# B Temperature regulation in Earth orbit

#### Treva31

If I had an object in an orbit around the Earth that I kept continually shaded from the sun, how cold would it get?
Assuming the shade device was not physically connected to and was sufficiently far away from the object to not radiate any heat to it.

And if I actively adjusted how much shade the object was under could I regulate the temperature to anything I wanted?
Between -100 and +260 celcius I would guess?
Assuming an orbit in continuous sunlight.

Would it be any different on the Moon?

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#### Andrew Mason

Homework Helper
Between -100 and +260 celcius I would guess?
Assuming an orbit in continuous sunlight.
According to NASA (based on the ISS experience) it would be 121C to -157C:

Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C).

Would it be any different on the Moon?
It would be different on the moon depending on the location. It can get pretty cold on some parts of the polar regions of the moon that are sheltered from the sun and the earth. See this NASA temperature map of the moon.

AM

#### Anand Sivaram

I could understand 121C as it the Stefan Boltzman equivalent temperature for the Solar flux density of 1360W/m2. But, how are we getting -157C, that I did not understand.

#### russ_watters

Mentor
That's not actually an answer to the OP's question. That's the answer for a particular system, if unregulated. The OP is asking what you can get with purpose-designed passive regulation (a heat shield and insulation). In theory, the answers should be:

1. As cold as arbitrarily close to the CMB temperature
2. As hot as arbitrarily close to the sun's temperature.
[3. Anything in between.]

I'm not sure why anyone would test the upper limit, but googling gives me one example down to 60K (-213C):

#### russ_watters

Mentor
I could understand 121C as it the Stefan Boltzman equivalent temperature for the Solar flux density of 1360W/m2. But, how are we getting -157C, that I did not understand.
When shaded from the sun and nearby planets, the only thing an object radiates against is the CMB.

#### Anand Sivaram

CMB has a temperature of 2.7K or -270C. But, Moon craters are one of the coldest places in the entire solar system at 26K. Any reason for this difference?

#### Filip Larsen

Gold Member
Don't forget, that in (low) orbit around Earth a little under a half-sphere of thermal radiation input comes from Earth. Also, for overall considerations one can usually safely ignore CMB thermal input when there are other (significant) sources around.

#### Anand Sivaram

That could be a valid point, the Earth has an average albedo of 0.3, that would mean that we are reflecting 30 percent of solar energy back to space.

#### russ_watters

Mentor
CMB has a temperature of 2.7K or -270C. But, Moon craters are one of the coldest places in the entire solar system at 26K. Any reason for this difference?
The moon is a big rock and it stores heat.
That could be a valid point, the Earth has an average albedo of 0.3, that would mean that we are reflecting 30 percent of solar energy back to space.
When you're trying to get something reeeaaally cold, that's a lot. Also it's not really about Earth's albedo, but its radiation temperature. If an object in LEO has a side facing Earth, Earth fills up most of its field of view on one side. So the minimum temperature on that side is the night-time temperature of Earth.

#### Filip Larsen

Gold Member
That could be a valid point, the Earth has an average albedo of 0.3, that would mean that we are reflecting 30 percent of solar energy back to space.
Its not just albedo radiation, there is also a thermal contribution from the atmosphere itself. For "first-order precise" work one can consider the Earth to be a spheric black body radiator at around -20 deg C. See [1] for a description relevant to a nano-sat and [2] for a similar question answered on stackexchange.

[1] http://webapp.tudelft.nl/proceedings/ect2012/pdf/chandras.pdf
[2] https://space.stackexchange.com/questions/7827/whats-the-typical-temperature-of-a-satellite-orbiting-the-earth

"Temperature regulation in Earth orbit"

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