Some of the moons of the outer planets of the solar system are large enough to be suitable places for colonization. Even small moons could be dug out to create habitats. They are not big, but they are many. Many of the larger moons contain water ice, liquid water, and organic compounds that might be useful for sustaining human life. Colonies in the outer solar system could also serve as centers for long term investigation of the planet and the other moons. In particular, robotic devices could be controlled by humans without the very long time delays needed to communicate with Earth. There have also been proposals to place robotic aerostats in the upper atmospheres of the gas giant planets for exploration and possibly mining of helium-3, which could have a very high value per unit mass as a