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Sunday, July 10, 2016

Bootstrapping into space with helium

Imagine you're a mountain climber whose ledge has just crumbled. As you look above you, there's no way up. Below there is no way down. Because of the rocky overhang above you, there's no way to lower a rope, and also no way a rescue helicopter can get close. You have pitons, but no rope left in your backpack. I envision this silly thought experiment as an example of bootstrapping a rope all the way from the ground, to high above where you are, where you need the rope to be.

Allow me to start with an apology. My analogy sucks. Sorry. My analogies have often been criticized. The funny thing about it is that even while people are criticizing my analogy, they're also getting my point. Understanding has been conveyed. A concept has been grasped. Communication has been achieved. So...entertain my silly thought experiment, I entreat.

There you are, stuck on a mountain. How will you get down? What if people on the ground floated a helium balloon up to you? It's a big helium balloon capable of carrying a thousand feet of strong thread. The balloon floats up and you are able to capture it. You carefully wind the thread around a makeshift spindle. As you pull the thread up, you notice it gets thicker. It gets stronger. More twinings are interwoven in this thread. After a while the thread is more like string. After a while the string is more like rope. You hammer in your piton, tie the rope securely on and then simply climb down. Imagine it. A simple helium balloon was the answer all along!
Helium is the second most abundant element in the universe. Sadly, most of it is in stars. On Earth, it’s formed from the radioactive decay of elements like uranium or thorium over millions of years. If not trapped, such as in rocks or underground caves, it is so light that it floats up through the atmosphere and slowly leaks into space. Also, unlike hydrogen, an even lighter gas, helium is inert: It almost never reacts with other chemicals. So it can’t get trapped in larger molecules—as hydrogen is trapped in water, for instance—that prevent its escape.
Imagine a helium balloon carrying a load towards space. The load that the lighter balloon is carrying, is a heavier helium filled balloon with extra strong fabric. Its extra bulky thick fabric could never float off the ground by itself, but because a much lighter helium balloon is towing it with a half-mile long tether it's able to float higher and higher. Once this bulky second balloon reaches a certain atmospheric density, it begins to expand and float upwards all on its own. The first balloon is jettisoned at a certain distance and it continues upward eventually reaching an atmospheric density where the bag pops releasing its stored helium. It plummets towards the Earth, but it's cargo—much denser, much stronger, is able to rise much much further into space. This second balloon carries a third balloon, one even tougher, even stronger, even heavier... and so on, until, finally, a tough strong helium filled balloon achieves escape velocity.
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.

1 comment:

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