Though the International Space Station is in low-Earth orbit at 350 kilometers up, that is high enough that Earth's horizon appears curved. The satellite, which orbits the Earth every 90 minutes, can often be seen from our planet's surface as a point of light drifting across the sky at sunset. The number of residents on the International Space Station varies, though three people are permanently on board. But when ships dock, there can be as many as 10 people aboard the spacecraft. It may someday serve as a way station for travelers to the moon and Mars. Here, astronaut Tracy Caldwell Dyson looks down on Earth on Nov. 15, 2010, from the station's Cupola window bay.