Here’s the sort of crazy idea that animates our office conversation at Scientific American. It all started with my colleague Michael Moyer’s joke that a certain politician could build his moon base using a balloon: just capture the hot air and float all the way up. Ha ha, we all know that balloons don’t work in outer space.

But is that really true? Why couldn’t they?

The more I thought about it, the more confused I got, so let me float it as a trial balloon and see whether you can shoot it down. Ground rules: no weaselly appeal to “feasibility” or “practicality” allowed. You have to argue from pure physics.

What makes a space balloon conceivable is that space is not a true vacuum. Even intergalactic space is filled with matter, albeit tenuous; by its standards, Earth’s extended atmosphere is a thick soup. As long as the balloon’s interior density is lower than the ambient density, it should rise—no matter how low the ambient pressure is. Drag force will limit the balloon's ascent velocity, but shouldn’t stop it altogether and can be minimized by choosing a prolate rather than spherical shape.

As the balloon rises, it will expand in inverse proportion to the ambient pressure and, neglecting temperature, density. At launch, the interior and exterior pressure is equal, and the interior density is lower; during the ascent, the pressure remains equalized, so the interior density will always be less than the ambient. Neglecting temperature is probably not a bad approximation: the absolute temperature will vary at most a couple of orders of magnitude, whereas the pressure and density fall off much more drastically, and in any event we can include a politician to regulate the temperature difference between interior and exterior.

The material tension would rise in proportion to radius. It has units of force, and the maximum possible force in nature, the Planck force, is 1044 newtons, so the balloon could get bigger than the known universe before it absolutely has to pop. The balloon walls would become extremely thin and porous, but because of the scaling of area and volume, they should always remain able to confine the gas.

Bottom line: if you release a helium balloon on the ground, it should rise forever! It will float up until Earth's atmosphere dovetails with the interplanetary medium, then float up and out of the solar system, then reach interstellar space and float out of the plane of the galaxy like the bubbles blown by supernova, and ultimately settle in one of the voids of large-scale cosmic structure.

Unless I’m missing something, it is a myth that balloons are inherently unable to work in space. The limit is set not by physics, but by trifling engineering problems such as material strength and permeability. Another caveat is that the laws of gas dynamics assume a continuum, an approximation that already fails in Earth’s upper atmosphere.

Now, someone, tell me what I’m missing.