The International Astronautical Congress is in full swing in Prague today, with regular updates flowing over #IAC2010 on Twitter and the first session of interstellar import now in progress as I write this. It’s a session on interstellar precursor missions that includes, in addition to Ralph McNutt (JHU/APL) on the impact of the Voyager and IBEX missions, a series of papers from the Project Icarus team ranging from helium-3 mining to communications via the gravitational lens of both the Sun and the target star (no specific target has yet been chosen for Icarus).
Claudio Maccone will be summarizing where we stand with the FOCAL mission, envisioned as the first attempt to exploit gravitational lensing for astronomical observations. But I’ll turn today to Marc Millis, who will wrap up the precursor session with a discussion of the first interstellar missions and their dependence on things we can measure, such as energy. The notion here is to look at the energy required for an interstellar mission and to weigh this against predictions of when those energy levels will be accessible and available to be used for space purposes.
Energy Needs Drive Spaceflight
How to get a handle on energy growth trends? Millis uses annual data on the world’s energy production from 1980 to 2007, calculating the ratio of each year’s energy production to the preceding year, then finding the average and standard deviation of all 27 of these years. How soon until Earth becomes a Kardashev Type I civilization — one capable of mastering all the energy reaching the Earth from the Sun? Acknowledging the wide span of uncertainty in the result, Millis pegs the earliest year this could occur as 2209, with a nominal date of 2390 and a latest date of 6498. A constant growth rate is assumed, which balances depletion of natural resources against unforeseen advances in new energy sources, leaving growth rates relatively stable.
Fascinating as they are in their own right, I won’t go through all the numbers (I’ll link to the paper when it becomes available online). But note the key factors here, which are the total amount of energy produced by our species and the proportion of that energy devoted to spaceflight. For the latter, Millis compares the annual Space Shuttle launch rate against the total annual energy consumed by the United States, finding that the maximum ratio of Shuttle propulsion energy to total US energy consumed occurred in the year 1985, equaling 1.3 x 10-6. The average ratio over the years 1981 to 2007 is 5.5 x 10-7. Millis then takes the maximum ratio (over an order of magnitude greater than the average ratio) to calculate the earliest opportunity for future missions. What he calls the Space Devotion Ratio is thus 1.3 x 10-6.
The Alpha Centauri Calculation
When could we launch a 104 kilogram interstellar probe to Alpha Centauri based on these calculations? Assume 75 years as the maximum travel time that might be acceptable to mission scientists and assume a rendezvous rather than a flyby mission, acknowledging the need to acquire substantial amounts of data at the destination. Millis extrapolates from existing deep space probes to arrive at a putative mass, adding the needed margins to ensure survival over a 75-year transit and the substantial communications overheard to relay information to Earth.
As to propulsion options, Millis works with two possibilities, the first being an ideal case that assumes 100% conversion of stored energy into kinetic energy of the vehicle (think ‘idealized beam propulsion’ or even some kind of space drive), the second being an advanced rocket with an exhaust velocity of 0.03c. We thus wind up with two sets of figures, again based on energy availability. Millis then converts the propulsion energy figures into equivalent world energy values, using the Space Devotion Ratio he first calculated earlier for US space involvement.
The result: The earliest launch for a 75-year probe is 2247, with a nominal date of 2463. This assumes idealized propulsion; i.e., a breakthrough technology like a space drive. Fall back on advanced rocket concepts and the energy requirements are much higher, with the nominal launch date of the probe now becoming 2566, the earliest possible date being 2301.
Strategies for Interstellar Research
The play in the numbers is huge, the uncertainty in the results caused by the wide span in possible energy production growth rates. Interestingly, Millis’ finding that the earliest interstellar mission will not be possible for two centuries coincides with earlier estimates from Bryce Cassenti and Freeman Dyson based on economic and technological projections. We can, obviously, adjust the numbers based on our projections of technological growth, and as with any projection, sudden changes to world economic patterns would be a substantial wild card.
But Millis argues that in the absence of a single technological solution, it would be premature to focus on specific propulsion options to the exclusion of other, more theoretical alternatives. For that matter, it would be foolish to be inhibited by the ‘incessant obsolescence’ postulate (a term that Millis himself coined), noting that earlier missions may well be overtaken by faster ones launched at a later date. Instead, what he calls ‘cycles of short-term, affordable investigations’ targeting key questions whose answers we can hope to find today are the best way to proceed. And that means continuing our investigations of everything from the already operational solar sails to technologies that today seem impossible, such as travel faster than the speed of light.