is a dynamically elevated ring placed around the Earth that rotates at an angular rate that is faster than orbital velocity at that altitude, stationary platforms can be supported by the excess centripetal acceleration of the super-orbiting ring (similar in principle to a Launch loop), and ground-tethers can be supported from stationary platforms. The Bernal sphere is a proposal for a spherical space colony with a maximum diameter of 16 kilometers. It would have gravity at the equator, and gradually turn to zero G at the poles. Rotating wheel space stations, such as the Stanford torus, are wheel-like space station which produce artificial gravity by rotation. Typical designs include transport spokes to a central hub used for docking and/or micro-gravity research. The related concepts, O'Neill and McKendree cylinders, are both pairs of counter-rotating cylinders containing habitable areas inside and creating 1g on their inner surfaces via centripetal acceleration. The scale of each concept came from estimating the largest 1g cylinder that could be built from steel (O'Neill) or carbon fiber (McKendree). Hollowed asteroids (or Bubble worlds or Terraria) are spun on their axis for simulated gravity and filled with air, allowing them to be inhabited on the inside. In some concepts, the asteroid is heated to molten rock and inflated into its final form. A stellaser is a star-powered laser or maser. Trans-orbital structures A skyhook is a very long tether that hangs down from orbit. A space elevator is a tether that is fixed to the ground, extending beyond geostationary orbital altitude, such that centripetal force exceeds gravitational force, leaving the structure under slight outward tension. A space fountain is a dynamically supported structure held up by the momentum of masses which are shot up to the top at high speeds from the ground. A launch loop (or Lofstrom loop) is a dynamically supported 2000 km long iron loop that projects up in an arc to 80 km that is ridden by maglev cars while achieving orbital velocity. StarTram Generation 2 is a maglev launch track extending from the ground to above 96% of the atmosphere's mass, supported by magnetic levitation. A rotovator is a rotating tether where the lower tip is moving in the opposite direction to the tether's orbital velocity, reducing the difference in velocity relative to the ground, and hence reducing the velocity of rendezvous; the upper tip is likewise moving at greater than orbital velocity,