Voyager 1, our civilization's furthest and fastest emissary into space. Traveling at 17 kilometers per second, Voyager 1 still would take some 73,000 years to reach the nearest star.
Yesterday, we talked about which stars might be the most important ones for the near future of the search for habitable and inhabited planets. All the stars I mentioned are relatively close by and pretty bright, and some of them are already known to have planets. If and when potentially Earth-like worlds are found around these or other nearby stars, astronomers will begin lavishing them with attention in a process of discovery that will span generations. In all likelihood, entire careers and even subdisciplines of astronomy and planetary science will emerge from studying all the data we can remotely gather from a handful of promising worlds scattered among the nearest stars. If we are extremely lucky, and find signs of not only extraterrestrial life but also extraterrestrial intelligence, the consequences will spread beyond our sciences to shape and change our religion, philosophy, literature, and art.
And, if we did locate another pale blue dot circling a nearby star, for many people the next logical step would be to attempt to send people or machines there for direct investigation. It sounds simple enough, to send a spacecraft from point A through mostly empty space to point B. The Moon hangs shining in the sky along with the stars, and we've already sent explorers there, as well as robotic emissaries to all the solar system's planets. Reaching the stars shouldn't be that much harder—but it is.
Consider the problem from the simple viewpoint of velocity. It's easy to forget that until very recently, the fastest anyone had ever traveled on planet Earth was almost certainly about 200 kilometers per hour (kph), the terminal velocity of a plummeting human form past which air resistance impedes further acceleration. But then our species learned to build machines that use the fossilized sunlight in coal, gas, and oil to go even faster.
In 1906, a Bostonian named Fred Marriott finally surpassed the millennia-old record—and lived to tell about it—traveling over 200 kph in a steam-powered car across the sands of Daytona Beach, Florida. Scarcely forty years later, a West Virginian test pilot named Chuck Yeager flew a rocket-propelled plane at more than 1,000 kph, faster than the speed of sound. A decade after that, gargantuan rockets were accelerating men and machines to nearly 28,000 kph, fast enough to orbit the Earth and gain a god's-eye view of the planet. That's how we sent astronauts to the Moon, and robotic probes to other planets. Surely we can go even faster, and undertake interstellar voyages.
But space is vast, and even the distance to the nearest star is mind-boggling. Let's say the Sun is the size of a large orange, 10 centimeters in diameter. Place the orange on the ground, walk a bit more than 10 meters away, and you're in Earth's orbit. Finding our planet might prove challenging—it would be the size of a millimeter grain of sand. The walk out to Pluto, a speck of dust ten times smaller than our sand-grain Earth, would be nearly a half-kilometer, and along the way you'd be lucky to encounter any of the planets: Even the largest, Jupiter, would be no bigger than a small marble.
From Pluto in this scale model, to reach the nearest star system, Alpha Centauri, you'd have to travel some 2900 kilometers: roughly the distance between Memphis and San Francisco, or about how far you'd have to dig straight down into the Earth before reaching its outer core. At this scale, light, the fastest thing in the universe, would travel through space at just over 2 centimeters per second. In actuality, light travels at 300,000 kilometers per second, and requires nearly four and a half years to reach Alpha Centauri from our solar system.
Today, the fastest humans on Earth and in history are three elderly Americans, all of whom Usain Bolt could demolish in a footrace. They're the astronauts of Apollo 10, who in 1969 re-entered the Earth's atmosphere at a velocity of 39,897 kph upon their return from the Moon. At that speed you could get from New York to Los Angeles in less than six minutes. Seven years after Apollo 10, we hurled a probe called Helios II into an orbit that sends it swinging blisteringly deep into the Sun's gravity well. At its point of closest approach, the probe travels at almost 253,000 kph—the fastest speed yet attained by a manmade object. The fastest outgoing object, Voyager I, launched the year after Helios II. It's now almost 17 billion kilometers away, and travels another 17 kilometers further away each and every second. If it were headed toward Alpha Centauri (it's not), it wouldn't arrive for more than 70,000 years. Even then, it wouldn't be able to slow down. Of the nearest 500 stars scattered like sand around our own, most would require hundreds of thousands of years (or more) to reach with current technology.
