Galileo Galilei (1564-1642) was
the first to recognise that a massive object moving in a straight
line with a constant velocity would continue to do so unless it
is acted upon by some external force. It had previously been
thought that such an object would eventually come to rest after
some period of time had elapsed. This was the Aristotelian world
view which had prevailed for almost two millennia before
Galileo's ideas took hold. Arguably, this was the single most
important observation, indeed the cornerstone, of all the great
discoveries (in classical mechanics) which were to follow in
The next great step came with sir Isaac Newton (1642-1727). He was the first to give quantitative meaning to the concepts of inertia, relative motion and the term action-at-a-distance as a description of the possible mode of action of forces such as gravity and magnetism. He made clear the fact that there is no way to distinguish absolute rectilinear motion from relative rectilinear motion and thus done away with the notions of absolute motion and absolute space. These views were clearly the precursors to modern relativity theory. He was also the first to suggest how the 'fixed stars' may act as a kind of reference against which the absolute motion of the earth, through space, could be determined. He formulated the concept of angular momentum and put forward his three laws of motion, the third of which (all action is accompanied by an equal but opposite reaction) is, today, recognised as the only physical law of the three. He new that the concept of mass could not be defined explicitly but rather characterised in terms of its inertial and gravitational properties. That is, mass cannot be described independently of the notion of force. Newton's works encompassed many disciplines, the corpuscular theory of light, spectroscopy, mathematics and even alchemy. He developed the idea of mathematical induction and proof by contradiction.
However, it is a well known fact that Newton was not very kind or even mildly altruistic in character, witness his litigious attacks on his scientific contemporaries, especially his run-ins with Leibnitz and Hook. However, his greatest contribution to science was probably his work on universal gravitation, i.e. he discovered the mathematical relationship which determines the forces which exist between masses, small or large, throughout the universe, and correctly identified this force as the one and the same force which holds us all firmly on the ground, gravity.
Today, with hindsight, we can clearly trace the successful development of spaceflight to these early brilliant contributions to science, made over the last two to three centuries of scientific discovery, and the subsequent explosive advancement of technology which followed throughout the late nineteenth and early twentieth century.
Although the history of space travel is a short one, surprisingly, there exist accounts of flying machines able to reach the stars, (so the accounts go), to be found in ancient Hindu manuscripts and other written and pictographic records from various past cultures. Testament, if nothing else, to man's deep seated desire to conquer the cosmos. This dream is as pervasive and deep rooted as the wish for the fountain of youth, cutting across all cultural and religious beliefs and spanning many millennia. However, for the purposes of this discussion, it suffices to consider only our own century since this is the only time in history for which we have the crucial and decisive evidence of space travel, albeit of a limited sort, necessary to allow an accurate scientific extrapolation of possible future developments.
Before progress can be made towards a new paradigm in space travel we must realise and accept one very important precept, namely, that the current use of the chemical rocket as the primary means of propulsion has reached an optimal design plateau and any further improvements in rocket technology, while not inconceivable, are severely limited by the huge inefficiencies related to the fuel-to-payload ratio. This fundamental catastrophe has no real, or easy solution and one is not expected any time soon.
Indeed, a quick survey of all the available alternatives leaves much to be desired. One major problem not obvious at first is the enormous power output required to counter the weight of any payload and sustain escape velocity in order to reach orbit. This kind of power output has only been possible with solid and liquid propellant rockets. Ion propulsion falls short of this requirement by more than ten orders of magnitude, and is thus only useful in long duration space missions or long term station keeping. Another scheme is the proposed use of fissible material in order to explosively heat hydrogen fuel for thrust. But this is just a revamped rocket which, putting aside all safety concerns, would require a great deal of redevelopment cost, while providing only marginal improvements in efficiencies over conventional rocket propulsion. Furthermore, it does nothing to address the biggest and most important problem of all, for deep space missions, that of the need to eliminate having to carry reaction mass (in the form of fuel).
Briefly, some of the more conventional schemes are; the solar sail, skyhook, laser sail, the fusion rocket, the matter-antimatter rocket, the magnetic sail and the list goes on. Similarly, some of the less conventional schemes which have been proposed, at some time or other, are; the warp or hyperspace drive (Alcubierre), worm hole travel, teleportation (a la stargate) and many others too numerous to list. However, one of the most intriguing ideas proposed by Robert L. Forward, is that of the very elegant, but so far hypothetical, Negative Matter Propulsion. Based on the early work by Herman Bondi, in 1957, this scheme relies on the existence of negative matter. That is, matter which displays the curious property of negative mass. Negative matter (not to be confused with antimatter), if it exists, would behave very differently to the matter we are used to. It's most important property, for our discussion, is that of it's negative gravitational potential i.e. it displays a gravitationally repulsive field. Forward has studied many of the possible positive mass/ negative mass system configurations and has determined that such systems would make ideal reactionless space drives. Furthermore, he speculates that negative matter may be found in the huge intergalactic 'bubbles' or voids and the possibility of designing experiments to detect negative matter. All this, as you would have noticed, is highly speculative conjecture. So what, if anything, can we achieve in the mid to long term ?