Two vital questions often get debated in the interstellar community, how much will an interstellar mission cost and when is it likely to happen? To place this question in context, let us first consider some big program costs in history. For the Vietnam War costs vary between $30 billion in 1969 to later estimates between $133-$584 billon. For interest, the number of Americans that served in the Vietnam War was around 2.1 million people – that’s ignoring how many people worked behind the scenes back home. For Project Apollo, at its peak it employed around 400,000 people. The cost of the project varies depending on the source but ranges between $20 and $25.4 billion in 1969 dollars (approximately $150 billion in 2012 dollars). Incidentally the original costs for Apollo were around $7 billion but the then Administrator James Webb increased it to $20 billion when he informed President Kennedy , a factor 2.8 times the original $7 billion estimate. Another large project is the International Space Station. Estimates for the total cost vary between $35-160 billion. Under the current US Administration of President Obama it has an extended lifetime through to 2020, although it could remain operational until 2028. The European Space Agency has estimated that the total costs for what is assessed to be a 30-year mission is around 100 billion euros. For interest, estimates for the US 2010 Health care bill are quoted to be in the region of around $1 trillion over 10 years, a cost similar to the likely lower bound costs of an interstellar probe mission, based on the assessments discussed below.
The costs associated with the development of the Apollo spacecraft were of order $83 billion in today’s money. If the total structure + payload mass for the Saturn V was around 6,000 tons then this equates to a development cost of around $14 million/ton. It is difficult to say what the true cost of an interstellar probe like the British Interplanetary Society’s 1970s Project Daedalus  would be, but if we multiply this by the structure + payload mass of Daedalus, 2,670 tons, it would equate to a development cost of around $37 billion, cheaper than Saturn V. Although this increases to $737 billion if one includes the 50,000 tons propellant. However, at today’s prices the typical costs of developments in the Aerospace industry is around $100 million/ton which would mean that to develop something like Saturn V today would cost around $600 billion and to develop Daedalus would cost around $267 billion for the structure and $5,267 billion for the entire vehicle.
We can also think about the cost of Daedalus from the perspective of global energy consumption. If we assume that the global energy consumption is around 4.75×1020 Joules of energy per year and note that Daedalus had the equivalent of around 2.88×1021 Joules in its propellant tanks, which is around 6 times larger than the global amount. For this simple calculation, we can ignore other sources of energy consumption such as nuclear, gas and coal, but instead focus on oil. The US uses around 21 million barrels of oil per day at a cost of around $80 per barrel, which is a cost per annum of around $1.68 billion/day or $613 billion/year. Let us assume that this represents around 40% of the total US consumption then the total actual consumption is around $1.5 trillion/year. If the total US consumption is around 25% of global consumption, then we can estimate the global annual energy consumption to be around 4 times the US, which equates to around $6 trillion. Then if the total energy consumption for Daedalus is 6 times the global amount, this means that the cost of the propellant for Daedalus would be around $36 trillion.
The only conclusion we can draw is that if the first unmanned interstellar probe is along the lines of Project Daedalus, then it would likely be in the trillions to tens of trillions of dollars in cost, if not a lot more. Based on these rough assessments, Daedalus would be a massive and costly undertaking and is suggestive of a Massive Infrastructure requirement. Any country would need significant justification before committing to such an enterprise, e.g. discovery of life on another planet or significant asteroid threat. This would all change if the vehicle was a lot smaller or less massive; alternatively if the design and build took place over a much longer time period than 10 years. The costs would also be hugely impacted by the arrival of “disruptive technology” such as the British Reaction Engines Skylon spaceplane or the wide scale use of nanotechnology in space applications.
In order to understand the true cost of an interstellar mission it is necessary to examine the nominal Gross Domestic Product or GDP. This is a measure of the economic output from one nation as a result of products and services sold annually, regardless of the specific application. Estimates vary for the GDP between different organizations such as the International Monetary Fund and the World Bank. However, for the year 2009 the GDP for the United States was around $14 trillion. This compares to some of the other nations with $5 trillion for China, $5 trillion for Japan, $2 trillion for the United Kingdom and just over $1 trillion for Russia, the other country that has a major space infrastructure. The European countries tend to co-operate on space projects since the formation of the European Space Agency in 1975 and so it is worth also knowing that the GDP for the European Union in 2009 was around $16 trillion. We can also consider the GDP for the entire globe, assuming an interstellar mission would be a collaborative international venture. The total global GDP for the same year was around $58 trillion. If the first interstellar probe mission cost was of order ~$100 trillion, then the cost would be equivalent to approximately two years global GDP or around seven times the GDP for the United States in 2009.
