away is the least efficient. If all the other stuff on the space shuttle was working I believe it would be more than enough to keep the astronauts warm (computers and such can generate a fair amount of heat).

ETA: "The payload bay doors are opened shortly after orbit is achieved to allow exposure of the environmental control and life support system radiators for heat rejection of the orbiter’s systems. " SOURCE - http://spaceflight.nasa.gov/shuttle/reference/shutref/structure/baydoors.html

10

What sources are these, and how did they describe the AC as working? If they had an AC on that ship (which I doubt), it would have been the first thing turned off.

11

From Boeing.

This is in reference to Apollo 11, but I assume apollo 13 was the same.

So they did have A/C, but why would it be on?

12

It depends on what kind of powered devices are running on your spacecraft, but in general there is a large problem in getting rid of heat rather than keeping a spacecraft warm enough. There is a short story by Frederik Polh (I believe it is “The Mapmakers”) that works on this general premise; a starship, unable to navigate to a planet to obtain air as a coolant, is slowly dying of increasing temperature as it can’t rid itself of its own waste heat from internal processes.

There are three different modes of heat transfer: convection (a moving fluid cares heat energy), conduction (heat flux through a continuum), and radiation (heat energy is transmitted directly via electromagnetic radiation). Radiation is the only way that heat is carried away in the vacuum of space (unless you are prepared to cool your vessel by evaporative cooling, which is a form of convection in which heat-containing coolant is expelled), and it is generally the least effective method of rejecting excess heat for a given exposed surface area. Unlike convection and conduction, heat transfer by radiation can only be radiated away normal to the surface; in other words, unlike finned radiators for high density convective flow, the most effective radiator is a smooth, flat or convex surface, which limits the amount of radiation that can be emitted. Another problem with radiation is temperature; although the background temperature of space is very low (2.7 Kelvin) radiation effectiveness is governed by both the absolute difference in temperature and the “bandwidth” of energy that can be carried by electromagnetic radiation of a given range, even assuming that the radiator is a perfect Planck blackbody. At temperatures we can survive (less than ~300 K) the amount of energy radiated just isn’t that great. If you can up the temperature to several thousand Kelvin then radiation becomes more effective, but the only way to do this is some kind of heat pump cycle to go from the ambient cabin temperature to the desired high temperature, which also creates heat and is ultimately self-limiting in terms of efficiency.

Several references have been made to the Odyssey capsule of Apollo XIII. It is true that one of the major problems encountered during that mission (in which bad wiring insulation going to a stir fan caused a fire that ruptured one of the liquid oxygen tanks which caused the fuel cells to be unavailable for power) was freezing. However, the reason for this is because all power was shut down to the vessel so there were no heat-generating processes of any kind save for the astronauts themselves. (Aquarius, the LEM, was designed to operate in sunlight conditions and so the major problem was warding away heat via radiation from sunlight; as such, it had a much smaller heater that was inadequate to keep the joined vessels warm for the duration of the roundabout trajectory back to Earth.) One would assume that an interplanetary spacecraft, particularly one functioning in the outsystem, would have some far more powerful energy source than just H[sub]2[/sub]-O[sub]2[/sub] fuel cells–something like a nuclear fission pile or subcritical fission-fusion reactor–which would produce significant waste heat that would have to be rejected. (Original model drawings for the Discovery spacecraft in Arthur C. Clarke’s 2001: A Space Odyssey actually had large radiating fins going out to each side, but director Stanley Kubrick vetoed them as he was concerned that the audience would believe them to be aerodynamic control surfaces–wings–that would be purposeless in vacuum and would ridicule the film for this perceived inaccuracy.)

As Whack-a-Mole mentions, heating is a major problem with the American STS Orbiter (which spends the bulk of its orbit in direct sunlight) which is why it is almost always nearly seen on-orbit with the cargo bay doors open; radiators are mounted on the interior of the doors to reject waste heat. And this is a craft that doesn’t even have any kind of high energy reactor; just fuel cells for internal power. One of the original problems with Skylab, too, was a protective flexible solar shield that wouldn’t properly deploy; the crew members of SLM-1 had to erect a temporary heat shield (dubbed a “parasol”) until they could get the stuck solar panel/radiation shield to deploy correctly. The Soviets had similar problems with early Almaz and Salyut stations (mostly resolved by the time they started doing long term habitation missions).

So far from freezing to death, you’re more likely to cook if your HVAC/radiator system isn’t working properly, especially as you move into the insystem (inside the orbit of Jupiter) where solar radiation becomes significant (>25 W/m^2) and your ship is incapable of naturally re-radiating the incoming solar energy.

Stranger

13

You realize that speed is relative and the “speed” of a spaceship has no bearing on how strong it needs to be (until you hit atmosphere).

14

It depends on, loosely speaking, the color of your spaceship. The whiter the better. Actually it’s mostly the “whiteness” in a certain range of the infrared that matters. I suppose a mirrored exterior would work too.

15

What you are talking about is its absorptivity and emissivity, which are dimensionless qualities that describe how far the body is away from being a perfect black-body.

Stranger

16

Stepping back for a moment, a few extra thoughts.

The question is probably too tightly posed. So, ignoring the issues of just what a spacecraft would be doing, and how the internal systems would be designed, what are the heat issues?

That far from the sun things are going to be different from at the Earth’s orbital distance. The sun is a tiny dot in the sky, and planets have liquid methane seas. It is cold. So you might as well just think of it as being close enough to inter-stellar space. The question then becomes: how long could you maintain a livable temperature on a spacecraft buildable with current technology in interstellar space, with no functioning internal energy sources?

