Spacecraft and sunlight

I understand that sunlight is orders of magnitude more intense in space than on earth. Doesn’t sunlight cause problems in spacecraft that have windows, like the space shuttle? Do they have automatic shutters, or do they just take care to always point their windows away from the sun?

Not for objects in earth orbit such as the Shuttle. With the sun overhead on a clear day, something like 73% of incident solar energy reaches the earth’s surface. So in low-earth orbit, you’d have to deal with around 37% more solar energy.

However, objects on the ground can transfer heat to the environment in lots of different ways. They touch the ground, they touch the air, water falls on them, and so on. Also, the sunlight they get is much more frequently eclipsed by the Earth than for objects in space. Objects on the ground spend almost half their time shielded from the Sun by the mass of the Earth.

But all this isn’t “orders of magnitude”. Even if you’re in shadow half the time, that means you get half the sunlight, not an order of magnitude less.

Doesn’t the atmosphere filter away much of the sunlight, too? I imagine that a sunny noon in mid-summer isn’t nearly as hot as a few minutes of unshielded sunlight in the vacuum of space.

Think of it this way - the effective depth of the atmosphere is about 10km (if you squish it down so it is all at sea level pressure). If you stand on a hill you can easily see for 10 km with little to no loss in apparent brightness. This is a worse test than looking down through the atmosphere as the dust and particulates are concentrated in the lower levels.

Where I think things are confusing is that the atmosphere is good at filtering out the far UV light. This seems to translate to a notion that the unfiltered sunlight must be significantly more “strong” in some way. However the Sun is a simple black body radiator where the chromosphere is roughly 5780 K. The energy from the sun peaks at 5013nm, (which is in the middle of the visual range) and drops off after this. Although there is significant energy in the UV, it is less than that in the visual wavelengths. Similarly the relative intensity in the IR is also less than the visual wavelengths. (The effective colour of sunlight is 6500K because the blue sky shifts the balance of the spectrum to the blue - so sunlight as we see it at ground level is actually bluer than in space.)

This isn’t to say that thermal control of spacecraft isn’t hard - it is both difficult and critical to get right. But the problem is that there is no atmosphere to convect energy away, not that the incoming energy is hugely greater than on the ground.

The ionosphere is responsible for absorbing the far UV, and this doesn’t form part of the 10km equivalent absorbtion, but we can’t see in the far UV, and the black body radiation spectrum show that the actual intensity of far UV is much less than intensity of the visual wavelengths.

An object in low earth orbit typically finds the sun eclipsed by the earth once per orbit, or around 16 times as often as an object on the ground.

Which is pretty close to what the Shuttle does.

If around 37% includes about 34%.

When I divide 100 by 73, the answer comes up as 1.369 - which, rounded, is a 37% increase.

Indeed. I have no idea what I did to get 34%, but as it’s wrong it doesn’t really matter.