History shows that the trends in the US space agency NASA budget since 1957 (the year that Sputnik 1 was place into orbit by the Russians) until the present, using US government data and inflation records , plotted with 4-year increments corresponding to Presidential terms. The data shows that the peak in expenditure for NASA of around $40 billion occurred during the mid to late 1960s, largely as a consequence of Project Apollo. What is interesting is that this peak is under 5% and only of short duration, thereafter constantly declining until the late 1970s where it then remains near constant in terms of overall percent. If we examine 5% of the US $14 trillion GDP in 2009, this is $0.7 trillion and for a $100 trillion interstellar probe project, would take around 143 years to fund; clearly not acceptable. However, if we examine 5% of the global $58 trillion GDP in 2009, this is £2.9 trillion and for the same interstellar probe project would take around 35 years to fund. Assuming the design, build, launch and mission flight of the first interstellar probe lasts for 100 years for the $100 trillion cost, this is $1 trillion per year expenditure for a century, equivalent to 7.1% of the US 2009 GDP or 1.7% of the global 2009 GDP. This analysis leads to two clear conclusions (1) international co-operation in the pursuit of space exploration is the favourable way that an interstellar mission will ever be launched (2) the cost of such a mission must be bought down significantly before any single nation state could even consider the project.
The British born physicist Freeman Dyson addressed starship economics in a Physics Today article, connected with Project Orion . Dyson had estimated that the cost of a 10,000 tons payload would be around $1011 or the equivalent of $10,000 / kg. In 1968 the Gross National Product of the United States was around $1 trillion which means that the cost of the probe during this period was ~0.1×GNP. But then Dyson assumed a growth rate of 4%, which would imply a GNP around one thousand times larger within 200 years reducing the cost of the probe to 0.0001×GNP, and costs approaching that of the Saturn V during Project Apollo. He concluded on the basis of this assessment that the first interstellar probes would be built around 200 years from the time of his prediction in 1968 – in the year 2168 AD.
It is interesting that Dyson’s estimate of 200 years hence is supported by two independent calculations. One author, Brice Cassenti  has considered maximum velocity trends throughout history from the 1800s and projected them into the end of the 22nd century. He looked at sailing ships (1800 AD, 7-14 m/s), early race cars (1900 AD, 55 m/s), earth escape velocity (1960 AD, 11.1 km/s) and solar escape velocity (1980 AD, 12.5 km/s). The author concluded that missions on the order of 10 light years should be possible 200 years from his projection in 1982 (i.e. 2182 AD). Another author, Marc Millis  has examined historic energy trends, societal priorities and required energies for an interstellar mission. He based his assessment on a minimal 107 kg colony ship with an irrelevant destination and sending a minimal 104 kg probe to Alpha Centauri with a mission duration of 75 years. He concluded that the colony ship cannot be launched until around the year 2200 AD and the probe cannot be launched until around 2500 AD, so that the earlier interstellar mission was around two centuries away. The reason the colony ship was deemed to be launched sooner was due to its much smaller energy requirement, which is proportional to the velocity, squared; the colony ship travelled at a cruise velocity of 0.00014c compared to the probe which travelled at a cruise velocity of 0.22c. In hard economic times, justifying the costs of an interstellar mission are not easy. With the earliest estimates putting the first launch in the 2200 time frame, this all indicates humanity will not achieve interstellar society status for many centuries yet. In order to buck this conclusion, clear changes in the way we work are needed today.
 Bizony, P, The Man Who Ran The Moon: James Webb, JFK and the Secret History of Project Apollo, Icon Books Ltd, 2006.
 Bond, A & A Martin, Project Daedalus: The Mission Profile, JBIS, S37-S42, 1978.
 Source of data includes US Inflation Calculator (www.usinflationcalculator.com) and Guardian online article ‘NASA budgets: US Spending on Space Travel Since 1958 Updated’ (www.guardian.co.uk/news/datablog/2010/feb/01/nasa-dugets-us-spending-space-travel).
 Dyson, F, Interstellar Transport, Physics Today, 1968.
 Cassenti, BN, A Comparison of Interstellar Propulsion Methods, JBIS, Vol.35, pp116-124, 1982.
 Millis, M, Energy, First Interstellar Missions, Considering Energy and Incessant Obsolescence, JBIS, 2011.