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23. Going Interstellar
Traveling at highway speeds, it would take 50 million years to reach the nearest star, Proxima Centauri.I But suppose you could put your foot on the accelerator and boost your speed to over 38,000 miles per hour. How long would it take then? The most distant human-built object, Voyager 1, is traveling right now at that hard-to-imagine pace into the depths of space, but even Voyager 1 wouldn’t get to Proxima for 75,000 years—if it were headed in the right direction (it’s not). The fastest object ever built, NASA’s Parker Solar Probe, is currently building up speed as it plunges toward the Sun and will eventually reach a breathtaking 430,000 miles per hour, hundreds of times faster than a speeding bullet. Yet even that mind-boggling pace represents only around 0.064 percent of the speed of light, and would get us to Proxima in no less than 6,500 years.
Travel to another star in a human lifetime quickly runs into a hard wall of physics. Kinetic energy is proportional to the square of velocity,II so reaching a speed a thousand times faster than the Parker Solar Probe—still a six-and-a-half-year trip to Proxima Centauri, not counting acceleration and deceleration—would require a million times more energy. Sending a spacecraft on a fifty-year voyage to another star would take more energy than is consumed in the United States during a year, somehow crammed into a container the size of a spacecraft. It’s not simply a matter of adding more fuel. You have to carry the mass of whatever fuel you add, increasing the thrust required and resulting in a death spiral of diminishing returns. Conventional rockets are limited because they get their energy from chemical bonds, and you can only put so much fuel in a tank. If we want to get to another star in a reasonable time frame, we’ll need alternative forms of propulsion.
What other types are available? Ion engines use electric fields to accelerate charged particles of propellant to extremely high velocities. Many interplanetary spacecraft have been equipped with them, and they’re highly efficient because they can fire for a long time (often days or weeks) without consuming much fuel. However, ion engines do have some drawbacks. They have extremely low thrust (equivalent to breathing on a sheet of paper), consume a lot of electricity, and on an interstellar trip would still be limited by their propellant supply. Nuclear thermal engines are also possible. These heat propellants to high temperatures in a nuclear reactor and then fire the propellants through a rocket nozzle at high velocity. Nuclear thermal engines have never been used operationally, but several were tested from the 1950s up to the early 1970s, and were even planned for use on later variants of the Saturn V rockets that went to the Moon.III
A rocket’s application of Newton’s third law—for every action, there is an equal and opposite reaction—is not fundamentally different from the recoil of a gun. It’s like pushing against water to swim forward, except in space there is nothing to push against, so we push against propellant ejected from a rocket instead. Chemical rockets push out vast quantities of propellant, generating a lot of thrust. Ion engines and nuclear thermal engines push out smaller quantities of propellant but at very high speeds, generating less thrust but maximizing efficiency over time. Nevertheless, they’re still limited ultimately by the size of their fuel tanks. This means that while they’re great for cruising around the solar system, neither is truly suitable for travel to another star. Existing rockets just can’t carry enough fuel.
What about using no engine at all? In a letter to Galileo written in 1610, Johannes Kepler observed that a comet’s tail doesn’t point away from its direction of motion, but away from the Sun. This implies that the Sun must be exerting some kind of “heavenly breeze” that could be captured to sail through the void of space.IV Indeed, this is precisely what can be done. Solar pressure is measurable even on spacecraft without sails, to the extent that it must be accounted for in orbital and trajectory planning. However, the force is vanishingly small. The solar pressure captured by a solar sail a square mile wide would add up to less than a pound. Extremely thin films around the thickness of a human hair are used to make solar sails to minimize the mass. A large solar sail could propel a craft to Jupiter within a few years, but current sails are overdesigned because they have to survive launch to orbit and unfurling by mechanical means.
An ultrathin sail of lithium (the lightest solid element) could theoretically be built in orbit at a tenth the thickness of sails today—one five-thousandth the thickness of a sheet of paper. Using such a sail, a spacecraft could reach Pluto within a year or two. But for interstellar travel there is the problem that sunlight declines rapidly as a spacecraft moves farther from the Sun, and in pushing beyond our solar system a spacecraft quickly gets too far away. Suppose, though, that we could create our own “wind”? By aiming a laser at a sail out in deep space, we could push it along even once the sunlight had faded. The craft would have to be small, and it could take a lot of energy to focus a laser far enough into space. At best, we might get to Proxima Centauri in fifty years with a tiny probe—but that would require a sail sixty-two miles across pushed by a laser consuming twenty-six thousand gigawatts: around double Earth’s entire power generation.
