This space settlement concept proposed by American physicist Gerard K. O'Neill in his 1976 book The High Frontier: Human Colonies in Space. Gerard O’Neill’s iconic cylinder concept is a pair of massive counter-rotating cylindrical habitats in low-Earth orbit, each 5 miles in diameter and 20 miles long. An O'Neill Cylinder consists of two cylinders which counter-rotate around each other to create simulated gravity by centripetal force. The cylinders themselves would have six consecutive sections, half of them are the ground that people will live on, while the other half are windows to outer space. Behind each window, a mirror is installed to direct the sunlight into the cylinder and simulate daytime. Night could be simulated by simply moving the mirrors to reveal the blackness of space. For energy, the mirrors will be perpetually aimed at the Sun to collect solar energy and distribute it to the inhabitants of the Cylinder. The cylinders would be designed to recreate Earth’s gravity and house millions of people for work and play, with the goal of solving the major concerns facing Earth such as hunger, overpopulation, limited resources and war. However, O’Neill’s concept was significantly ahead of its time, and the immense structures would be too expensive to make. While the concept of the O’Neill cylinder has gained wide popularity in the realm of science fiction, its realization won’t occur until the distant future. One of the biggest problems we face today is climate change. Overpopulation, increasing carbon emissions, deforestation, and the burning of fossil fuels, are some of the man-made factors that contribute to the deteriorating conditions on planet Earth. Scientists have asked what planet Earth will look like a hundred years from now and how humans will cope with the extreme conditions and the ongoing depletion of resources. Decades ago, some scientists believed the solution lies in outer space. Perhaps the O'Neill cylinder could someday help solve Earth’s overpopulation problem. Although the construction of the O'Neill cylinder is still decades if not centuries away, space habitats will no longer be science fiction as some engineers are already building real prototypes. For example, the Orbital Assembly Corporation plans to start building the first space hotel by 2025 which could possibly open by early 2027. The space hotel named the Voyager Station will have artificial gravity and could host up to 400 guests. The space hotel will feature individual pods that are attached to a rotating wheel, and guests will have access to a movie theater, a health spa, gyms, restaurants, libraries and concert venues. Guests will also be able to watch planet Earth from their space lounge areas.