Although people have been recommending that I read Charles Stross’ novel Accelerando for some time now, I haven’t found the time and now wish that I had. I recently read a fascinating speech that Stross gave at a Munich tech conference discussing, among many other things, how advances in computer storage will change our lives. And now his essay The High Frontier, Redux is exciting controversy, asking whether many of our ideas about spaceflight need to be reassessed in light of the enormity of the challenges we face.

Much of what Stross has to say is true, and I hope that those who haven’t read the essay will give it a look. The reason why we don’t want to minimize the magnitude of the problems we face in interstellar flight is that a clear-eyed view is needed to begin to conceive the radical technologies that may one day solve the problem. And I know too many people who learn how far away the stars really are, even the closest of them, and then throw the whole concept out as wishful thinking.

But I don’t think Stross is doing this. Have another look at the essay and you’ll note what he’s up to. He wants to “…highlight the problems I face in trying to write believable science fiction about space colonization.” Fair enough! And what exercises him as a writer is that interstellar travel requires a) outrageous amounts of energy; or b) highly efficient robot probes; or c) a magic wand. Stross doesn’t find option A easy to work with, and B isn’t as interesting for many a writer, since putting humans into interstellar space is prime science fiction material. That leaves C, and the problem there is that the day of the deus ex machina is long past. No good science fiction writer wants anything to do with springing absurd contraptions out of closets to save the day.

Now I would venture that anyone reading Centauri Dreams already has a pretty good idea of how far the stars are. Stross reduces an astronomical unit — the distance between the Earth and the Sun — to a centimeter and describes the scary result even for nearby stars. On that scale, Proxima Centauri, the nearest star, is fully 2.6 kilometers away (Stross says 2.3 — one of us has the math slightly wrong, and I wouldn’t be surprised if it were me).

But I like British astronomer William Herschel’s far more homely (and much less accurate) way of reading the distance to Centauri A and B. Herschel (1792–1871) was the first to make a serious attempt to measure this value, and he put it in terms his contemporaries could understand: “…to drop a pea at the end of every mile of a voyage on a limitless ocean to the nearest fixed star, would require a fleet of 10,000 ships of 600 tons burthen, each starting with a full cargo of peas.”

Yep, better have plenty of peas for that Centauri trip. As Stross says, the distances are mind-numbing. But I’ll differ slightly on his take on Proxima, which he describes as “…a poor choice, if we’re looking for habitable real estate. While exoplanets are apparently common as muck, terrestrial planets are harder to find.” He may well be right, but we do have planet hunter Greg Laughlin (UC Santa Cruz), who just the other day said this in his systemic blog in the context of a proposed HARPS search for Proxima planets:

“Proxima is effortlessly old, adequately quiet, and metal-rich. If our understanding of planet formation is first-order correct, it has several significant terrestrial-mass planets.”

Now let’s be careful here. Laughlin is talking about planets of terrestrial mass, rocky worlds that may or may not be in the habitable zone. The odds for any given star are that such planets are not in that zone. On the other hand, we can’t completely rule the possibility out, nor can we rule out the even more interesting scenario around the primary stars Centauri A and B, each of which may have planets within roughly 4 AU of the parent, with possibilities for the delivery of volatiles to the inner system involving Proxima itself. See this older Centauri Dreams post for more.

The energy cost is indeed staggering. But let’s assume no magic wands. When I was at Marshall Space Flight Center in Huntsville several years ago talking to NASA solar sail researchers for my book, Greg Matloff was there doing consultant duty and participated in the interviews. Matloff, the author of the highly regarded The Starflight Handbook (about which Robert Forward had the ultimate dust jacket compliment — ‘Don’t leave the Solar System without it!’), talked about solar sail missions that don’t invoke any off-the-charts wizardry.

“When you look at technology such as Tim Knowles’ stuff [a reference to ultra-lightweight carbon-carbon sail material], you realize how close we are to being able to build a generation ship. You have to start saying, with planets being discovered all over the place, if we discover an Earth-like planet around one of the three Centauri suns, and we might well find that, is there going to be a push to do that. Because we can do it. You have to look into the weird stuff, I believe, to show it doesn’t work.”