Space and Ships
Part of the problem is rocketry—an inescapable fact of accelerating by venting material out of a nozzle is that it's not terribly efficient. Not even accounting for food, water, and other consumables, you must carry all your fuel along with you, and the faster you wish to go, the more fuel you'll need—fuel that itself requires additional amounts of fuel to accelerate the additional mass. We've already almost maxed-out the velocities attainable through Apollo-style chemical rockets. But even so, there are no insurmountable physical barriers preventing people and machinery from going much, much faster than the pioneers of forty and fifty years ago. A small, scattered vanguard of idealistic scientists and engineers around the world still obsessively concoct new ways of harnessing more energy, of achieving more velocity, of going faster and farther than anyone has ever gone before. Maybe the stars are within reach.
We already know how to build speedier and more efficient rockets powered by electricity instead of chemicals, but they won't do much to get us to nearby stars. For that, only a handful of schemes could suffice. Some researchers suggest building rockets fueled by antimatter, an energy source so potent that the amount required to send you on a month-long crossing to Mars would be measured in grams. Others call for constructing gossamer-thin thousand-kilometer-wide "sails" in space, which would ride on powerful laser or particle beams out of solar system. These options and their more exotic variations theoretically offer velocities that are a significant fraction of the speed of light.
Sadly, while the physics may be on our side, the economics aren't: Based on present production rates and costs, producing and storing enough antimatter to fuel an interstellar mission would quite possibly bankrupt the planet. As for an interstellar sail, such an endeavor would dwarf the largest single piece of space-based infrastructure yet built, the International Space Station, a construction project that has so far cost an estimated $150 billion. Constructing an interstellar sail would probably cost far more—and that's not including the truly astronomical electric bill associated with powering the multimillion-gigawatt laser that would need to shine on the outbound sail for years on end.
At present, the most economically viable fast boat out of the solar system would probably be a spacecraft propelled by regular pulses of detonating atomic explosives. We do, after all, already have plenty of nuclear bombs lying around for no other real purpose than destroying civilization. Perhaps it's not unreasonable to co-opt them for a more productive endeavor. The US government actually funded a study of the concept in 1958, an ambitious program called Project Orion that seriously proposed, among other things, building a nuclear-pulse spacecraft that could send humans to the moons of Saturn as early as the 1970s. But legitimate concerns over radioactive fallout and the dual-use possibilities of miniaturized thermonuclear explosives forced Project Orion's eventual cancellation.
A more recent effort to design a nuclear-pulse spacecraft began in September 2009, and is called Project Icarus. Though, to be fair, Icarus itself is based on another highly regarded study, the 1970s-era Project Daedalus, named after the mythological craftsman who flew free from imprisonment on wings he constructed from feathers and wax. Both projects plan spacecraft that would voyage to the stars propelled by thermonuclear fusion.
Fusion occurs when nuclei of light elements like hydrogen or helium are slammed together with such force that they merge, releasing a flood of energy. It's a process that creates and destroys: It's what gives hydrogen bombs their fearsome power, but it also is how stars shine, glomming together light nuclei in their cores to form heavier elements. Stellar fusion is what made the calcium in your bones, the carbon in your DNA, and the oxygen that you breathe.
If fusion reactions could somehow be used in a propulsion system, they could accelerate a spacecraft to perhaps 10 percent the speed of light. Daedalus envisioned replicating that pressure and heat via arrays of high-powered lasers that would focus on small fuel pellets, compressing them past the fusion threshold and channeling the resulting plasma through a magnetic nozzle to produce thrust. The Icarus team is considering that approach, but has yet to decide on its thermonuclear propulsion method of choice.
Like Daedalus before it, Icarus is a project run entirely by volunteers, scientists and engineers who spend their idle time dreaming of starflight and performing laborious calculations to learn how it might be practically achieved. But unlike the Daedalus volunteers, who relied on the liberal use of slide-rules, brand-new HP-35 calculators, and an occasional sketch on the back of a bar napkin, the Icarus team is leveraging the power of more than 30 additional years of technological progress. Our advances in velocity may have petered out over the past few decades, but our prowess in information processing and communication has steadily accelerated.
Each individual volunteer on the Icarus team today could marshal more computing power than was available to most nation-states in the 1970s, and can electronically access the bulk of the world's accrued scientific and technical knowledge within seconds. Rather than gathering in pubs, they are formulating their starship design via internet telephony, private messaging forums, and the occasional post on the official Icarus blog. Still, while anyone can crunch numbers, actually building a starship would consume a large chunk of the Earth's entire economy, and likely would require creating massive economies off-world.
