This book takes the reader on a journey through the history of extremely ambitious, large and complex space missions that never happened. What were the dreams and expectations of the visionaries behind these plans, and why were they not successful in bringing their projects to reality thus far?
 
As spaceflight development progressed, new technologies and ideas led to pushing the boundaries of engineering and technology though still grounded in real scientific possibilities. Examples are space colonies, nuclear-propelled interplanetary spacecraft, space telescopes consisting of multiple satellites and canon launch systems.
 
Each of these enormous projects was both technically and socially linked to the time it was conceived. The large O’Neill space colonies for instance made sense in the 1970s, when people started to try and find solutions for the limitations the Earth’s resources were starting to put on human expansion. They would also enable free environments for various social community experiments, a hot topic in that decade. The idea of launching an interplanetary spacecraft from Earth using nuclear power could only have been developed in the 1950s: before then it would technically not have been possible, while afterwards environmental concerns inhibited such dangerous and polluting projects. Similarly, giant space stations for weather observation and relaying communications signals made sense in the 1950’s and early 1960’s, but were then quickly rendered obsolete by advances in electronics and satellites. Large multi-spacecraft astronomical observatories are only now seriously considered, because advances in micro-propulsion and attitude control have recently made them technically possible, and because economic realities inhibit the development of giant single-element space telescopes. As such, each project described in this book says something about the dreams and expectations of their time, and their demise was often linked to an important change in the cultural, political and social state of the world.
For each mission or spacecraft concept, the following will be covered:

• Description of the design.
• Overview of the history of the concept and the people involved.
• Why it was never developed and flown
o Analysis related to current technology
o What it would take/cost today
o Validity of pro- and con- arguments, both at the time of the project was proposed, today and in the future (i.e., whether the idea is now obsolete, economically not viable and/or technically unfeasible). These analyses will be performed with help from various international experts on the subject.
o Lessons learned and technologies obtained from the design and partial development of the concept concerned, and possible smaller derivatives that were further developed.
• What if the mission was actually carried out – consequences, further developments, etc.