Spacecraft are held in orbit by the gravity of the planet which they are orbiting. Habitability for hypothetical planets orbiting brown dwarfs has been studied. This discovery was the first known observation of satellites orbiting another planet. Astronomers can also calculate Earth's mass by observing the motion of orbiting satellites. The exosphere contains most of the satellites orbiting Earth. The mission was designed to maximize the probability of detecting planets orbiting other stars. They were prototypes intended to prove various concepts behind the large constellation of orbiting satellites. For example, in the Solar System the planets and planetoids are orbiting the Sun. Approximation is also used to analyze the motion of several planets orbiting a star. These estimates are applicable to an array of satellites orbiting the Earth. The Earth is also orbiting around the Sun in an orbital revolution. Part of an orbiting satellite's broadcast included its precise orbital data. Contact is maintained by spaceships shuttling between the stations and large orbiting space stations. HD 49674 is a star in Auriga with one planet orbiting it. The third phase is visitation by space probes, by either orbiting or flyby.