Beyond: Our Future in Space

by Chris Impey

  Figure 50. NASA’s version of the Project Orion concept, where pulsed nuclear fusion projects the power. The design combines high thrust and high exhaust velocity. No current technology can harness nuclear explosions in this way.

  The energy requirement of interstellar travel is formidable. Sending a 2,000-ton, Space Shuttle–size craft to Alpha Centauri in fifty years (a tenth the speed of light) costs 7 x 1019 Joules, assuming a perfect conversion of energy into forward motion, which isn’t true of any real propellant. That’s the energy consumption of the entire United States for six months. If that energy came from nuclear explosions, it would take a thousand Hiroshima bombs. The energy requirement can only be reduced by having the smaller payload or by traveling more slowly and taking longer to get there.

  A clever alternative avoids carrying and accelerating all that fuel.

  An interstellar ramjet would employ a magnetic “scoop” a thousand kilometers across to grab protons from the near vacuum of space and fuel a nuclear reactor. The idea originated with American physicist Robert Bussard in 1960.13 He was assistant director of the program to develop fusion power under the Atomic Energy Commission in the 1970s, and his ramjet concept was quickly coopted to become a staple of science fiction. There are huge engineering issues in realizing this concept. The physical challenge is to gather enough fuel from the sparse interstellar medium—the scoop has to sweep the equivalent of the volume of the Earth just to get one kilogram of hydrogen—while producing enough thrust to overcome the drag of the fuel collected. Slowing down at the destination is another problem, as yet unsolved.

  Solar sails are still more promising. The appropriately named Robert Forward developed a similar concept in the mid-1980s where a 10 million gigawatt laser shines through a 1,000-kilometer Fresnel lens onto a 1,000-kilometer sail. Unfortunately, 10 million gigawatts is a hundred times the energy consumption of all countries on Earth. Unfazed, Forward retooled his idea into a 10 gigawatt beam of microwaves that push on a kilometer-wide grid of fine wires. His “modest” proposal could be done with the energy output of ten large electrical generating plants.14 Forward was a dapper engineer, known for his shock of white hair, owlish glasses, and eye-popping vests. He died in 2002, but his ideas are still very influential.

  In the late 1980s, Dana Andrews and Robert Zubrin came up with the concept of the magnetic sail.15 A solar sail is driven by radiation from the Sun while a magnetic sail is driven by the solar wind, a diffuse plasma of charged particles streaming out from the Sun. The plasma would be harnessed by the magnetic field created by a large loop of superconducting wire. The magnetic sail has the disadvantage that the solar wind carries thousands of times less momentum than sunlight, but its big advantage is that the momentum is gathered by a massless magnetic field rather than a large physical sail.

  An alternative to using the Sun to propel a sail is to beam the energy to it from the Earth, with quick enough acceleration that it could coast to the destination. Two heads are better than one for this idea. James Benford, president of Microwave Sciences, believes that a microwave beam is superior to a laser for accelerating a solar sail. His lab experiments show that high-intensity microwave beams could be developed, but the sail material has to be extremely light and robust and could only withstand the temperature of 2,000 degrees by being highly reflective.16 His twin brother, Gregory Benford, is professor of physics at the University of California in Irvine and a noted science fiction writer. They collaborate on this project, and on gathering together the hard-science experts and science fiction visionaries to brainstorm the future of interstellar travel.17

  The 100 Year Starship project is funded by NASA and the Defense Advanced Research Projects Agency (DARPA). In 2012, a million-dollar grant was awarded to former astronaut Mae Jemison and the nonprofit organization Icarus Interstellar for work toward interstellar travel in the next hundred years. It’s important to realize that the majority of the speculative research on interstellar travel is being undertaken by professional physical scientists and engineers, with the work published in scholarly journals and books.18