There’s only one existing technology that might get us to another star in a reasonable time frame, but it sounds a bit wacky. Back in the 1950s, a group of scientists studied the possibility of using nuclear bombs to propel a spacecraft. Called Project Orion, the idea was simple: you push nuclear bombs out the back and ride the detonation shockwaves on a specially designed pusher plate. The advantage is that your acceleration comes not from chemical bonds but directly from nuclear reactions, thus liberating millions of times more energy. There’s no upper size limit for ships powered by nuclear blasts, since a larger ship could just carry more bombs and better survive the shock waves. The scientists who worked on Project Orion envisioned interplanetary and eventually interstellar ships the size of cities, but the project was killed in 1963 by a treaty banning nuclear weapons in space—and also, probably, by the fact that even Apollo’s price tag was tough to swallow, let alone the anticipated cost of city-sized nuclear spaceships. Yet, the basic principle was sound, and such a ship might be able to reach 5 percent of the speed of light, getting to another star in around a century: still a long time to wait, but at least a human lifetime, more or less. There is, of course, the problem of launching large numbers of miniaturized high-yield nuclear weapons into space, an activity that seems self-evidently hazardous. While this method of propulsion is technically feasible, it’s hard to imagine the world’s politicians signing off on the idea any time soon.
Assuming we’re unwilling to ride nuclear bombs to the stars, what else is on the horizon? Fusion power has the potential to generate ten million times more energy than chemical reactions. Theoretically, fusion power could solve all of Earth’s energy problems, providing a sustainable, clean energy source. The only problem is that we haven’t quite gotten fusion power to work yet—at least, not to the point where we can produce more energy than what must be put in to start the reaction. It’s one of those technologies that’s always twenty years away—but if we do get it to work, we could use fusion to power rockets. Depending on the type of reactor used, a fusion rocket could either produce energy to drive a very efficient ion engine or simply direct the fusion reaction exhaust products out the rocket’s rear. Someday, fusion rockets could get us to another star, but the trip would still probably take decades and the rockets would have to carry a thousand tons of hydrogen fuel: the mass of a small navy warship, or the yearly launch capacity of all nations on Earth.
Or, instead of carrying hydrogen to fuel the fusion reactor, why not collect it along the way? Although space is extremely sparse, it’s not completely empty. Interstellar space contains around one atom per cubic centimeter. This is less than a quintillionth the density of air (1 followed by eighteen zeroes), but, with an enormous scoop, hydrogen atoms could be collected and concentrated into a fusion reaction. Proposed in 1960 by physicist Robert Bussard, this mega-scoop, “Bussard ramjet”V would use a magnetic field several kilometers wide to gather stray hydrogen atoms into its enormous maw. Although this technology could theoretically work, it’s unclear if deep space has enough atoms to sustain fusion reactions, or if the ramjet could overcome the solar wind of the star it was traveling toward. So fusion rockets? Probably, yes. But interstellar fusion ramjets are a definite maybe.
A thousand times more energetic than fusion power is antimatter, whose reaction with normal matter results in the maximum possible conversion of mass to energy (Einstein’s E = mc2). It sounds strange, but every particle has an opposite “antimatter” equivalent with the same mass but opposite quantum numbers. These atomic doppelgangers are created sporadically in nature but don’t last long because matter and antimatter instantly annihilate each other when they meet, releasing an intense burst of energy. As with fusion power, we could use antimatter to power an electric engine, but to achieve maximum efficiency, we’d have to find a way to direct the reaction products out the back of a rocket to produce thrust.VI Since antimatter exhaust would travel at relativistic speeds, rockets could accelerate to a significant fraction of the speed of light. This would allow travel to another star in a matter of years—if we could solve the production, containment, and lethal gamma ray problems. Currently we can’t produce more than a few atoms of antimatter, and they quickly react to annihilation with, well, everything, because everything else is normal matter. Antimatter is the most expensive product in existence. NASA estimates that it would cost $60 trillion to produce a single gram of the stuff, and we’d need a lot more than that.
Is there any way to get to another star faster? There isn’t within our understanding of the laws of physics, but that doesn’t mean there never will be. One way to get around the awkward problem that nothing can move faster than light would be to keep a spaceship stationary but “warp” space around it. This is the solution adopted by Star Trek, in a vague notional sort of way, but it is consistent with at least one property of the universe. The universe is “only” 13.8 billion years old, but it has a visible diameter of around 93 billion light-years. This seems to imply that parts of the universe somehow moved away from each other faster than the speed of light—but what it actually means is that space itself is expanding, not from a central location, but in many (possibly infinite) locations. (Picture a ball of dough with raisins as “galaxies.” When you bake the dough, it stretches out, moving the “galaxies” apart.) Since space can expand and compress, instead of traveling faster than light, could we ride a bubble by compressing space in front of a ship and expanding it behind?
In 1994, Mexican physicist Miguel Alcubierre proposed a way to create a warp bubble by producing an energy-density field lower than that of a vacuum. This could theoretically work, except it would rely on large quantities of either negative energy or negative matter, which may not even exist.VII There are other complications, including the inability to steer or stop the ship because signals could never penetrate the bubble, and the fact that Hawking radiation would obliterate everything inside.VIII Even Alcubierre now thinks his idea is impossible, although we can’t entirely rule out the chance that these barriers will someday be overcome. But even if we can someday create a warp bubble in a laboratory, this would be a far cry from turning it into a practical transportation system. Thus, for now “warp drives” must remain in the “maybe someday, maybe never” category.
