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Viewing Single Post From: The giant space ship example
Chris Ho-Stuart
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Here's why I think this is an excellent problem.

The giant starship gives a realistic (or at least conceptually sensible) completely defined problem, that naturally avoids many of the additional complications that would be involved on planet Earth.

There are some additional constraints that are needed to get an answer. We need to know the atmospheric composition. If other people have alternative methods to answer the problem, then they should indicate what additional assumptions are needed and propose sensible constraints.

To solve this problem we don't need to worry about any of the following complications.

  • There's no problem with summer and winter, or day and night. We really can treat the moon sized ship as homogenous all over its surface.
  • There's no incoming solar radiation where we need to worry about atmospheric absorption.
  • We can pick a very straightforward atmospheric composition.
  • We can assume a dry atmosphere, with a simply calculated lapse rate.

I am proposing to give an estimate for power required to maintain the starship surface temperature, using the conventional atmospheric greenhouse effect as taught in conventional physics or meteorology courses. Critics of atmospheric greenhouse theory may propose an alternative theory, and show how they would answer the question posed for the topic.

I was struck at how effectively Gbaikie's problem managed to strip away various complications and give a clear and concrete case for which a reasonably direct calculation is feasible; but would which still requires a non-trivial application of physical theory for the impact of an atmosphere.

The easy solution is with a hypothetical idealized "grey" gas, with the same absorptivity across the spectrum.

A realistic but still pretty straightforward solution could use an atmosphere of only Oxygen and Nitrogen, plus just one real greenhouse gas, with CO2 being an obvious choice.

I think that about 2% CO2 by volume is the maximum credible concentration to allow passengers to live and work "outside" for extended periods without harm. (On Earth the concentration is about 0.04%.) So I am hoping to calculate answers for various compositions of the starship atmosphere, using 21% Oxygen, and Nitrogen with Carbon Dioxide for the rest. I'll use a range of concentrations, from 0 to 2%, for the Carbon Dioxide.

I'm not ready to give answers yet, but I love the problem!

People who think it is sufficient to use the lapse rate, or the specific heat in the atmosphere, should try this example also. I think they will find they really do need to consider additional properties of the atmosphere; in particular the optical depth at various frequencies in the infra red.
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The giant space ship example · Physical theory for climate