  Thomas Jefferson thought it would take a thousand years for the American frontier to reach the Pacific. It happened in less than one-tenth of that time. And technology advances swiftly; the first nuclear reactor in 1942 in Chicago generated half a watt, but within a year a reactor was constructed that could power a small town. In the first fifty years of its development, the most powerful laser has increased in intensity by a factor of 1020. Returning to the analogy with information technology, linear progress in propulsion technology will not be sufficient to reach the stars; there will have to be technological leaps. Physicist Andreas Tziolas, president of Icarus Interstellar, says, “I have faith in our ingenuity.”19

  Here Come the Nanobots

  The nearest Earth-like planet is likely to be in our cosmic backyard in a galaxy a hundred thousand light years across. Remote sensing might indicate life on that planet, but the evidence will be limited and possibly ambiguous. Going there ourselves will, for the foreseeable future, be fiendishly difficult and ruinously expensive. Costs are variable, but they run upwards of a hundred trillion dollars, the value of current world GDP. Is there another strategy?

  Nanobots could reduce the cost and the energy requirements drastically. The US military’s smart motes for the battlefield give a sense of the possibilities. Space researchers can piggyback on a relentless push for miniaturization that’s motivated by medical applications. We imagine a fleet of baseball-size spacecraft, each crammed with sensors and a small camera, sailing toward the nearest Earth-like planet. As they arrive and drift down through the atmosphere, they transmit video back to Earth. There’s redundancy, so if some are lost in transit or fail to make it to the surface, the mission isn’t lost. We’d send nanobots in waves, so they could pass information back down the route of travel, like a bucket brigade at a fire. That reduces the power requirement for the transmitters on each nanobot. The mission would take a generation, but we can imagine expectation building as the fleet reaches its destination: Huge screens in city centers around the world carry the video feeds and crowds gather as the first images reveal details of an exotic new world.

  Going from tons to kilograms makes everything easier, but it’s not a slam dunk. Tony Dunn has crunched some numbers using solar sails for propulsion. With existing materials like Mylar, a kilogram nanobot can only reach a terminal speed of 80 kilometers per second, just five times faster than the Voyager spacecraft and hopelessly inadequate for the task. Making the sail larger than a hundred square meters means all the energy is going into accelerating the sail rather than the payload. Sail materials a million times lighter than Mylar would be needed to reach 10 percent of the speed of light. Using a laser to beam power from Earth directly to the sail helps. Now the solar sail needs to be only one meter across. The difficult trick is aiming the laser at such a small target when it’s far away. At the distance of Neptune, the laser would have to be targeted 100,000 times more accurately than the Hubble Space Telescope. A readily available 30-kilowatt laser could propel the kilogram probe to Alpha Centauri in forty years. The cost of the electricity: $800 million, assuming a residential rate of 15 cents per kilowatt hour. For a fleet, the price tag climbs to $100 billion. That’s steep, but doable.20

  Figure 51. NASA is collaborating with Tethers Unlimited on a space fabrication system. In this mockup, a space robot 3-D–prints the backbone for a mile-wide solar array. Creating structures in orbit is far cheaper than sending them there by rocket.

  Miniaturizing a spacecraft is a logical strategy, but it’s unimaginative. Nanotechnology suggests other possibilities: self-assembly and self-replication. In 2012, a company named Tethers Unlimited won a NASA contract to develop a system called SpiderFab.21 Spiderfab aims to use 3-D printing and robotic assembly to fabricate components in orbit—solar arrays, trusses, and shrouds that are ten times bigger than those that can currently be put in orbit (Figure 51). In the lab, self-assemblin
g machines are showing great promise. MIT researchers have created cubes no larger than dice that hold sensors, magnets, and a tiny flywheel. Identical cubes can all be commanded to move, snap together, and form arbitrary shapes.

  An even more exciting capability is self-replication. Eric Drexler talked about it in his prescient 1986 book on nanotechnology, Engines of Creation. Even earlier, in lectures at Princeton University, physicist Freeman Dyson described thought experiments involving large-scale replicating machines. In one, spacecraft traveled to Saturn’s small moon Enceladus, mined material to replicate themselves, and also launched spacecraft powered by solar sails to carry ice to Mars and begin to terraform the red planet. As with self-assembly, self-replication has made more progress in the lab than in space.

