Popular Mechanics just published an article entitled How Many People Does It Take to Colonize Another Star System? Apparently the number needed to maintain sufficient genetic diversity is very large indeed – 40,000 would be ideal! Unfortunately, if you do the math, the amount of energy it would take to transport that many people to another star system, even allowing a couple of thousand years for the voyage, is enormous. As several commenters pointed out, by the time our technology advances to the point that such missions are feasible, it will also be feasible to send the necessary “genetic diversity” along in the form of frozen eggs and sperm with carefully chosen DNA sequences, complete libraries of human alleles that can be fabricated and inserted into DNA sequences as needed, etc. It might not even be necessary to send anything as bulky as fully formed humans on the voyage. Self-replicating robots could be sent in advance to create housing, farms, and birthing facilities prepared to receive fertilized eggs. The first humans born would have robotic “parents.”
It’s always fun to speculate on what we might be able to do assuming our technology becomes sufficiently advanced. The question is, what can we do now, or at least in the foreseeable future with existing technologies, or ones that seem accessible in the near future? “Existing technologies” means travel times of 25,000 years, give or take. In other words, we must rule out our own species, at least for the time being. It will be necessary for us to send some of our relatives. For some of them – other species – such lengthy interstellar voyages are feasible now. As I wrote in an earlier post,
The 32,000 year old seed of a complex, flowering plant recovered from the ice was recently germinated by a team of scientists in Siberia. Ancient bacteria, as much as 250 million years old have been recovered from sea salt in New Mexico, and also brought back to life. Tiny animals known as tardigrades have survived when exposed to the harsh environment of outer space. We might choose the species from among such candidates most likely to survive the 50,000 to 100,000 years required to journey to nearby stars with conventional rocket propulsion, and most likely to evolve into complex, land-dwelling life forms in the shortest time, and send them now, instead of waiting 100’s or 1000’s of years for the emergence of the advanced technologies necessary to send humans. Slowing down at the destination star would not pose nearly the problem that it does for objects traveling at significant fractions of the speed of light. The necessary maneuvers to enter orbit around and seed promising planets could be performed by on-board computers with plenty of time to spare. Oceans might be seeded with algae in advance of the arrival of organisms that feed on it (and breathe the oxygen it would release).
Why would we want to do such a thing? Survival! Morality exists only because animals equipped with it were more likely to survive. We are one such animal. There is no such thing as an objective “ought.” However, given the reason that morality exists to begin with, the conclusion that nothing can be more immoral than failing to survive does not seem unreasonable. It one accepts that logic, it follows that our first priority “should” be the survival of our own species, and our second should be the preservation of biological life. It’s really just a whim, but I hope that many others will share it. The alternative is to accept the fact that one is a defective biological unit, resigned to extinction, which I personally don’t find an entirely pleasant thought.
Let’s assume that a canonical voyage will last 25,000 years. Conventional rockets are capable of reaching the nearest star systems in that time. By using nuclear propulsion of the type that was successfully tested 50 years ago, we should be able to reach stars within a distance of a dozen light years or so within the same period. As noted above, there are life forms that could survive the voyage. The particular ones chosen would be those most compatible with the conditions existing on candidate planets. Needless to say, the conditions of our own atmosphere, oceans, etc., have been drastically altered by the long existence of life on our planet. Finding such conditions on reachable planets is most unlikely, and our biological voyagers must be chosen accordingly.
It will be necessary to develop certain technologies that we do not as yet possess. Fortunately, they are all within reach, and nowhere near as demanding as, say, fusion or anti-matter propulsion systems. For example, we will need a timing device that can keep “ticking” for 25,000 years, and, when necessary, signal the rest of the interstellar package to “wake up.” The Long Now Foundation has made some interesting starts in this direction, in the form of giant mechanical clocks that are designed to run for 10,000 years. Of course, those designs aren’t exactly what we’re looking for, but if one can conceive of a 10,000 year mechanical clock, than a 25,000 year digital clock must be feasible as well. A similar problem was solved by John Harrison more than two centuries ago, in the form of a clock that kept time exactly enough to keep track of a ship’s longitude. If he succeeded in solving the British Navy’s problem with the technology that existed then, we should be able to succeed in solving our own clock problem with a technology that is now far more advanced.
It will be necessary to develop systems that will perform reliably over extremely long times. As it happens, that, too, is a problem that has already been taken in hand by earth-bound scientists. The relevant acronym is ULLS (Ultra Long Life Systems), and some of the required technologies are discussed in a NASA presentation entitled, Technology Needs for the Development of the Ultra Long Life Missions. Some of the ideas being considered include,
Generic Redundant Blocks – redundant components that are generic and can be programmed to replace any type of failed component. An example might be field-programmable gate arrays (FPGA’s).
Adaptive Fault Tolerance – Working around failures instead of replacing failed components with spares.
Self-repair components – Including self-repair with nano-technologies and self-healing with biologically inspired technologies.
Regenerative systems – Modular regrow with biologically inspired technologies.
In interesting presentation on the subject by NASA scientist Henry Garrett, who happens to prefer Project Orion-type interstellar missions with propulsion by few kiloton nuclear devices, may be found here (sorry about the long-winded introduction). Dave Reneke recently posted an interesting if somewhat speculative article on various types of self-replicating interstellar probes entitled How Self-Replicating Spacecraft Could Take Over the Galaxy.
Of course, none of this fine technology will work without a reliable power supply that needs to last, potentially, for upwards of 25,000 years. It so happens that we have just the isotope – plutonium 239. You might call it the ultimate dual use material – life or death. It is ideal for making nuclear bombs or carrying life across interstellar distances. Of course, another isotope of plutonium, plutonium 238, has already been used to power many spacecraft, including the Voyagers and New Horizons. Unfortunately, with a radioactive half-life of only 87.7 years, there would only be a few atoms of it left after 25,000 years. Pu-239, on the other hand, has a half-life of 24,100 years – just about what’s needed. Of course, it could only provide a tiny fraction of the power of Pu-238 via radioactive decay. Not much is required, though – only enough to keep the clock going. At key points in the mission, of course, a great deal more power will be necessary. And that’s what brings us to the reason that Pu-239 is ideal – it’s fissile. In other words, it’s an ideal fuel for a nuclear reactor. When high power is needed, the plutonium can be assembled into a critical mass, serving as either a conventional reactor or a space propulsion system.
I am convinced that all of the above can be accomplished in a matter of a decade or two instead of centuries if we can somehow again achieve the level of collective willpower we reached during the Apollo Program. Of course, this old planet of ours could easily go on supporting high tech human civilizations until we master the art of interstellar travel on our own. It might – but why take chances?