Space being hot vs cold

Apollo 13 tried to freeze over, but on the moon people had to have cooling in the suits.

The suits were so small that the heat got trapped better? The capsule was less insulated? It seems kinda of contradictory.

I’m sure an actual physicist will be along to give a better answer, but as I understand it (and I Am Not A Physicist), space isn’t “hot” or “cold”. It’s a vacuum (or pretty close to it), and a vacuum doesn’t have a temperature. There is, by definition, nothing to store or transfer heat.

Which means, if you don’t have a heat source, you get really, really cold. But if you do have a heat source, like sunlight, you get really, really hot.

Apollo 13 spent a good chunk of its flight in shadow. Without direct sunlight, and with no internal heat source other than the astronauts’ body heat, it got pretty cold.

In contrast, the missions that reached the Moon spent their time on the surface during the lunar day, very much by design so they’d have plenty of light to see by. But that also means they’re absorbing a lot of solar radiation, and reflected radiation from the Moon’s surface, but without any air to shed excess heat to, so they got really hot, and keeping them cool was a major engineering challenge.

In the vacuum of space, the only heat is radiant heat*, and the single largest source of radiant heat remains the Sun.

The temperature difference between sunlight and shade in open near-Earth space is 220 degrees Celcius.

So, are you in the sunlight, or the shade?

As for Apollo, their spacecraft was well enough insulated that sunlight on the hull didn’t warm the interior very well. Still, the interior never got below freezing, even as it might have within its own shadow.

The thermal balance of the spacecraft allowed it to shed excess heat generated by the on-board electronics and maintain a “shirt-sleeve” working temperature. Shut off the electronics to conserve electricity and the thermal balance is wrong.

*Technically, you could have conductive heat transfer, if you’re touching a heat source, but that doesn’t enter into this.

In the shadow of what?

Its own shadow would be enough. And even orienting the spacecraft to allow sunlight into the cabin through its small windows wasn’t enough. Most of the heat remained on the surface.

It helps to be more specific than that. Most things cannot be in their own shadow exactly. Do you mean that one piece was in the shadow of another? The lander was shadowing the flight capsule or whatever?

Actually everything is always in its own shadow assuming it casts one.

If you cast a shadow then the side closest to the shadow is in shadow. Such as the side of the Earth that is in nighttime. Or the bottom of a turtle. Or (potentially) half of a spacecraft.

Stating that half an object is not on the sunward side is obvious. I hope that that is not the intent of the phrase, in its own shadow.

“Temperature” is the measurement of how quickly molecules of some substance are moving.

Strictly speaking, the temperature of outer space is very, very cold – just a few degrees above absolute zero. But since space is so empty, the heat from direct sunlight vastly overpowers the cold on a local scale. The same thing applies to a person’s body heat. And also because space is so empty, there is no good way to get rid of excess heat; it’s not like there’s a cooling breeze.

Thus, you have the paradox of needing to cool things down, even though those things are in an environment far colder than Antarctica can ever be.

Seems a reasonable statement to me. It’s night time where I am presently. What shadow am I in? The shadow of the Earth. I.e., I am in the Earth’s own shadow.

I wonder if empty space itself can have a temperature at all. Could the virtual particles that appear and disappear constantly be hotter or older, and could this, if so, be measured at all? Probably not, I would guess.

If it’s completely empty, then it has no temperature. If it contains, say, photons, then you can talk about the temperature of the photons.

Your hope is dashed.

Any set of particles can have a temperature. it’s even more general than that, electronic signals can be assigned a temperature (I.E., the signal itself, the electrons carrying the signal may have a DIFFERENT temperature). Temperature is just a parameter is a particular distribution in the same way that sigma is a parameter of the normal distribution. Therefore, I’m sure it is entirely reasonable for virtual particles to have a temperature (though I’ve never studied field theory to know for certain).

As Chronos said, space is filled with photons called the “cosmic microwave background” which can be assigned a temperature. But this ISN’T a temperature something like a spacecraft would “feel”.