This comes down to really three things. How much heat you had on board, how good your thermal insualation is, what your emissivity is.

Heat on board, comes down to mass. If we assume that you had a nice fat spacecraft that was mostly at a human happy temperature, you might have a reasonable amount of heat. Big tank of water would be a nice start. In general of course spacecraft themselves are designed to minimum wieght, so you probably won’t get much help from the craft proper.

Insulation. Since we are in a vacuum, we don’t need to worry about conduction or convection. So it is only a matter of trying to keep the radiated heat in. The answer here is going to be multiple layers of reflective material. A really nice example is the solar shield on the recently launched Hershel telescope, or best of all, the shield on the James Webb Telescope The Sunshield Webb/NASA. These are capable of sitting at Earth orbital distances and yet maintaining cryogenic temperatures on the other side. Indeed the temperature extremes are greater than interstellar to here. However the ultimate extremes are dependant upon the evaporation of liquid Helium, so this counts as active cooling. But the principle is sound. The same design could keep a large fraction of the heat inside your spacecraft.

Part of the mechanism by which the sun shields work is low emissivity. They are highly polished, and emit vastly less energy than a matt or a black surface. The outermost surface of the shield is what radiates the heat out into space where it is lost. So its emissivity is the final determinant of heat loss.

So, the answer? Hard to say. My expectation is that you will still freeze. Especially if the scenario was one where the loss of heating was unexpected. But would it be possible to design a spacecraft with modern, existing, technology that could survive? Very likey. But it would not be trivial. Live humans create some heat. But enough? Hard to say.

For completeness, the issues with heat and heat rejection are almost backwards at Earth - Sun distances. At interstellar distances a black surface will become cold very quickly, and a polished silver one will lose heat much slower. Nearer the sun, the black body will absorb solar radiation quickly and heat up dramatically, whereas the polished one will adsorb much less. The matt white of the Shuttle and spacesuits turns out to be a bit of a happy medium, and stabilises at a temperature that is within the bounds of the suit or spacecraft’s systems to be able to keep the interior within habitable bounds. A suit for visiting Venus would be silver, a suit for Mars probably black, and a suit for visiting Titan silver again. Roughly.

17

They had “A/C,” not in the sense that most people use the word (a system that cools air), but in the sense that it modified a large number of cockpit air parameters: heating, cooling, humidity, CO2 levels, etc.

It is laughable to think that if the system were left turned on, it would somehow cool the cockpit down to near freezing in the absense of the usual heat loads; this presupposes that those wacky NASA engineers neglected to include a thermostat - or that the astronauts simply sat there shivering, wondering aloud why it was getting so goddam cold in there, never realizing that someone had left the air conditioning system cranked.

18

The cooler you can keep the exterior skin of the spacecraft, the less heat will be lost via radiation. Insulation will help keep heat in the interior of the spacecraft from being conducted/convected to the exterior surface.

19

Not exactly IMO

IF they HAD to keep the whole “AC” system on to do things like keep the air breathable, yet by turning off all non critical systems that generated heat that the system was designed to get rid off, yes, they would be in a somewhat absurd situation. They would realize the absurdity of it, but they couldnt really do anything about it. If its breathe and keep some critical systems working or keep warm, the choice is clear.

Of course, such a statement really depends on the fine engineering details of the whole ship, which I don’t have, to determine if thats the case in reality.

20

The trick here is to ask, what sort of insulation? It isn’t the same set of problems as we have in an atmosphere, so what you would ordinarily think of as insulation doesn’t work. The best insulating material you can have is a vacuum. Anything else is worse.

Insulation we use on the ground is almost entirely concerned with limiting convection, as this is the dominant heat transfer mechanism in air. Rock wool, foams, double glazing, even aerogels, these all work by constraining a layer of air in a manner that it doesn’t move much. In space there is no air to constrain. A layer of insulating material will simply act as a heat transfer medium, and won’t help a great deal at all. (There are vacuum double glazing and aerogel insulators, but these are rather more exotic.)

The best insualation system we have on the ground is a thermos flask. Two reflective walls separated by a vacuum. Pretty much the only place it leaks energy is at the stopper. We keep cryogenic fluids in such flasks - and call the Dewars. This is the insulating mechanism we need on the spacecraft. Luckily we don’t need a special vacuum layer, because space is already a vacuum. Except for the first one, the walls don’t need to be capable of supporting themselves with a vacuum on one side and atmospheric pressure on the other, because the pressure is the same on both sides is the same - i.e. none. So we can make the walls out of really thin material. And we can add multiple layers with essentially zero penalty. And, guess what? We have just designed the solar shield as used on Herchel and the James Webb telescopes.

Each layer is a thin film (kapton) coated with a highly reflective material (silicon) that is also very low emissivity. (The two go hand in hand.) The outward facing surface radiates very little energy - because of its low emissivity - and that energy it does radiate is mostly reflected back by the back of the next layer. That little that does get absorbed heats up the next layer, but since it is also very low emissivity, it radiates very little, to the next layer, and so on. Five layers for the James Web solar shield. Full sunlight on one side, 50 Kelvin on the other, which is pretty impressive.

The final outer layer will be as cool as the intervening layers allow it to be. And we make sure the outer layer is as low emissivity as we can, same as the intervening layers. Which means they are all highly reflective.