Dark Skies is the first work to assess the full impacts of space expansion, past, present, and future. Thinking about space, and the visions fervently promoted by the global space movement, is dominated by geographic misperceptions and utopian illusions. The parts of space where almost all activity has occurred are part of the planet Earth, its astrosphere, and, in practical terms, are smaller than the atmosphere. Contrary to frontier visions, orbital space is already congested and degraded with dangerous space debris. The largest impact of actual space activities is an increased likelihood of catastrophic nuclear war stemming from the use of orbital space and space technology to lob nuclear weapons at intercontinental distances. Building large-scale orbital infrastructures will probably require or produce world government. The ultimate goal of space advocates, the colonization of Mars and asteroids, is promoted to guarantee the survival of humanity if major catastrophes strike Earth. But the spread of humanity into a multiplanet species will likely produce an interstate anarchy highly prone to total war, with Earth having many disadvantages. Altering the orbits of asteroids, a readily achievable technology vital for space colonization, also makes possible “planetoid bombs” with destructive potentials millions of times greater than all nuclear weapons. The biological diversification of humanity into multiple species, anticipated by space advocates, will further stoke interworld wars. Astrocide—the extinction of humanity resulting from significant space expansion—must join the lengthening list of potential threats to human survival. Large-scale space expansion should be relinquished in favor of an Earth-oriented space program of arms control and planetary security.