In the year 1956 the science fiction writer Robert Heinlein published his memorable novel “Time for the Stars”. This was a tale of two twin brothers, one who stayed at home and the other who went off on a fantastic journey to the stars in a so called ‘Torchship’. They stayed in constant communication through the power of telepathy; the ultimate long distance communications device. The mission was put together by the Long Range Foundation, a non-profit organization that funds expensive, long term projects for the benefit of human kind. Moving at sub-light speeds the vessel takes many years to get to its destination. When we read science fiction, it all makes star travel seem so near and within our grasp. Our minds conceive of countless technologies, devices, and machines by which the journey can be launched. But when is it really “Time for the Stars”? By this question, we mean of course when will we will be in a position to actually launch the first Starships towards those distant lights of Suns that shine like candles in the dark of our Cosmos? Well, several authors have actually looked at the problem, and it’s interesting to see what they have got to say about it.

The American born Robert Enzmann derived his ‘Ezmann Starship’ concept sometime in the 1960s. This would be a fusion powered vehicle carrying a colony of humans on a 60 year trip to the stars. It would travel at around 9 percent of the speed of light using its nuclear pulse based engines. Although Enzmann never wrote up his ideas, a recent 2012 Journal of the British Interplanetary Society (JBIS) study reviewed the Starship in it’s entirely in a paper titled “The Enzmann Starship: History and Engineering Appraisal” by Adam Crowl, Kelvin F.Long and Richard Obousy. The American author (and pioneer of model rockets) G.Harry Stine later look these ideas as a basis for his 1973 article in Analog magazine titled “A Program for Star Flight” in which he described an enormous ten-ship colony mission launched to the stars beginning in the year 1990. The entire program would involve ten vessels and cost something like $100 billion spread out over a couple of decades.

One of the first people to try and estimate when star flight would be possible using credible scientific methods was the British born physicist Freeman Dyson. In his 1968 paper titled “Interstellar Transport” published in Physics Today, he looked at 4 percent economic growth rate scaling and concluded that this implied a Gross National Product for the United States of around one thousand times larger within 200 years. This meant that the first mission would be feasible from around the year 2168.

The American Brice Cassenti examined the problem in a 1982 JBIS in his paper “A Comparison of Interstellar Propulsion Methods”. He looked at velocity trends throughout history from the 1800s including sailing ships, early race cars, Earth escape velocity and solar escape velocity.  He concluded that the first missions on the order of 10 light years distance would be feasible around the year 2182.

During the British Interplanetary Society Project Daedalus Study of the 1970s Alan Bond and Anthony Martin considered the clear need for large space based infrastructure in order to facilitate the design, construction and assembly of a large multiple Mega-tons Starship. The (unmanned) fusion powered Daedalus would travel at 12 percent of the speed of light reaching its target stars in around half a century. They predicted it would not be feasible until around the year 2100 at the earliest although most likely much later.

Bond and Martin also lead the follow up 1984 World Ship studies published in “World Ships – An Assessment of the Engineering Feasibility” and “World Ships – Concept, Cause, Cost, Construction and Colonisation”. These were enormous vessels tens of kilometres in length, capable of carrying thousands of people and included an ocean. They would travel at a slow few percent of the speed of light towards the distance stars in journey times measuring centuries. They concluded, optimistically, that such vessels might be feasible around the year 2200.

The American physicist Robert Forward was renowned for his efforts to develop propellantless propulsion systems. He pioneered studies in solar sails, laser sails and even antimatter powered engines. In his 1976 JBIS publication titled “A Programme for Interstellar Exploration”, Forward constructed a plan for going to the stars as part of a strategic roadmap which saw the first light-sail based missions being launched around the year 2000.

A more recent examination of the problem was performed by the American physicist Marc Millis in his JBIS publication titled “First Interstellar Missions, Considering Energy and Incessant Obsolescence”. He looked at energy scaling trends in history from the year 1980 to 2007 using an assumed average growth rate in world energy production of less than 2 percent. He then compared that to the energy you need to push either a robotic probe or a small colony ship towards the stars. His analyses lead to the predicted dates of 2200 (for the colony ship) and 2500 (for the probe). The reason the colony ship was deemed to be launched sooner was due to the much smaller energy requirement.

The British science fiction author Stephen Baxter also discussed the problem in his 2010 JBIS paper titled “Project Icarus: Three Roads to the Stars”. Baxter examined three scenarios for the next few centuries including an energy constrained Earth, and energy rich Earth and an interplanetary Mankind. Although he did not make a prediction on a specific date, Baxter’s research leads him to conclude that a Starship could be launched by a society closer to ours in nature, and closer in time, than is often imagined.

As reported in my own JBIS paper published in 2011 titled “Project Icarus: The First Unmanned Interstellar Mission, Robotic Expansion and Technological Growth”, many of these (excellent) projections miss out the important role of “disruptive technologies” which will alter the assessment on when the first missions are possible. In particular, it was shown how for a 100 year flyby space probe mission to Alpha Centauri 4.3 Light years away, and assuming an ideal cruise velocity of 2,700 AU/year (1Astronomical Unit = mean distance between the Earth and the Sun), that if we started from an assumed mission capacity of 5 AU/year in the year 2020 then assuming a greater than 8% technology grown annually in mission capacity (as measured by the attainment of cruise speeds) it may be possible to launch an interstellar probe by around the year 2100.

All of these efforts are worthwhile attempts to essentially predict the future, although note the large range in projected first launch dates and thereby uncertainties in the prediction of interest; ergo – we do not know. But many of them rely on linear extrapolation techniques. If history has taught us anything, it is that technology does not usually follow linear trends in this way. The science fiction writer Arthur C Clarke made a similar point in his 1975 Analog essay published in “Man, Space and Destiny” which was also presented to the US House of Representatives. He said:

 “It is a cliché that we often tend to overestimate what we can do in the near future and grossly underestimate what can be done in the more distant future. The reason for this is obvious, though it can only be explained with a certain amount of hand-waiving. The human imagination extrapolates in a straight line; but in the real world, as the Club de Rome and similar organizations are always telling us, events follow a compound interest of exponential law. At the beginning, therefore, the straight line of the human imagination surpasses the exponential curve; but sooner or later the steeply rising curve will cross the straight line, and thereafter reality outstrips imagination. How far ahead that point is depends not only on the difficulty of the achievement but also upon the social factors involved”.

When thinking about the future there is a constant tension between our imagination and our reality. Our imagination believes in the infinite extent of possibilities that lay before us. Our reality grounds us to what nature really allows, to the well understood laws and constraints of physics. As we strive to predict the future, we would be sensible to keep a good balance between our speculative musings, and our rigorous derivations, between our subjective desires and our objective analyses. Going to the extreme of one way or the other will result in wildly under predicting or over predicting what is possible. It is the rare person indeed, who can follow in the footsteps of people like Arthur C.Clarke and get the balance right.

As the current starship design endeavours (e.g. Project Icarus, Project Dragonfly) head towards their final quarter in deriving our Starship design, we too will be asked to make projections on when we think such a probe could be built and indeed, how much it would cost. In some ways, these questions are more difficult to answer than addressing the nuts and bolts engineering of the probe itself, because the uncertainties on our assessments are so large. All we can do is extrapolate using worst case and best case techniques and hope that the real answer lies somewhere in between. But ultimately, does it matter if our projections are wrong or right? For it is the pursuit of the game itself that will get us there and as long as we continue to look forward and out to those distant worlds of light, the time of the stars will someday surely come. Meanwhile, here’s hoping for 2063 and Zefram Cochrane’s warp drive. Anyone want to take some bets on that prediction?