interstellar space travel could be possible in as little as 200 years. The paper calculates the amount of energy required for two types of interstellar missions, and uses estimates of world energy output growth to determine when the required energy might be available to such missions. Millis bases his calculations on the fraction of energy made available to current space missions, accounting for various technological innovations and broader considerations that might accelerate or delay serious consideration of interstellar missions. The math suggests that an interstellar spacecraft colony could be achievable in approximately 200 years, and a probe to Alpha Centauri could be launched within 500 years.
Millis' estimate is both exciting and disappointing. Space enthusiasts want desperately to see an interstellar mission in our lifetime, but of course most of us realize that's probably not in the cards. At the same time, some scientists have said it may be a thousand years or more before we are capable of interstellar space travel (that is, fast interstellar space travel), so a few hundred years is actually good news. The world is likely to be a far different place in a thousand years, but 200 years is only 10 generations or so. The English we speak today will probably remain intelligible to those lucky people in 2211. We are just barely missing the wonders that may be in store for the future, just as the great scientists of the 18th and 19th centuries barely missed the extraordinary breakthroughs that would come in the 20th century. They laid the groundwork for our world, though, and now it is up to us to lay the groundwork for tomorrow.
But where are we going, and why? Will we be diversifying our interests, spreading the seed of humanity beyond the solar system? Or are we simply going to investigate our nearest stellar neighbor and radio back the results?
Millis explores both of these options -- an interstellar probe bound for Alpha Centauri, and a 500-passenger colony ship, destination unknown, with a mission only to take a piece of humanity far beyond the domain of our Sun. It is here that Millis approaches the practical considerations, but doesn't quite explore the implications fully (though, to be fair, it is really beyond the scope of his paper... his purpose is to speculate on when interstellar travel could be possible, not to predict when it will actually be achieved). In a few hundred years, we can expect the capabilities of our space telescopes to have expanded considerably, so a mere flyby or rendezvous with the Alpha Centauri system may tell us little; it would be of great academic interest to send a probe there, but what will we learn that we could not learn from observations with our telescopes? Of course, that is the great question! But if there are any planets to be found there, we should be able to detect them long before such a mission is possible. It remains to be seen, but the existence of planets in the Alpha Centauri system would immediately make this mission more appealing.
If we are considering the colony ship for the purpose of placing our eggs in other baskets, a whole mess of other questions come to mind. Why not simply send our colonists to other planets or moons in our own solar system? Well, we may have already done that. Interestingly, though, Millis' calculations indicate that the colony ship will actually be feasible long before the interstellar probe to Alpha Centauri, about 260 years earlier. This is based on the required delta-v for the probe to rendezvous with Alpha Centauri, and the requirement to reach a speed consistent with tolerable limits on mission duration (Millis selects 75 years). But what good does it do setting 500 people on a course to nowhere? What do they do if they need to slow down, or land somewhere? (There is no built-in delta-v to slow down this craft in Millis' calculation). Will we really be able to build a spacecraft large enough to carry 500 people, indefinitely, in the next 200 years? Again, these questions are beyond the scope of Millis' paper, but they are relevant to the question of when we will achieve human interstellar travel. A self-sustaining human colony floating endlessly in space, set adrift from the rest of humanity, is a somewhat unappealing notion, and besides, we can hardly assume a spacecraft launched in 2211 will be able to house human beings safely for centuries or longer in the vast depths of space. There are bound to be minor malfunctions, electronic failures, and occasional hull damage, but spare parts will be hard to come by. And eventually, one would think that they'd want to settle on another world.
With these considerations, and taking into account the anemic funding afforded space travel these days, it's easy to see that Millis' estimates for achieving these goals, while perfectly reasonable, may represent an exceedingly optimistic time frame. In other words, Millis has shown us what is possible, but it's far from assured that we will meet the mark. We will need to see some remarkable advances in propulsion technology if the interstellar probe is to be realistic anytime soon, and though the energy may be available to launch a colony ship in the next two hundred years, from a practical standpoint it is beyond the pale. In 200 years we may be sending many humans to live on Mars or elsewhere, but I doubt we will be sending any to drift in interstellar space forever. In fact, the only reason we would have to escape the Solar System, rather than simply take refuge on another planet or moon in our vicinity, would be the death of our Sun. But that's not going to happen for another 5 billion years or so.
While Millis' estimates and their implications may be disappointing, a realistic evaluation of these time scales is useful. When we fully appreciate the great distances between our solar system and others, we begin to understand just how important it is to learn about our own planetary neighborhood, where travel time is relatively short, and where we may one day set up human colonies. There are lots of places left to explore, and it is still a good idea to put our eggs in other baskets. But this ultimately brings us back to the most basic concern: our own survival on this planet. The Earth is the only place in the Universe, so far as we know, where we can live perfectly comfortably. We have been sculpted over billions of years of evolution to be perfectly tuned to our environment -- we are accustomed to the pressure, temperature and content of our atmosphere, and the nutrients of the soil are passed on to us through the food chain. Here on Earth, we have all the water we could ever need, and the Sun banishes the cold and darkness of space. It is truly a paradise. But this paradise is by no means promised to us, and today natural and man-made terrors threaten to destroy the planet, or at least extinguish the human species. Whether we are talking about nuclear war, global warming, or a cosmic collision, we must do everything in our power to avoid these calamities. We owe it to our ancestors and descendants alike. For those who dreamed of visiting the stars, and for those who may one day reach them, it's up to us to get there.