would have been more likely, based on astronauts' training for the task. Martin Marietta won the contract for US$26 million to design the apparatus. TRS would contain about three tons of propellant. The remote-controlled booster had TV cameras and was designed for duties such as space construction and servicing and retrieving satellites the shuttle could not reach. After rescuing Skylab, the TRS would have remained in orbit for future use. Alternatively, it could have been used to de-orbit Skylab for a safe, controlled re-entry and destruction. In two shuttle flights, Skylab would have been refurbished. In January 1982, the first mission would have attached a docking adapter and conducted repairs. In August 1983, a second crew would have replaced several system components. In March 1984, shuttle crews would have attached a solar-powered Power Expansion Package, refurbished scientific equipment, and conducted 30- to 90-day missions using the Apollo Telescope Mount and the Earth resources experiments. Over five years, Skylab would have been expanded to accommodate six to eight astronauts, with a new large docking/interface module, additional logistics modules, Spacelab modules and pallets, and an orbital vehicle space dock using the shuttle's external tank. The first three phases would have required about US$60 million in 1980s dollars, not including launch costs. Other options for launching TRS were Titan III or Atlas-Agena. After departure After a boost of by Skylab 4's Apollo CSM before its departure in 1974, Skylab was left in a parking orbit of by that was expected to last until at least the early 1980s, based on estimates of the 11-year sunspot cycle that began in 1976. NASA had first considered the potential risks of a space station reentry in 1962, but decided not to incorporate a retrorocket system in Skylab due to cost and acceptable risk. The spent 49-ton Saturn V S-II stage which had launched Skylab in 1973 remained in orbit for almost two years, and made a controlled reentry on January 11, 1975. The re-entry was mistimed however and deorbited slightly earlier in the orbit than planned. Solar activity British mathematician Desmond King-Hele of the Royal Aircraft Establishment (RAE) predicted in 1973 that Skylab would de-orbit and crash to Earth in 1979, sooner than NASA's forecast, because of increased solar activity. Greater-than-expected solar activity heated the outer layers of Earth's atmosphere and increased drag on Skylab. By late 1977, NORAD also forecast a reentry in mid-1979; a