To see a world in a grain of sand And a heaven in a wild flower, Hold Infinity in the palm of your hand And Eternity in an hour.

William Blake, from Auguries of Innocence

SO what's new? Many of the ideas expressed in this book are not new. It is only now, however, after decades of fundamental research and laboratory experimentation that many of these technologies are at the point where they can be seriously considered for real applications in space. For the foreseeable fUture, space exploration technologies will be based on ideas that are as old as the Space Age itself, if not older. There were 60 years of laboratory research between Hermann Oberth's discussion of electric propulsion in 1939 and the 1998 flight of an ion engine as the primary propulsion system for the Deep Space 1 mission. It took 90 years for Konstantin Tsiolkovsky's solar sail to go from an interesting idea to a realizable propulsion system. As we move closer to fielding systems such as these, it is rightful to ask if there are any great new ideas in the pipeline that might be available for the next generation of space explorers.

Not so very long ago, interplanetary travel by rocket was a fantasy, considered by most to be unworthy of serious consideration. People who publicly speculated about life beyond the confines of Earth's biosphere risked scorn, ostracism or worse.

We have indeed come a long way. But it is worthy of note that the approaches presented here to interplanetary travel and development rest heavily upon early twentieth-century physics and mid-twentieth-century technological projections. It may be a good idea to follow the poet William Blake and attempt to "Hold Infinity in the palm of your hand'' and speculate about possible future developments that may ease the road ahead.

Many of the suggested exotic approaches to space flight that we are about to consider may prove to be physically impossible or impractical to implement. But it must be remembered that only one need succeed. Even if they all fail, the seed of a realistic breakthrough concept may be planted by this discussion in the fertile soil of a reader's mind.

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