  The RepRap Project began in 2005 with the goal of designing a 3-D printer that could create most of its own components. It started at the University of Bath in England, but the code for computer-aided design and manufacture is open source, so the project has spawned a large developer community. In 2008, the RepRap machine “Darwin” produced all the parts needed to make an identical “child” machine. The project will make its technology freely available to anyone, with the goal of helping people make artifacts for everyday life.22

  The ultimate expression of self-replication is a von Neumann probe. This is a spacecraft that could go to a neighboring star system, mine materials to create replicas of itself, and send those out to other star systems. Using fairly conventional forms of propulsion, these probes could spread through a galaxy the size of the Milky Way in less than a few million years. The probes could investigate planetary systems and send information back to us on the home planet.23

  The concept is named after the Hungarian mathematician and physicist John von Neumann. He was one of the major intellectual figures of the twentieth century, making important contributions to mathematics, physics, computer science, and economics. Noted physicist Eugene Wigner recalled that von Neumann’s unusual mind was like a “. . . perfect instrument whose gears were machined to mesh accurately within a thousandth of an inch.” But he was less perfect in the real world. As a driver, he had numerous accidents and a few arrests, usually because he was distracted or reading. He overate, told off-color jokes, and did his best work in noisy and chaotic environments.

  In the 1940s, von Neumann figured out the logical requirements for self-replication. He described a computational “machine” that could make copies of itself, allow for errors, and evolve. This remarkable work preceded computers and anticipated the later discovery of DNA and the mechanisms of life. His work was theoretical, but it created a roadmap for building actual self-replicating machines.24

  Perhaps this is the way we will eventually explore the galaxy. Diffusing through interstellar space and exploring distant worlds with a fleet of self-replicating probes sounds fantastical, but it could be achieved with a reasonable extrapolation of our current technology. Which raises the question: Has any other civilization done this?

  Warp Drives and Transporters

  No fundamental obstacle prohibits the creation of a propulsion system that can accelerate a payload to a significant fraction of the speed of light. The highest speed ever reached by a spacecraft was 165,000 mph or 25 miles per second, when the probe Juno used Earth’s gravity to catapult toward Jupiter. That’s fifty times faster than a bullet, but only 0.01 percent of the speed of light. Reaching the nearest stars in less than fifty years would require speeds a thousand times faster, or 10 percent of the speed of light.

  Let’s now venture beyond the bounds of projected capabilities based on well-established science, into the realm of speculation and science fiction.25

  Two staples of science fiction, and routine occurrences on Star Trek, are warp drive and teleportation. A warp drive enables faster-than-light travel. Einstein’s theory of special relativity posits the speed of light as an absolute limit for the transmission of matter, energy, or information of any kind. Special relativity is a foundational principle in physics, so that would appear to kill the possibility of a warp drive. Tachyons—fundamental particles that travel faster than light—were hypothesized in 1967, but no evidence for them has ever been seen.26 In 1994, physicist Miguel Alcubierre proposed a theoretical solution for faster-than-light travel based on negative mass.27 The consensus among physicists is that a warp drive is not possible under the known laws of physics, but the idea got some attention at the 100 Year Starship Symposium at the Johnson Space Center in 2012.

  What about teleportation? Imagine this situation. You’re about to step into a device that will deconstruct your atoms into an energy pattern, beam the information to a remote target, and rematerialize you.

  In “Realm of Fear,” the 128th episode of the TV series Star Trek: The Next Generation, Lieutenant Reginald Barclay develops a fear of the transporter that’s used to beam crew members down to the surface of a planet. He becomes obsessed with all the things that could go wrong when the 1028 atoms in his body are dismantled and then reassembled.28 Eventually, his fear becomes debilitating.

  There’s no formal term for this condition.

