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Basic Information

"The Dirty Snowballs" of our solar system, Comets are small bodies of rock and ice that orbit our Sun. They are similar to asteroids, but distinguished by their fuzzy atmosphere and sometimes tail(s). The atmosphere, called a "coma", is an irregular envelope of gas and vapor that steams off and surrounds the comet when it passes near the sun. The tail is an extension or appendage of the coma, and always points directly away from the sun. In addition to the gassy tail, there's often a second tail of dust particles that the expanding gases pushed away from the comet. This second tail will trail and curve as the comet moves, having more mass and thus more inertia to resist the magnetic field that pushes the gassy tail away from the Sun. Note that the coma and tails only appear when the comet is reasonably close to the sun - many comets have orbital cycles that go years without a coma, followed by just a few months with one. The rocky nucleus of the comets in our solar system range from 100 meters to 50 kilometers in diameter, but their tails can (under the right circumstances) extend to be longer than the diameter of the sun.

Within the scientific community, there's something of a debate over comet composition. Originally they were thought to be mostly ice. Then it was suggested that they may in fact be mostly rock (like an asteroid) with interior ice that vents out of the rocky shell. A typical "ingredient list" for a comet probably includes rock, dust, water ice, and frozen ammonia and organic materials such as carbon monoxide, carbon dioxide, and methane. These organic components may explain why some comets have a black (described as asphalt-like) surface when viewed via telescopes.

While all currently-known comets orbit our sun, presumably other stars and star systems have similar bodies. More than 3,000 comets have been detected and catalogued in our system.

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Game and Story Use

  • Disaster plotline: Nemesis (Star) knocks a bunch of comets from the Oort Cloud onto collision-courses with the Earth. The impact will be worse than the Tugunska Event.
  • Asteroid Mining isn't just for asteroids anymore.
    • A comet may have rare complex organic compounds worth mining.
    • In a system with a Desert Planet, it may be worth the expense of mining ice from comets.
  • While Stealth In Space is pretty tricky and hard to pull off, I suppose one option would be to cut your engines and glide in behind a comet, hoping that the coma and tails would make it hard to detect your reduced heat signature. Probably somewhat dangerous, and it requires a lot of patience and planning (or very fortuitous coincidental timing) to pull off.
  • Habitable comets! A comet might be effectively terraformed by a genetically engineered Dyson Tree.
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