Beyond the Known: How Exploration Created the Modern World and Will Take Us to the Stars - Hardcover

Über die Autorin bzw. den Autor

Andrew Rader is a Mission Manager at SpaceX. He holds a PhD in Aerospace Engineering from MIT specializing in long-duration spaceflight. In 2013, he won the Discovery Channel’s competitive television series Canada’s Greatest Know-It-All. He also co-hosts the weekly podcast Spellbound, which covers topics from science to economics to history and psychology. Beyond the Known is Rader’s first book for adults. You can find him at Andrew-Rader.com.

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23. Going Interstellar

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?...