by Neil deGrasse Tyson
From Natural History Magazine, June 2005
This is part one of a two-part article.
In daily life you rarely need to think about propulsion, at least the kind that gets you off the ground and keeps you aloft. You can get around just fine without booster rockets—simply by walking, running, rollerblading, taking a bus, or driving a car. All those activities depend on friction between you (or your vehicle) and Earth's surface.
When you walk or run, friction between your feet and the ground enables you to push forward. When you drive, friction between the rubber wheels and the pavement enables the car to move forward. But try to run or drive on slick ice, where there's hardly any friction, and you'll slip and slide and generally embarrass yourself as you go nowhere fast.
For motion that doesn't engage Earth's surface, you'll need a vehicle equipped with an engine stoked with massive quantities of fuel. Within the atmosphere, you could use a propeller-driven engine or a jet engine, both fed by fuel that burns the free supply of oxygen provided by the air. But if you're hankering to cross the airless vacuum of space, leave the props and jets at home and look for a propulsion mechanism that requires no friction and no chemical help from the air.
One way to get a vehicle to leave our planet is to point its nose upward, aim its engine nozzles downward, and swiftly sacrifice a goodly amount of the vehicle's total mass. Release that mass in one direction, and the vehicle recoils in the other. Therein lies the soul of propulsion. The mass released by a spacecraft is hot, spent fuel, which produces fiery, high-pressure gusts of exhaust that channel out the vehicle's hindquarters, enabling the spacecraft to ascend.
Propulsion exploits Isaac Newton's third law of motion, one of the universal laws of physics: for every action, there is an equal and opposite reaction. Hollywood, you may have noticed, rarely obeys that law. In classic Westerns, the gunslinger stands flat-footed, barely moving a muscle as he shoots his rifle. Meanwhile, the ornery outlaw that he hits sails backward off his feet, landing butt first in the feeding trough—clearly a mismatch between action and reaction. Superman exhibits the opposite effect: he doesn't recoil even slightly as bullets bounce off his chest. Arnold Schwarzenegger's character the Terminator was truer to Newton than most: every time a shotgun blast hit the cybernetic menace, he recoiled—a bit.
Spacecraft, however, can't pick and choose their action shots. If they don't obey Newton's third law, they'll never get off the ground.
Realizable dreams of space exploration took off in the 1920s, when the American physicist and inventor Robert H. Goddard got a small liquid-fueled rocket engine off the ground for nearly three seconds. The rocket rose to an altitude of forty feet and landed 180 feet from its launch site.
But Goddard was hardly alone in his quest. Several decades earlier, around the turn of the twentieth century, a Russian physicist named Konstantin Eduardovich Tsiolkovsky, who earned his living as a provincial high school teacher, had already set forth some of the basic concepts of space travel and rocket propulsion. Tsiolkovsky conceived of, among other things, multiple rocket stages that would drop away as the fuel in them was used up, reducing the weight of the remaining load and thus maximizing the capacity of the remaining fuel to accelerate the craft. He also came up with the so-called rocket equation, which tells you just how much fuel you'll need (assuming you won't be stopping at any filling stations en route) for your journey through space.
Nearly half a century after Tsiolkovky's investigations came the forerunner of modern spacecraft, Nazi Germany's V-2 rocket—
V for Vergeltungswaffen, or
Vengeance Weapon. The V-2 was conceived and designed for war, and was first used in combat in 1944, principally to terrorize London. The brainchild of Wernher von Braun and hundreds of other scientists and engineers working with the Nazis, the V-2 was the first ballistic missile and the first rocket to target cities that lay beyond its own horizon. Capable of reaching a top speed of about 3,500 miles an hour, the V-2 could go a few hundred miles before plummeting back to Earth's surface in a deadly free fall from the edge of space.
To achieve a full orbit of Earth, however, a spacecraft must travel five times faster than the V-2, a feat that, for a rocket of the same mass as the V-2, requires no less than twenty-five times the V-2's energy. And to escape from Earth orbit altogether, and head out toward the Moon, Mars, or beyond, the craft must reach 25,000 miles an hour. That's what the Apollo missions did in the 1960s and 1970s to get to the Moon—a trip requiring at least another factor of two in energy.
And that represents a phenomenal amount of fuel.