In the Daedalus plan, for instance, constructing a nearly 200-meter-long, 4,000-metric-ton spacecraft in Earth orbit was actually the easy part, nevermind that such a ship would be roughly the same size as one of the UK's Queen Elizabeth-class aircraft carriers. The harder task was acquiring 50,000 metric tons of the necessary thermonuclear fuel, an isotope of helium that is vanishingly rare on Earth. The Daedalus solution was to harvest the fuel from gas-giant planets like Jupiter, by building and operating a fleet of balloon-borne robotic extraction factories in their atmospheres. In other words, the easiest way the Daedalus volunteers found to fuel their starship was, in effect, the industrialization of the outer solar system.
Additional obstacles abound. Traveling at a significant fraction of light-speed can be compared to staring down the barrel of a gun: Running into a small piece of dust, or, heavens forbid, a sand grain, could cause catastrophic damage. The preferred Daedalus countermeasure was a 50-ton beryllium shield placed at the ship's prow. Even if no damaging impacts occur, a starship on a mission of decades or centuries would still require maintenance as parts and components wore out or broke down. For Daedalus, the solution was to pack a number of autonomous robotic wardens onboard the spacecraft to repair damage as it occurred. Creating such artificially intelligent robots capable of tending a starship for decades on end might be a bit more difficult than designing a Roomba to autonomously vacuum your living room.
And all that effort would only send a 500-ton payload, sans humans, strictly on a one-way flyby of a star. There would be no slowing down, stopping, or returning home. The Daedalus probe would fly through the alien star system in only a matter of hours, gradually trickling data homeward via a parabolic radio antenna. After absorbing untold treasure, time, and talent to reach another star, the Daedalus starship would have sent back scarcely more than the cosmic equivalent of a postcard. Icarus has upped the ante: The team intends to design a starship that can enter orbit around its target star, perhaps to monitor any potentially habitable planets there, and then, somehow, send large amounts of data back to Earth.
Suffice to say, engineering at these scales makes "rocket science" look like child's play.
Even further, consider the disruptive, unanticipated effects of the technologies a project like Icarus currently uses—not even including the ones it hopes to eventually employ for its starship: The ubiquitous computing and information networking that now allows Icarus to break out of local pubs and stretch across the world also seems to be turning many people's focuses inward, simultaneously connecting and unweaving the world. The velocity of our technology may ultimately be too fast, rather than too slow, and like the Daedalus probe of yore, could accelerate past its target in a flash, never to return. In other words, we could all too easily become lost in the virtual worlds we make for ourselves, and lose interest in the stars. Or, more probably, we could squander our resources and experience profound and irreversible technological regression. Sometimes, I pessimistically hold with some combination of these two extremes.
Given the magnitude and number of extreme technological and economic challenges that must be overcome to achieve starflight, it's difficult to imagine what, in fact, a civilization capable of interstellar travel would look like. Probably not much like us–more than anything else, projects like Icarus and Daedalus seem to tell us that we are presently as distant from interstellar travel as the stars are from Earth. And, at least until our culture's prioritization of short-term profit once again aligns with pushing the limits of the ultimately possible, that's likely to remain the case.
Perhaps someday one of these starship designs will take us out of the solar system on voyages to other living planets, other cosmic oases, strewn among the stars. Or maybe all the methods conceived today will in the fullness of time bear no more resemblance to actual starships than airplanes have to birds. Either way, it's worth remembering that the 100,000-year duration of interstellar voyages we can undertake right now is but the blink of an eye in cosmic terms. It may actually be more effective to adapt our expectations to those timescales, and to attempt to master such long-term planning rather than trying to brute-force our way to Alpha Centauri.
In expanding outward into space, patience, not velocity, may be the greatest virtue. After all, we're already on an interstellar spacecraft called the Earth, sailing with the Sun and its retinue of other planets around the Milky Way in circuits lasting 250 million years. Only by carefully preserving and cultivating the relatively bountiful and accessible resources of our planet and the solar system will we ever escape their confines. For now, it's wise to reflect that in our headlong rush to go ever faster and farther, we may only be fooling ourselves.