ヒル球
An astronaut could not have orbited the Space Shuttle (with mass of 104 tonnes), where the orbit was 300 km above the Earth, because its Hill sphere at that altitude was only 120 cm in radius, much smaller than the shuttle itself. A sphere of this size and mass would be denser than lead. In fact, in any low Earth orbit, a spherical body must be more dense than lead in order to fit inside its own Hill sphere, or else it will be incapable of supporting an orbit. A spherical geostationary satellite, however, would only need to be more than 6% of the density of water to support satellites of its own.