Beyond: Our Future in Space (30 page)

If we look beyond chemical energy, the best source is mass itself. The implication of Einstein’s iconic equation E = mc
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is that mass is frozen energy. Because the speed of light is a large number, a tiny amount of mass converts into a huge amount of energy. Mass can be liberated into energy in nuclear reactions with an efficiency of 0.1 percent for fission, 1 percent for fusion, and 100 percent for matter–antimatter annihilation (
Figure 49
). That represents energy storage of 10
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MJ/kg for fission, 10
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MJ/kg for fusion, and 10
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MJ/kg for matter–antimatter annihilation. Surely an efficiency millions of times better than chemical fuels can get us to the stars?

Yes, but not without the necessary technology development. A rocket needs thrust (or force) so it can exert a big push, but it also needs a high specific impulse, which is the force delivered per kilogram of fuel per second, analogous to fuel efficiency. Chemical rockets have high thrust but lousy specific impulse. The ideal interstellar rocket must score well for both quantities. Remember the rocket equation? It says that the final speed of a rocket depends on the fuel exhaust speed and the ratio of the fuel mass to payload mass. Having a nuclear fuel means less mass is needed, but that ratio is inside a logarithm, which suppresses its influence on the final speed, so it’s just as important to increase the exhaust speed.

None of the rocket engines about to be described have ever been built. They all rely on bleeding edge technology, though they are comfortably within the realm of known physics. Let’s look at the potential performance of rockets that don’t depend on chemical energy.

For nuclear fission, the simplest concept is to put a reactor on top of a rocket nozzle. Conventional fission and fusion concepts—recall that fusion hasn’t yet been used to generate energy on Earth—have ten to twenty times better performance than chemical rockets. This is a key limitation, since the practical gain in a rocket ends up far less than the theoretical gain based on energy density.

Fusion would still require 10
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kilograms of fuel, equal to a thousand supertankers, to get to Alpha Centauri in less than a thousand years. In the 1960s, Project Orion was developed by Stanislaw Ulam, a brilliant mathematician who had worked on the Manhattan Project. The idea was to use a series of controlled nuclear explosions to propel the spacecraft forward. In the 1970s, the British Interplanetary Society amended the design to use a large number of microfusion explosions (
Figure 50
).
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With matter–antimatter engines, we enter the realm of speculation, since antimatter, the quantum shadow partner of matter, has only been produced and contained in tiny quantities. An antimatter engine has a performance gain of a factor of a hundred, and the fuel requirement drops to about 100,000 kilograms or ten railway tankers of propellant to get there in less than a millennium. These numbers double because the spacecraft will need fuel to decelerate when it reaches its destination. Gathering this much antimatter will be impossible for the foreseeable future. At the moment, it would cost $100 billon just to create one milligram of antimatter.
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For those wanting to try this at home—the calculation, not actually building an interstellar rocket—the RAND Corporation used to sell the nifty (and very retro) Rocket Performance Calculator, dating from 1958.
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This circular slide rule incorporates the rocket equation and it can still be found occasionally on eBay, making for a great conversation piece.

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 10
¹⁹
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.
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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.
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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.
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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.
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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.
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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.
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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 10
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. 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.”
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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.
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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.
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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-assembling 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.