A spaceship works by burning fuel to produce high-speed exhaust gases, then riding the reaction force in the opposite direction. That core principle, Newton’s third law of motion, gets a craft off the ground and into orbit. But propulsion is only part of the story. Keeping a crew alive, maintaining the right temperature, communicating across millions of kilometers, and surviving the trip home all require interlocking systems working simultaneously.
How Rockets Generate Thrust
Every rocket engine, whether it burns liquid hydrogen or solid fuel, operates on the same basic physics. A fuel and an oxidizer are combined inside a combustion chamber, where they ignite and produce enormous volumes of exhaust gas at extreme temperature and pressure. That gas blasts out of a nozzle at the bottom of the engine. For every force pushing the exhaust downward, an equal and opposite force pushes the rocket upward. That’s Newton’s third law in action, and it’s the reason rockets work even in the vacuum of space, where there’s no air to “push against.” The exhaust itself is what the rocket pushes against.
The amount of thrust depends on two things: the mass of exhaust leaving the engine each second and the speed at which it exits. Engineers optimize both by choosing propellant combinations that burn hot and produce lightweight gas molecules, which naturally move faster. Liquid oxygen paired with liquid hydrogen is one of the most efficient combinations, which is why it has powered upper stages of rockets for decades.
Getting Into Orbit and Beyond
Reaching space isn’t just about going up. A spacecraft needs to go sideways fast enough that, as gravity pulls it back toward Earth, it keeps missing the surface. To stay in low Earth orbit, roughly 200 to 400 kilometers up, a spacecraft needs to reach about 7.8 kilometers per second, or around 28,000 kilometers per hour. That’s fast enough to circle the entire planet in about 90 minutes.
Leaving Earth entirely requires even more speed. Escape velocity from the surface is approximately 11.2 kilometers per second. Interestingly, that number drops dramatically with altitude. A spacecraft already parked in a high orbit around 36,000 kilometers up only needs about 4.35 kilometers per second to break free of Earth’s gravity altogether. This is why missions headed to other planets often begin from a parking orbit rather than launching directly from the surface in one shot.
Once in space, a ship doesn’t need to keep its engines running. Objects in orbit are essentially in free fall, perpetually falling toward Earth but moving sideways fast enough to keep clearing the horizon. Short engine burns adjust speed and direction as needed.
Breathing in a Sealed Metal Box
A crew needs a constant supply of oxygen and a way to get rid of the carbon dioxide they exhale. On the International Space Station, oxygen comes from splitting water molecules into hydrogen and oxygen using electricity, a process called electrolysis. The station’s Oxygen Generation Assembly runs water through a stack of electrochemical cells that separate the two gases. The oxygen goes into the cabin air, while the hydrogen is either vented overboard or fed into other systems.
Carbon dioxide is the more urgent problem. Exhaled CO2 builds up quickly in an enclosed space and becomes toxic at surprisingly low concentrations. During the Apollo missions, NASA used canisters filled with lithium hydroxide, a chemical that absorbs CO2 permanently. The drawback: once a canister was saturated, it was useless, and the crew needed a fresh one. Modern spacecraft use regenerable systems instead. The station’s current CO2 scrubber pushes cabin air through beds of a mineral called zeolite, which traps CO2 molecules on its surface. When the bed is full, heaters bake the trapped gas off, venting it into space, and the bed cycles back into service. This approach means the system can run indefinitely without replacement cartridges.
Staying the Right Temperature
Temperature control in space is counterintuitively difficult. On Earth, you shed heat by convection: air carries warmth away from your body and from machines. In the vacuum of space, there’s no air, so the only way to dump heat is by radiating it as infrared energy. Meanwhile, surfaces facing the sun can reach extreme temperatures, and surfaces in shadow plunge far below freezing.
Spacecraft solve this with active thermal control systems. Pumped fluid loops circulate a coolant through the interior, absorbing heat from electronics, crew areas, and other equipment. That warm fluid then flows out to large radiator panels on the exterior of the spacecraft, where the heat radiates into the cold of space. The Space Shuttle’s radiator system contained over 5,500 feet of fluid tubing across more than 400 separate tubes, all pumping coolant to panels mounted inside the cargo bay doors. The ISS uses an even larger system with external radiator panels that are oriented edge-on to the sun whenever possible, minimizing the solar energy they absorb while maximizing the heat they can shed.
For larger stations, engineers have moved toward heat pipe radiators built from modular panels. Each panel operates independently, so if one fails, the rest continue working, and new panels can be added as the station grows.
Talking Across the Solar System
Spacecraft communicate using radio waves, the same fundamental technology as a car radio but operating at much higher frequencies and with far more precision. NASA’s Deep Space Network, a collection of giant dish antennas in California, Spain, and Australia, provides continuous coverage for missions beyond Earth orbit. The three sites are spaced roughly 120 degrees apart around the globe, so at least one station can always see a given spacecraft regardless of Earth’s rotation.
For missions more than 2 million kilometers from Earth, the network uses dedicated frequency bands. S-band signals (around 2,100 to 2,300 megahertz) were the workhorses of early deep space communication. Modern missions increasingly use X-band (around 7,100 to 8,450 megahertz) and Ka-band (around 32,000 to 34,700 megahertz), which can carry more data per second. Even so, the speed of light imposes hard limits. A signal from Mars takes anywhere from 4 to 24 minutes to reach Earth depending on orbital positions, making real-time conversation impossible. Spacecraft must carry enough onboard intelligence to handle routine operations and emergencies on their own.
Surviving the Trip Home
Getting back to Earth’s surface is arguably the most violent phase of any mission. A spacecraft returning from orbit hits the upper atmosphere at roughly 7.8 kilometers per second. At that speed, the air in front of the vehicle can’t move out of the way fast enough and compresses into a superheated shockwave. Temperatures on the leading surfaces can reach thousands of degrees Celsius.
The solution is a heat shield. Most crewed capsules use ablative shields, materials designed to slowly burn away and carry heat with them as they go. NASA’s PICA (Phenolic Impregnated Carbon Ablator) material, and SpaceX’s PICA-X variant, are engineered to handle heating intensities that would melt steel in seconds. As the outer layer chars and erodes, it absorbs enormous energy and carries it away in the airflow, keeping the spacecraft’s interior at survivable temperatures. The back face of the shield, the side touching the actual spacecraft structure, stays around 250°C even while the front face endures far more extreme conditions.
The shape of the capsule matters too. A blunt, rounded heat shield creates a wider shockwave that pushes the hottest gases further from the surface. This is why returning capsules look like upside-down cones or gumdrop shapes rather than sleek, pointed vehicles. The blunt design trades aerodynamic elegance for thermal survival.
Power Generation
Every system on a spacecraft, from life support to communications to the thermal control pumps, needs electricity. Close to the sun, solar panels are the standard solution. The ISS carries eight large solar array wings that together generate enough power for the station’s systems and experiments. Each wing tracks the sun to maximize energy capture, and batteries store excess power for the 45 minutes out of every 90-minute orbit that the station spends in Earth’s shadow.
For missions headed to the outer solar system, where sunlight is too dim for practical solar panels, spacecraft carry radioisotope thermoelectric generators. These devices convert the heat from decaying plutonium into electricity. They produce far less power than solar arrays, typically a few hundred watts, but they work reliably for decades regardless of distance from the sun. Voyager 1 and 2 have been running on this technology since 1977.