and scientific gain." Design iterations Following the presidential announcement, NASA began a set of studies to determine the potential uses for the space station, both in research and in industry, in the U.S. or overseas. This led to the creation of a database of thousands of possible missions and payloads; studies were also carried out with a view to supporting potential planetary missions, as well as those in low Earth orbit. Several Space Shuttle missions in the 1980s and early 1990s included spacewalks to demonstrate and test space station construction techniques. After the establishment of the initial baseline design, the project evolved extensively, growing in scope and cost. "Power Tower" (1984) In April 1984, the newly established Space Station Program Office at Johnson Space Center produced a first reference configuration; this design would serve as a baseline for further planning. The chosen design was the "Power Tower", a long central keel with most mass located at either end. This arrangement would provide enough gravity gradient stability to keep the station aligned with the keel pointed towards the Earth, reducing the need for thruster firings. Most designs featured a cluster of modules at the lower end and a set of articulated solar arrays at the upper end. It also contained a servicing bay. In April 1985, the program selected a set of contractors to carry out definition studies and preliminary design; various trade-offs were made in this process, balancing higher development costs against reduced long-term operating costs. Revised Baseline Configuration (1987) At the same time, late 1986, NASA carried out a study into new configuration options to reduce development costs; options studied ranged from the use of a Skylab-type station to a phased development of the Dual-Keel configuration. This approach involved splitting assembly into two phases; Phase 1 would provide the central modules, and the transverse boom, but with no keels. The solar arrays would be augmented to ensure 75 kW of power would be provided, and the polar platform and servicing facility were again deferred. The study concluded that the project was viable, reducing development costs while minimizing negative impacts, and it was designated the Revised Baseline Configuration. This would have a development cost of US$15.3 billion (in FY1989 dollars) and FEL in the first quarter of 1994. This replanning was endorsed by the National Research Council in September 1987, which also recommended that the long-term national goals should be