Skyhook (structure)

Different synchronous non-rotating orbiting skyhook concepts and versions have been proposed, starting with Isaacs in 1966, Artsutanov in 1967, Pearson and Colombo in 1975, Kalaghan in 1978, and Braginski in 1985. The versions with the best potential involve a much shorter tether in low Earth orbit, which rotates in its orbital plane and whose ends brush the upper Earth atmosphere, with the rotational motion cancelling the orbital motion at ground level. These "rotating" skyhook versions were proposed by Moravec in 1976, and Sarmont in 1994. This resulted in a Shuttle-based tether system: the TSS-1R mission, launched 22 February 1996 on STS-75 that focused in characterizing basic space tether behavior and space plasma physics. The Italian satellite was deployed to a distance of from the Space Shuttle. In 2000 and 2001, Boeing Phantom Works, with a grant from NASA Institute for Advanced Concepts, performed a detailed study of the engineering and commercial feasibility of various skyhook designs. They studied in detail a specific variant of this concept, called "Hypersonic Airplane Space Tether Orbital Launch System" or HASTOL. This design called for a hypersonic ramjet or scramjet aircraft to intercept a rotating hook while flying at Mach 10. In 2007, a student-built satellite called Young Engineers' Satellite 2 (YES2), part of ESA's Foton-M3 microgravity mission, deployed a 31.7 km tether. This was the longest tether ever deployed in space and officially set the Guinness world record While no skyhook has yet been built, there have been flight experiments exploring various aspects of the space tether concept in general. ==Rotating skyhook==

curve. At the lowest point it is momentarily stationary with respect to the ground, where it can 'hook' a payload and swing it into orbit. By rotating the tether around the orbiting center of mass in a direction opposite to the orbital motion, the speed of the hook relative to the ground can be reduced. This reduces the required strength of the tether, and makes coupling easier. The rotation of the tether can be made to exactly match the orbital speed (around 7–8 km/s). In this configuration, the hook would trace out a path similar to a cardioid. From the point of view of the ground, the hook would appear to descend almost vertically, come to a halt, and then ascend again. This configuration minimises aerodynamic drag, and thus allows the hook to descend deep into the atmosphere. == Similar concepts ==

The capture-ejector rim is a variation that consists of a rim- or ring-shaped structure. Like a rotating skyhook, it would rotate in a direction opposite to its orbital motion, allowing a spacecraft at suborbital velocity to attach to its lower portion and later be flung into orbit from its upper portion. It would be easier for a spacecraft to attach to the lower portion of a capture-ejector rim than to attach to the end of a skyhook (which would only point downwards for a brief period of time). ==See also==