Because of Tsiolkovsky's unforgiving rocket equation, the biggest problem facing any craft heading into space is the need to boost
excess mass in the form of fuel—most of which is the fuel required for transporting the fuel it will burn later in the journey. And the spacecraft's weight problems grow exponentially. The multistage vehicle was invented to soften this problem. In such a vehicle, a relatively small payload—such as the Apollo spacecraft, an Explorer satellite, or the space shuttle—gets launched by huge, powerful rockets that drop away sequentially or in sections when their fuel supplies become exhausted. Why tow an empty fuel tank when you can just dump it and possibly reuse it on another flight?
Take the Saturn V, a three-stage rocket that launched the Apollo astronauts toward the Moon. Designed by von Braun (among others), it could almost be described as a giant fuel tank. The Saturn V and its human cargo stood thirty-six stories tall, yet the three astronauts returned to Earth in an itty-bitty, one-story capsule. The first stage dropped away about ten minutes after liftoff, once the vehicle had been boosted off the ground and was moving at about 9,000 feet per second (more than 6,000 miles per hour). Stage two dropped away about ten minutes later, once the vehicle was moving at about 23,000 feet per second (almost 16,000 miles per hour). Stage three had a more complicated life, performing several episodes of fuel burning: the first to accelerate the vehicle into Earth orbit, the next to get it out of Earth orbit and head it toward the Moon, and a couple more to slow the craft down so that it could pull into lunar orbit. At each stage, the craft got progressively smaller and lighter, which means that the remaining fuel could do more with less.
Since 1981, NASA has used the space shuttle for missions in
low-Earth orbit—a few hundred miles above our planet. The shuttle has three main parts: a stubby, airplanelike
orbiter that holds the crew, the payload, and the three main engines; an immense external fuel tank that holds more than half a million gallons of self-combustible liquid; and two
solid rocket boosters, whose two million pounds of rubbery aluminum fuel generate 85 percent of the thrust needed to get the giant off the ground. On the launchpad the shuttle weighs four and a half million pounds. Two minutes after the launch, the boosters have finished their work and drop away into the ocean, to be fished out of the water and reused. Six minutes later, just before the shuttle reaches orbital speed, the now-empty external tank drops off and disintegrates as it reenters Earth's atmosphere. By the time the shuttle reaches orbit, 90 percent of its launch mass has been left behind.
Now that you're launched, how about slowing down, landing gently, and one day returning home? Fact is, in empty space, slowing down takes as much fuel as speeding up.
Familiar, earthbound ways to slow down require friction. On a bicycle, the rubber pincers on the hand brake squeeze the wheel rim; on a car, the brake pads squeeze against the wheels' rotors, slowing the rotation of the four rubber tires. In those cases, stopping requires no fuel. To slow down and stop in space, however, you must turn your rocket nozzles backward, so that they point in the direction of motion, and ignite the fuel you've dragged all that distance. Then you sit back and watch your speed drop as your vehicle recoils in reverse.
To return to Earth after your cosmic excursion, rather than using fuel to slow down you could do what the space shuttle does: glide back to Earth unpowered, and exploit the fact that our planet has an atmosphere, a source of friction. Instead of using all that fuel to slow down the craft before reentry, you could let the atmosphere slow it down for you.
One complication, though, is that the craft is traveling much faster during its home stretch than it was during its launch. It's dropping out of an 18,000-mile-an-hour orbit and plunging toward Earth's surface—so heat and friction are much bigger problems at the end of the journey than at the beginning. One solution is to sheathe the leading surface of the craft in a heat shield, which deals with the swiftly accumulating heat through ablation or dissipation. In ablation, the preferred method for the cone-shaped Apollo-era capsules, the heat is carried away by shock waves in the air and a continuously peeling supply of vaporized material on the capsule's bottom. For the space shuttle and its famous tiles, dissipation is the method of choice.
Unfortunately, as we all now know, heat shields are hardly invulnerable. The seven astronauts of the Columbia space shuttle were cremated in midair on the morning of February 1, 2003, as their orbiter tumbled out of control and broke apart during reentry. They met their deaths because a chunk of foam insulation had come loose from the shuttle's huge fuel tank during the launch and had pierced a hole in the shield covering the left wing. That hole exposed the orbiter's aluminum dermis, causing it to warp and melt in the rush of superheated air.
Here's a safer idea for the return trip: Why not put a filling station in Earth orbit? When it's time for the shuttle to come home, you attach a new set of tanks and fire them at full throttle, backward. The shuttle slows to a crawl, drops into Earth's atmosphere, and just flies home like an airplane. No friction. No shock waves, No heat shields.