What the other posters mean by Apollo being in it’s own shadow is that the skin of spacecraft is designed to reflect light. This is the ONLY way to cool an object in space. Therefore, the skin of Apollo shielded the body of the spacecraft from the heat (via light) from the sun. In this sense, the Apollo spacecraft was in it’s own shadow.

Because radiative heating and cooling is the only heating that matters in space. Objects can get VERY hot if they absorb lots of light from the sun, or very cold if they radiate a lot of light. This has nothing to do with how “hot” or “cold” space itself is (since, as discussed above, space doesn’t really have a temperature in the normal sense).

Not really. The sun, after all, is “getting rid of excess heat” all the time! Other than the fact that fans don’t help, the basic process for cooling in space is much the same as on earth. When active cooling is required on spacecraft, they pump heat out into radiators. The ISS has a whole bunch of them. Thermal energy is then removed through IR radiation, which dissipates in the form of photons.

I regret my choice of topic heading. All I wanted was an explanation of the seeming contradiction about astronauts being cold versus being hot. I’m not asking if space is actually cold or hot.

I think the important thing to remember is that while “space” is cold the only means of heat transfer is via radiation which is the least efficient method (conduction and convection being the others).

While space is colder than very cold water a person would freeze to death more quickly in very cold water than in space (ignoring all the other issues you would have being unprotected in space).

The Space Shuttle and ISS actually have to work to remove heat. When you have really well insulated spaces removing heat can become the priority and troublesome.

Imagine that “your house was really, really well insulated and you closed it up and shut off the air-conditioning,” said Gene Ungar, a thermal fluid analysis specialist at NASA’s Johnson Space Center. “Almost every watt of power that came through the electric wires would end up as heat.”

This is just what happens on the Space Station. Energy from the solar arrays flows into the ISS to run avionics, electronics … all of the Station’s many systems. They all produce heat, and something has to be done to get rid of the excess. SOURCE

I’ll try again. Keep in mind, I Am Not A Physicist, and I Am Also Not A Rocket Scientist, but here’s my layman’s understanding.

Knowing that space isn’t actually hot or cold is a key component to understanding what’s going on with the Apollo spacecraft and the astronauts on the Moon.

Our everyday intuitions about temperature and heat transfer are shaped by our experience of living on the surface of a rocky body, under a few miles of atmosphere. Those intuitions fail in outer space, on the surface of a rocky body with (effectively) no atmosphere.

In our everyday life, we’re surrounded by and in constant contact with atmospheric gases that exchange heat with us. We’re also surrounded by objects that themselves are continuously absorbing and radiating heat.

In outer space, the Apollo spacecraft is surrounded by a vacuum (or close enough). The astronauts on board, and the electrical and mechanical systems when they’re working, generate waste heat. When directly exposed to direct sunlight, the craft is also absorbing radiant heat from the Sun. It’s shedding heat through electromagnetic radiation - infrared light. But that’s relatively inefficient compared to the kinds of heat transfer we’re used to here on Earth, so the craft can get too hot to be survivable for the astronauts pretty quickly. To compensate, the capsule was designed to reflect as much incoming solar radiation as possible, and rotated so one side didn’t get too hot. Under normal operating conditions, in sunlight, the main concern is how to protect the craft from overheating.

But, the fire on the spacecraft shut down almost all the systems. At that point, the only internal source of heat was the astronauts’ body heat, which they were shedding to the capsule, which was in turn radiating it out into space. Because it had been designed to reflect solar radiation, it wasn’t absorbing enough to keep up with the radiative heat loss, and the capsule got cold.

It also spent part of its flight in the Moon’s shadow, and during that period, it had no heat source other than the astronauts’ body heat.

Compare that to the astronauts on the moon. While in their space suits, they only needed to heat up the suits, much smaller than the capsule. Between their body heat and the solar radiation during the lunar day, the main problem again was shedding excess heat. The lunar lander was also receiving direct solar radiation, and had operating systems on board generating waste heat, so dissipating that excess heat was again an issue.

If I got any of that wrong, I’m sure one of our resident physicists will be along to correct me.

Just who is this space being of whom you speak? Is it there with you now?

;-D

We know of at least one space being who’s OK with the airlessness of absolute space and the heatlessness of absolute zero.