m. c. de marco: To invent new life and new civilizations...

BDO of the Day: Balloon Worlds

Today’s Big Dumb Object (BDO) is a balloon world: a sphere of gas not unlike Virga, but without the carbon nanotube shell. Instead, a balloon world is mostly held together by gravity, with a lighter and more fragile skin for protection and pressurization. The usual skin design is asteroid rubble atop a thin, balloon-like membrane, making them quite unlike the hard shells of your typical BDO.

The basic balloon world is asteroid-sized, with an outer skin of some former asteroid’s rubble (asteroids are rarely as solid as they look on TV) protecting the gaseous interior. Smaller structures float around inside the bubble, rotating individually to create gravity. SciFi Ideas has a short article about building balloon asteroid colonies, but the anonymous scientist author maintains an entire blog devoted to the idea. You may recall the friction issues involved in rotating structures within an atmosphere from the megasphere; in this case they’ve been addressed with a novel solution involving flow dividers.

You can super-size your balloon world to sizes we’ve seen before in super-Virgas: Adam Crowl calculates that you can inflate a balloon of Earth-like atmospheric gas to about 21,600 miles in diameter before the self-gravitation that’s holding your balloon together starts working against you and the sphere collapses under its own gravity. If you change your atmosphere to a still-breathable but much lighter heliox mix, you can stretch out to about 285,000 miles in diameter (enclosing 12 quadrillion cubic miles). A balloon world at 250,000 miles in diameter weighs only about one Earth mass between the atmosphere and the shell, yet encloses eight quadrillion cubic miles of space, or two dozen Jupiters by his estimate.