  The TV series and subsequent films were sketchy on how transporter technology works. It’s supposed to transport objects accurately at the level of individual atoms, using something called a Heisenberg compensator to remove uncertainty from subatomic measurements. When technical adviser Michael Okuda was asked how it works, he said, “It works very well, thank you.” On the original Star Trek show, the special effect for the transporter was created before computer animation existed, so it was low tech: a slow-motion camera was turned upside down and it filmed backlit grains of aluminum powder falling in front of a black background.

  Classical teleportation measures every atom in the human body, encoding that information into photons, sending the photons to a remote location, and using the information to reconstruct a perfect replica of the body. That’s just an engineering problem. But with 1028 atoms to deal with, it’s a very nasty engineering problem.

  For decades, it was thought that teleportation defied physics. Heisenberg’s uncertainty principle says that we can’t simultaneously and accurately measure all the properties of even a single atom, let alone vast numbers of them. Measuring any property of a subatomic particle changes its state. So there’s no way to convey that state to a remote location with high fidelity.

  Figure 52. Theoretical diagram for the quantum teleportation of a photon. In this Feynman diagram, two bits of information would move classically from A to B; in quantum teleportation, information is transmitted via a single entangled qubit.

  In 1993, physicist Charles Bennett and his team made a breakthrough. They realized that particles at two different locations could be induced into something called quantum entanglement, where information about their physical states was shared. The loophole that lets us circumvent Heisenberg’s uncertainty principle involves trying not to know too much. We disturb the particle before we measure it, so we never know its state. Then we can subtract that disturbance at the other end to re-create the original state of the particle (Figure 52).29 Think of entanglement as a black box that conceals but connects events at two locations remote from each other. It seems to violate causality because changes at the two locations occur instantly, but there’s a limit to what we can know or measure. Quantum entanglement has been demonstrated using photons, electrons, buckyballs, and even small diamonds. It’s pure quantum weirdness.

  Let’s personalize it. Alice wants to teleport something to Bob.30 An entangled pair of photons serves as the intermediary in their experiment. She measures a property of her photon where the outcome depends on the entangled state of the pair. She records her measurement and sends it to Bob. He can’t tell what the state of her photon was, because the entanglement used in the measurement hides the true nature of that state. What Bob can do, however, is use information from Alice to modify the state of his photon. Then he can re-create the exact state of the photon Alice origina
lly measured.

  Even though the entangled state spans two separate locations, Bob can’t complete the teleportation until she sends him the result of her measurement. So the special theory of relativity and causality aren’t broken. The process allows information to be copied with perfect fidelity, although teleportation doesn’t literally make a copy; it shifts quantum information from one place to another, destroying the original in the process.

  Progress is rapid in this exciting research field. Physicists first demonstrated quantum teleportation in the lab in 1998 over a distance of a meter. In 2012, a research group teleported information between two locations in the Canary Islands 143 kilometers apart. In 2013, worldwide teleportation was demonstrated.31 The reliability of teleportation is also improving dramatically. In 2009, transfer of quantum information over distances of a few meters succeeded only one in 100 million times. In 2014, scientists at Delft University in the Netherlands teleported the quantum state of two entangled electrons with 100 percent reliability.32

  The mechanism of quantum entanglement is being used for cryptography and it’s likely to play a role in developing faster computers, but most physicists think it’s unlikely we’ll ever be able to create and interrogate the quantum entangled state of more than a few thousand atoms. So transporters aren’t even on the far horizon.

  Star Trek’s Lieutenant Barclay was only worried about his atoms being scrambled so that he turned into a pile of unrecognizable goo. But he also should have been worried about the philosophical implications of teleportation. You are not your particles. The atoms in your body fall off and get replaced all the time. Toast turns into eyelashes. You and your thoughts and your genetic information are really patterns rather than piles of particles. So when a transporter disassembles you, it kills you; when it reassembles you elsewhere, it gives birth to you. Logically it could do that as many times and at as many locations as it liked. Where would that leave your sense of self?