In 1687, Isaac Newton published three laws of motion in his Philosophiae Naturalis Principia Mathematica. The third law is the one most people can recite from memory, even if they have forgotten everything else about physics: for every action, there is an equal and opposite reaction.
It sounds simple. Almost obvious. Two kids pushing off each other on ice skates, everyone gets that. But this “simple” principle is literally the reason we can travel through space. Every rocket that has ever launched, from the V-2 to the Saturn V to the Falcon Heavy, works because of Newton’s third law. Without it, leaving Earth would be impossible.
And here is the thing that trips people up: rockets do not work the way most people think they do.
The Most Common Misconception About Rockets
Ask someone how a rocket works, and many will tell you something like this: “The rocket pushes hot gas downward against the ground or the atmosphere, and that pushes the rocket up. Like a balloon releasing air.”
The balloon analogy is actually closer to correct than the first part. But the idea that rockets push against the air or the ground is wrong, and it matters that it is wrong, because the whole point of rockets is that they work in a vacuum. There is nothing to push against in space. No air. No ground. Nothing.
This misconception was so prevalent that when Robert Goddard, one of the pioneers of modern rocketry, proposed in 1920 that rockets could work in space, the New York Times published an editorial mocking him. The paper declared that Goddard “seems to lack the knowledge ladled out daily in high schools,” specifically, the knowledge that a rocket needs something to push against. The Times did not issue a correction until July 1969, the day after Apollo 11 launched for the Moon. Forty-nine years late.
The Times editors were wrong, and Newton, writing 233 years before Goddard’s paper, had already explained why.
How Rockets Actually Work
Newton’s third law does not say anything about pushing against an external medium. It says that forces come in pairs. If object A exerts a force on object B, then object B exerts an equal force on object A, in the opposite direction. That is it. No medium required.
Here is what happens inside a rocket engine:
- Fuel and oxidizer are mixed and ignited in a combustion chamber
- The combustion produces extremely hot, high-pressure gas
- This gas is expelled through a nozzle at tremendous speed
- As the rocket pushes the gas backward (action), the gas pushes the rocket forward (reaction)
The rocket and the exhaust gas are the two objects in Newton’s force pair. They push against each other. The surrounding environment, air, ground, vacuum of space, is completely irrelevant to the fundamental mechanism.
Think of it this way: if you are standing on a frozen lake (frictionless surface) and you throw a heavy bowling ball away from you, you will slide backward. You did not push against anything external. You and the ball pushed against each other. A rocket does exactly this, but instead of throwing a single bowling ball, it throws a continuous stream of gas molecules at extremely high velocity.
Why Exhaust Velocity Matters
The effectiveness of a rocket depends critically on two things: how much mass it expels and how fast that mass is moving. This relationship is captured in the Tsiolkovsky rocket equation, derived by the Russian scientist Konstantin Tsiolkovsky in 1903, the same year the Wright brothers flew at Kitty Hawk.
The equation shows that the change in a rocket’s velocity depends on the exhaust velocity of the gas and the ratio of the rocket’s initial mass (full of fuel) to its final mass (fuel spent). Higher exhaust velocity means more “push” per kilogram of fuel. This is why rocket engineers obsess over exhaust velocity and why different fuels produce different performance.
- Solid rocket boosters produce relatively low exhaust velocities but are simple and reliable
- Liquid hydrogen and oxygen produce high exhaust velocities, which is why the Space Shuttle’s main engines used them
- Ion engines produce extraordinarily high exhaust velocities by using electric fields to accelerate ions, but with very low thrust, useful for deep space probes, not for leaving Earth
In every case, the principle is the same: throw stuff backward, and by Newton’s third law, you move forward.
From Principia to the Launchpad
The historical path from Newton’s third law to actual space travel is long and fascinating. Newton himself mused about orbital mechanics in the Principia. He proposed a famous thought experiment, Newton’s cannonball, in which a cannon on top of a very tall mountain fires a ball with increasing velocity. At low speeds, the ball falls to Earth. At higher speeds, it travels farther before falling. At just the right speed, the ball falls at the same rate that the Earth’s surface curves away beneath it, and it orbits. Fire it even faster, and it escapes Earth entirely.
This thought experiment perfectly described what rockets would need to achieve, centuries before the technology existed to do it. Newton understood the physics of spaceflight in 1687. The engineering to make it happen took another 270 years.
The Pioneers
Three figures bridge the gap between Newton’s theory and actual rocket technology:
- Konstantin Tsiolkovsky (Russia, early 1900s): worked out the mathematical theory of rocketry, including the rocket equation, multi-stage rockets, and the use of liquid fuels
- Robert Goddard (United States, 1920s-1940s): built and launched the first liquid-fueled rocket in 1926, despite widespread ridicule
- Hermann Oberth (Germany, 1920s-1930s): independently derived much of the same theory and inspired the German rocket program that produced the V-2
All three understood Newton’s third law and its implications for space travel. All three faced skepticism from people who thought rockets needed air to push against. All three were right.
Newton’s Laws and Apollo
When NASA sent astronauts to the Moon, every aspect of the mission was governed by Newton’s laws. The third law propelled the Saturn V off the pad. The first law (an object in motion stays in motion unless acted upon by a force) kept the spacecraft coasting through the vacuum between Earth and Moon. The second law (force equals mass times acceleration) determined how much thrust was needed for every maneuver. And the law of universal gravitation, also from the Principia, governed the trajectories.
The Apollo missions are perhaps the most dramatic demonstration of Newtonian physics in action. The Apollo 13 Activation Checklist is a visceral artifact of this, the actual procedures astronauts followed to bring their spacecraft systems online, each step grounded in the physics Newton described three centuries earlier.
And the Apollo 11 Translunar Trajectory Plotting Chart shows the mathematical beauty of the path from Earth to Moon, a trajectory calculated using the same gravitational equations Newton published in 1687. The math had not changed. Only our ability to use it had.
Modern Rocketry: Still Newtonian
SpaceX’s Falcon 9, Blue Origin’s New Shepard, the Space Launch System, every modern rocket still operates on Newton’s third law. The engineering has advanced enormously. We have better fuels, better materials, better computers for guidance and control. SpaceX has even figured out how to land orbital-class boosters vertically, a feat that requires extraordinary precision in applying Newtonian mechanics in real time.
But strip away the technology and you are left with the same principle Newton wrote down over three centuries ago: every action produces an equal and opposite reaction. Throw mass one way, move the other way. That is all a rocket does. That is all it needs to do.
Even proposed future propulsion systems, ion drives, plasma thrusters, nuclear thermal engines, are still reaction engines. They still work by expelling mass. They still depend on Newton’s third law. Until and unless someone discovers entirely new physics, Newton’s insight from 1687 will continue to be the foundation of all space travel.
Why a 17th-Century Book Still Matters
There is something remarkable about the fact that a principle written in Latin in a book published in 1687 is literally what carries human beings into orbit today. The Principia is not just a historical curiosity. It is a living, working document. Its ideas are active in every rocket engine, every satellite orbit, every interplanetary trajectory.
Newton’s Principia in a handcrafted edition is not merely a beautiful book to display on a shelf, though it certainly is that. It is a chance to hold the ideas that made space travel possible. The very same pages that describe the third law are the intellectual blueprint for everything from the Saturn V to the Mars rovers.
Newton could not have imagined a Falcon 9 landing on a drone ship in the Atlantic. But he gave us the physics to build one. For every action, there is an equal and opposite reaction. Simple, profound, and powerful enough to carry us to the Moon and beyond.
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