Costs of an interstellar probeS

In the following analysis, Centauri Dreams discusses an oft forgotten factor that could hinder future space travel: price.

When does it make sense to build a starship? Back in the late 1960s, Freeman Dyson went to work on the question of how much an interstellar probe might cost. Extrapolating from nuclear pulse propulsion and the state of the art in spacecraft design as then understood, Dyson arrived at an estimate of $100 billion to build the craft, which translates into roughly $650 billion today. Though stark, that figure is by no means as eye-popping as one of the estimates drawn up by the original Project Daedalus team: $100 trillion in 1978 dollars.

These figures numb the senses, and you may recall the recent work by Ralph McNutt (Johns Hopkins Applied Physics Laboratory) and team, which pegged the cost of a series of human expeditions into the outer Solar System at $4 trillion. It's helpful to remember, though, that calculating when a project becomes fiscally feasible can be a useful undertaking in itself. Richard Obousy goes to work on these matters in a recent post in the Project Icarus blog, trying to figure out how long it would take before our civilization has grown to the point that the expenses we are talking about today would become manageable.

Starships as Percentage of GDP

Obousy's assumption is that it will become economically feasible (and just as important, politically viable) to construct an interstellar craft when the total cost represents no more than 1% of the Gross Domestic Product of the constructing nation. Now the GDP of the United States, a measure of the country's overall economic output in a given year, is currently $14.6 trillion. The GDP of the entire planet is now $61.1 trillion, and the option of consolidating the larger amount in these calculations is obvious, so Obousy looks at his figures in terms of the US economy and contrasts those results with the global perspective.

The outcome: The US economy becomes able to support a Dyson-class starship costing $650 billion by the year 2085; i.e., in that year, such a cost represents 1% of GDP based on a 2% growth rate per year. A Daedalus-style craft becomes feasible no earlier than 2340. In global terms, the Dyson starship could be built (assuming the global cooperation we at present do not have) within the next few years, whereas the Daedalus class craft would have to wait until 2268. Note that we're basing all this purely on percentage of GDP and an estimated cost for a craft whose core technologies have not yet been developed. In other words, we're brainstorming.

And if we achieve a compounding global GDP of not 2% but 4%? In that case, things move more quickly, and we might aim at a stripped down Daedalus (in the $20 trillion range) sooner:

Compounding the Global GDP at 4% returns a date of 2099 for when construction of the ‘Budget Daedalus' represents only 1% of the planet's GDP. Thus it is, in some sense of the word, possible that our transition from the 21st century into the 22nd century will be celebrated with the construction of Earth's first interstellar explorer. An exciting and delightful end to this century, one has to admit.

Powering Up a Lightsail

Projections for the cost of interstellar missions vary widely. Curt Mileikowsky, who at the time he delivered his talk on the subject in 1994 was serving as Sweden's director of technology, analyzed an interstellar probe mission that would travel at one-third the speed of light and be built around a laser-powered lightsail. His figures were likewise daunting, involving the need for 65,000 billion watts of installed electric power capacity to power up the laser and deliver the needed acceleration to the craft. The cost: $130 trillion for electricity alone.

But Mileikowsky, who spoke at the "Interstellar Robotic Probes: Are We Ready?" conference in New York hosted by Ed Belbruno, noted the same point Obousy does. Economies are not static entities, and these huge figures may one day be justifiable within the context of worldwide growth. No one should be sanguine about that growth, but on time-scales of centuries it is not reasonable to rule out projects that today's economy could hardly support.

We can hope, too, for the kind of conceptual changes that bring interstellar flight to a more manageable level. Speaking at the same conference as Mileikowsky, Dana Andrews (Andrews Space) analyzed the cost of fusion missions powered by deuterium from the outer planets, coming up with a future cost of $15 million per mission, which is the most optimistic scenario I've seen. Andrews runs through the analysis in "Cost Considerations for Interstellar Missions," JBIS 49 (1996), pp. 123-28, explaining the assumptions that got him there.

Interstellar Design and Infrastructure

But back to Obousy, who in the course of discussing motivations for eventual interstellar flight - and taking the long-term perspective so favored by Centauri Dreams - places our starship building in the context of a developing infrastructure in space:

Underlying all of these assumptions is the belief that the solar system will in time, become the domain of an all encompassing, profoundly well organized commonwealth, with a vast economic and productive capability. Perhaps the enterprise of starship building, gargantuan by today's standards, may be one of the few that is demanding enough to keep the community engaged. If history is to serve as a reliable yardstick, evidence indicates that new technological innovations tend to catalyze productivity, which is further used in the creation of armies, empires and opulent masterpieces of stone, steel and canvas. Starship construction may serve as a welcome alternative to these historic follies.

Or perhaps not so much an alternative as yet another option our species can use to continue the investigations that scientific curiosity and the human imagination have fueled. We are at the point in starship design where early 19th Century engineers were in imagining craft that flew in the air. Surely if birds flew by flapping their wings, so should an artificial aeroplane, and as for power, we have the admittedly heavy steam engine… Project Icarus is important because it seeks to push current technologies ill adapted for interstellar work to the next level, just as later 19th Century designers would re-think powerplants, camber and ailerons.

Cost, then, is just one of the interstellar imponderables. From our current vantage, we can get just a glimpse of the magnitude of the task, but must continue seeking ideas that will reduce not only the expense but the time of the journey. No one said building starships would be easy.

The Dyson paper is "Interstellar Transport," Physics Today 21 (1968), pp. 41–45. And you can find Curt Mileikowsky's work in "How and When Could We Be Ready to Send a 1,000 Kg Research Probe with a Coasting Speed of 0.3c to a Star?" JBIS 49 (1996), pp. 335-344.

This post originally appeared on Centauri Dreams.