But how much fuel would that take? Exactly as much fuel as it took to get the thing up there to begin with. And how might all that fuel reach the orbiting filling station that could service the shuttle's needs? Presumably it would be launched there, atop some other skyscraper-high rocket.
Think about it. If you wanted to drive from New York to California and back again, and there were no gas stations along the way, you'd have to drag along a fuel tank as big as a tanker truck. But then you'd need an engine strong enough to pull a tanker, so you'd need to buy a much bigger engine. Then you'd need even more fuel to drive the car. Tsiolkovsky's rocket equation eats your lunch every time.
In any case, slowing down or landing isn't only about returning to Earth. It's also about exploration. Instead of just passing the far-flung planets in fleeting
flybys, a mode that characterized an entire generation of NASA space probes, the spacecraft ought to spend some time getting to know those distant worlds. But it takes extra fuel to slow down and pull into orbit. Voyager 2, for instance—launched in August 1977—has spent its entire life coasting. After gravity assists from both Jupiter and then Saturn (the poor man's propulsion mechanism), Voyager 2 flew past Uranus in January 1986 and past Neptune in August 1989. For a spacecraft to spend a dozen years reaching a planet and then spend only a few hours collecting data on it is like waiting two days in line to see a rock concert that lasts six seconds. Flybys are better than nothing, but they fall far short of what a scientist really wants to do.
On Earth, a fill-up at the local gas station has become a pricey activity of late. Plenty of smart scientists have spent plenty of years inventing and developing alternative fuels that might one day see widespread use. And plenty of other smart scientists are doing the same for the world of propulsion.
The most common forms of fuel for spacecraft are chemical substances: ethanol, hydrogen, oxygen, monomethyl hydrazine, powdered aluminum. But unlike airplanes, which burn fuel by drawing oxygen through their engines, spacecraft have no such luxury; they must bring the whole chemical equation along with them. So they carry not only the fuel but an oxidizer as well, kept separate until valves bring them together. The ignited, high-temperature mixture then creates high-pressure exhaust, all in the service of Newton's third law of motion.
Bummer. Even ignoring the free
lift a plane gets from air rushing over its specially shaped wings, pound for pound, any craft whose agenda is to leave the atmosphere must carry a much heavier fuel load than does an airplane. The V-2's fuel was ethanol and water; the Saturn V's fuel was kerosene for the first stage and liquid hydrogen for the second stage. Both rockets used liquid oxygen as the oxidizer. The space shuttle's main engine, which must work above the atmosphere, uses 385,000 gallons of liquid hydrogen and 143,000 gallons of liquid oxygen.
Wouldn't it be nice if the fuel itself carried more punch than it does? If you weigh 150 pounds and you want to launch yourself into space, you'll need 150 pounds of thrust under your feet (or spewed forth from a jet pack) just to weigh nothing. To actually launch yourself, anything more than 150 pounds of thrust will do, depending on your tolerance for acceleration. But wait. You'll need even more thrust than that to account for the weight of the unburned fuel you're carrying. Add more thrust than that, and you'll accelerate skyward.
The space mavens' perennial goal is to find a fuel source that packs astronomical levels of energy into the smallest possible volumes. Because chemical fuels use chemical energy, there's a limit to how much thrust they can provide, and that limit comes from the stored binding energies within molecules. So, given those limitations, physicists and engineers have been looking into innovative alternatives.
After a vehicle rises beyond Earth's atmosphere, propulsion need not come from burning vast quantities of chemical fuel. In deep space, the propellant can be small amounts of ionized xenon gas, accelerated to enormous speeds within a new kind of engine. A vehicle equipped with a reflective sail can be pushed along by the gentle pressure of the Sun's rays, or even by a laser stationed on Earth or on an orbiting platform. And within ten years or so, a perfected, safe nuclear reactor will make nuclear propulsion possible—the rocket designer's dream engine. The energy it generates will be orders of magnitude more than chemical fuels can produce.
While we're getting carried away with ourselves, making the impossible possible, what we really want is the antimatter rocket. Better yet, we'd like to arrive at a new understanding of the universe, to enable journeys that exploit shortcuts in the fabric of space and time. When that happens, the sky will no longer be the limit.
Astrophysicist Neil deGrasse Tyson is the director of the Hayden Planetarium at the American Museum of Natural History. His latest book, co-authored with Donald Goldsmith, is Origins: Fourteen Billion Years of Cosmic Evolution (W. W. Norton, 2004).