Only 5,000, but look at the pyramids. They're cool to look at, and the construction is fun to think about, but they're utterly useless. For any sort of 10,000 y

Only 5,000, but look at the pyramids. They're cool to look at, and the construction is fun to think about, but they're utterly useless. For any sort of 10,000 year space project, there's little (1%) harm in having a 100-year cool-off period to see if it still makes sense and how far technology has come.