Colonization of the outer Solar System is subject to only 0.01 rem a day. When NASA carried out a study called "HOPE" (Revolutionary Concepts for Human Outer Planet Exploration) regarding the future exploration of the Solar System, the target chosen was Callisto. It might be possible to build a surface base that would produce fuel for further exploration of the Solar System. The Keck Observatory announced in 2006 that the binary Jupiter trojan 617 Patroclus, and possibly many other Jupiter trojans, are likely composed of water ice, with a layer of dust. This suggests that mining water and other volatiles in this region and transporting them
Planet Nine some combinations of semi-major axis and mass) and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected. Brown speculates that the predicted planet is most probably an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas. In fact, if it once orbited the region of the gas/ice giants, the planet probably