How long would it take to get to interstellar velocities? If we could sustain a steady acceleration equivalent to Earth’s gravity (1G), it would take less than a year (354 days) to accelerate to the speed of light.IX This modest level of acceleration is less than a fifth of that experienced during a rocket launch, and if it could be sustained for long periods of time, interstellar travel becomes possible. Our galaxy is vast, consisting of four hundred billion stars spread across a hundred thousand light-years, but even if we never approach the speed of light, humans could one day expand across it. At a mere 5 percent of the speed of light, we could settle the entire galaxy in two million years: a long time, but not much longer than our species has existed. Even at the fastest speeds we can reach today, we could travel across the galaxy in a few hundred million years: less than a tenth of the age of our planet, and not much longer than mammals have inhabited Earth. Still, a long time from the perspective of a human life.
Even at small fractions of the speed of light, there are some problems with interstellar travel. As ships accelerate to extreme velocities (with correspondingly high energy levels), individual atoms pose a deadly threat. Indeed, even at orbital velocities of several miles per second, small pieces of debris can rip through metal plates. (The International Space Station’s windows are scarred with streaks where tiny particles have cut the glass, and astronauts report hearing “pings” when tiny projectiles tear through the station’s solar panels.) Thus, for relativistic travel, we’ll need to devise a way to deflect sparse interstellar atoms.X This might be possible using some kind of laser to ionize atoms so they can be deflected with a magnetic field or channeled as with the Bussard ramjet. Another problem with interstellar travel is that communicating with a spacefarer’s home planet is tough since radio signals decay with the square of distance. Radio antennas two hundred feet wide, part of NASA’s Deep Space Network, are required to communicate with the New Horizons spacecraft out past Pluto. Proxima Centauri is thousands of times farther, meaning radio signals would decay by factors of millions. One way to get around the signal loss would be to communicate with extremely high-powered lasers, or even send back physical memory packets on tiny spacecraft—but it would still take years to transmit a message, let alone receive a round-trip reply.
How could we keep a crew alive for the many—possibly hundreds—of years it would take to get to another star? One option is to send a “generation ship,” where only the great-grandchildren of the people who set out would reach the destination. This could entail building a giant starship, or hitching a ride on an interstellar comet, whose materials could be harvested to build, sustain, and propel a space colony traveling to the stars. Or we could make pit stops at the many “rogue planet” islands floating in the space between stars. There may be many times more rogue planets than regular ones (several billion in our galaxy), but they’re tough to find. So far, around twenty are confirmed or strongly suspected. Dark and distant from any sun, some may nevertheless have moons that are heated by tidal forces from the host planet, like Jupiter’s moons, and might be able to support life. Like the vagabond comets plying the space lanes between stars, rogue planets could act as refueling stations or natural spaceships, furnishing resources for the journey. Multigenerational voyages aren’t entirely unprecedented. The humans who left Africa to eventually settle in North America took perhaps fifty thousand years to complete the journey, or several thousand generations.
Or perhaps we could preserve the crew? Creatures such as tardigrades,XI some insects, and some types of turtles and frogs can revive after being frozen. It’s unclear if we could do this with humans, but this hasn’t stopped hundreds of people from having their bodies frozen at the moment of death in case they can ever be safely thawed (presumably after a cure is discovered for whatever killed them). Easier than freezing would be hibernation. This would reduce body temperature, slowing metabolism to a comalike state where the crew could be monitored and fed intravenously. Many mammals and birds survive like this throughout the winter, and hypothermic therapy is already being used by hospitals to extend the life of patients in cardiac arrest for hours or days. An alternative to sending a preserved crew would be to send frozen embryos, to be raised on their new home planet by robot guardians. Combine preservation with fusion or antimatter rockets, and it’s possible to imagine a voyage to another star being initiated by the end of the century.
If humans are to survive in the long run, we’ll have to travel to another star. But without a major breakthrough in our understanding of the laws of physics, the challenges of interstellar travel mean that a voyage will take an awfully long time. Instead of hopping from star to star like in Star Trek or Star Wars, our future may include gargantuan interstellar colony ships setting out on epic one-way voyages across the depths of space. In the meantime, we’re already scouting for landing sites, as our telescopes scan the skies for planets around other stars. Soon, we’ll start sending robotic explorers, and this could enable virtual participation. Imagine simulated vacations to a distant point in the galaxy, perfectly reconstructed based on data transmitted by robotic pathfinders trillions of miles away. Yet, as enticing as this prospect is, I suspect it won’t satisfy us forever. We come from a long line of explorers. Someday a band of bold pioneers will leave the warm embrace of our Sun, if only because restlessness is part of being human.