Matloff has studied sail mission concepts using so-called Sun-diver trajectories that deploy a sail at perihelion. In a subsequent telephone interview, he added: “We’ve learned that it is quite possible to take both large ships and small probes to the nearest star within a thousand years or so. Using the sail alone. But it is very difficult to get the trip time down below 800 or 900 years.” Alan Bond and Anthony Martin would use a key Matloff paper on this subject for their own classic paper (see below for citations) discussing worldships and the two technologies they thought could power them — nuclear-pulse engines and hyper-thin solar sails.

None of which is to deny the enormous challenges that Stross talks about. But again, I catch his reference to his dilemma as a craftsman of words when he says, “…the conclusion I draw as a science fiction writer is that if interstellar colonization ever happens, it will not follow the pattern of historical colonization drives that are followed by mass emigration and trade between the colonies and the old home soil.”

Now colonization is quite another matter from robotic probes, and I can certainly believe that historical colonization drives, trading routes and the rest are not a likely analog for future star missions. Stross could be right that humans — as opposed to intelligent machinery — may not get to the stars, but even on that score I’m reminded again of Robert Forward. His vast lens in outer Solar System orbit focusing laser light from a power-rich installation near the orbit of Mercury could theoretically launch a crewed Epsilon Eridani mission with return capability, with travel time within a single human lifetime.

An engineering nightmare? To be sure, the magnitude of building installations that require lenses hundreds of kilometers across to focus a laser pushing a vast sail are almost beyond comprehension. But even today, we see the nano-technology revolution proceeding apace, and have to speculate about what effects it may have on the building of large structures in space. Give up on the notion of thousands of human workers in spacesuits welding joints into place and you’re left with the possibility of ‘growing’ a Fresnel lens inside the orbit of Uranus that could serve the purpose.

Far-fetched? You bet. But I think the future usually is far-fetched. If I were a practicing science fiction writer like Stross, I too would see how difficult it must be to craft a believable future within technology that is explicable to a broad readership. The man has a good point! But I believe that if we can come up with even faintly plausible scenarios that don’t drift far from understood physics today, then we can certainly assume better scenarios as some of today’s computing and nano-tech trends continue to develop.

We can also assume that the innate human impulse to do seemingly impossible things will keep theorists focused on this quest. Leave magic wands out of the equation and ask yourself whether, if we could build the infrastructure to launch a Forward-style lightsail with human crew, we would have volunteers to fly the mission. Ponder as well the energy resources that will have to be realized for this kind of journey and ask whether we really are gradually evolving into a Kardashev Type 1 civilization. Because if we are, and if we keep imagining and planning and doing, we may yet make this wildest of improbable dreams happen.

And if it takes a thousand years to build the tools needed to launch such a mission? Ten thousand? Then some of our descendants may just see the Centauri stars up close. What matters is not when but whether we go. I wouldn’t advise betting against a long-term optimism that may, by its own drive and pluck, find a way to push to Centauri and beyond. As for Charles Stross, I still plan to read Accelerando. This guy knows his stuff, and articles like his help to frame and focus the terms of the debate.

A final thought: Stross has one comment that has me baffled. As one who thinks we need to build a space-based infrastructure to help protect our planet from space debris, I found myself surprised to read this:

‘We can’t afford to keep all our eggs in one basket’ isn’t so much a justification as an appeal to sentimentality, for in the hypothetical case of a planet-trashing catastrophe, we (who currently inhabit the surface of the Earth) are dead anyway. The future extinction of the human species cannot affect you if you are already dead: strictly speaking, it should be of no personal concern.

Stross calls such arguments about future catastrophe ‘quasi-religious,’ for reasons that escape me. If we can identify an incoming danger and mount a mission to avert possible species extinction, shouldn’t we be working on the technology needed to do the job? And if the beneficiaries are our great-grandchildren instead of ourselves, what of that? Isn’t that the sort of commitment we should be making to our descendants?

Addendum: See the comments below for Stross’ response to these questions. We’re a lot closer on this than I had realized.

And now, the promised citations: Matloff’s paper with Eugene Mallove is “Solar Sail Starships: Clipper Ships of the Galaxy,” Journal of the British Interplanetary Society 34 (1981), pp. 371-380, and the related “The Interstellar Solar Sail: Optimization and Further Analysis” is in JBIS 36 (1983), pp. 201-209. The worldship article is Bond and Martin, “World Ships: Concept, Cause, Cost, Construction and Colonization,” JBIS 37 (1984), pp. 243–53. Also see their “Worldships: Assessment of Engineering Feasibility,”‘ JBIS 37, pp. 254-256 (1984).