Space travel is the use of spacecraft to leave Earth’s atmosphere and move through the vacuum of space, whether orbiting our planet, landing on another world, or simply crossing the boundary where the sky ends and space begins. Since Yuri Gagarin circled Earth in 108 minutes aboard Vostok 1 on April 12, 1961, humans have walked on the Moon, built a permanently crewed space station, and begun selling tickets for commercial flights above the atmosphere.
How Spacecraft Leave Earth
Earth’s gravity holds everything on the surface in place, and overcoming it is the first challenge of any space mission. Every object in the universe has an escape speed: the velocity needed to break free of its gravitational pull. For Earth, that speed at the surface is about 11.2 kilometers per second, or roughly 40,000 kilometers per hour. A rocket doesn’t need to hit that speed all at once, but it does need to generate enough sustained thrust to push a spacecraft into orbit or beyond.
Most rockets achieve this with chemical propulsion, burning liquid oxygen and a fuel like liquid hydrogen or kerosene to produce enormous thrust in a short time. Chemical engines are powerful but relatively inefficient, converting only about 30 percent of their fuel’s energy into useful motion. Ion thrusters, used on some deep-space probes, flip that equation: they produce a tiny amount of thrust but use fuel more than 80 percent more efficiently, making them ideal for long missions where a spacecraft can accelerate gradually over months or years. Chemical rockets get you off the ground; electric propulsion keeps you going once you’re already in space.
Where Space Begins
Low Earth orbit, where the International Space Station flies, extends up to about 2,000 kilometers above the surface. The ISS itself orbits at roughly 400 kilometers, circling the planet every 90 minutes. This is where most human spaceflight happens. Higher up, at about 35,786 kilometers, satellites sit in geostationary orbit, matching Earth’s rotation so they hover over a single point on the equator. At that altitude, the escape speed drops to just 4.35 kilometers per second, less than half of what’s needed at the surface, because gravity weakens with distance.
Beyond Earth orbit, spacecraft can travel to the Moon (about 384,000 kilometers away), to other planets, or out of the solar system entirely. The Voyager probes, launched in 1977, are now more than 20 billion kilometers from Earth and still sending data back.
What Space Does to the Body
Weightlessness feels liberating, but it quietly damages the human body. Without gravity pulling down on the skeleton, bones lose density at a rate of 1 to 1.5 percent per month, primarily in the hips and legs. Over a six-month stay on the ISS, astronauts typically lose 6 to 10 percent of their hip bone mass. Muscles atrophy too, especially in the legs and back, since they no longer need to support the body’s weight. Astronauts on the station exercise about two hours a day using resistance machines and treadmills to slow these losses, though they can’t fully prevent them.
Radiation is the other major health concern. On Earth, the atmosphere and magnetic field block most cosmic radiation. In space, astronauts absorb significantly more ionizing radiation, which raises their long-term cancer risk. NASA limits career exposure so that an astronaut’s additional cancer mortality risk stays below 3 percent. For missions to Mars, which could take two to three years round trip, staying within that limit becomes extremely difficult with current shielding technology.
Keeping Humans Alive in a Vacuum
The International Space Station is essentially a sealed bubble of breathable air surrounded by a lethal environment. Its life support system has three core components: water recovery, air revitalization, and oxygen generation. The water recovery system reclaims about 90 percent of all water on board, recycling everything from cabin humidity to crew urine. That reclaimed water passes through filtration beds and a chemical purifier before anyone drinks it.
Oxygen comes from splitting water molecules apart using electricity, a process called electrolysis that yields breathable oxygen and hydrogen gas. The hydrogen then combines with the carbon dioxide that the crew exhales inside a reactor, producing methane (which gets vented into space) and more water that feeds back into the cycle. It’s a remarkably closed loop, though resupply missions still deliver backup water, food, and spare parts.
The Cost of Getting to Orbit
Launch costs have dropped dramatically over the past two decades, largely because of reusable rockets. A SpaceX Falcon 9 carries about 22,800 kilograms to low Earth orbit for around $67 million, which works out to roughly $2,940 per kilogram. The larger Falcon Heavy brings that down to about $1,520 per kilogram. For comparison, the Space Shuttle era saw costs closer to $54,000 per kilogram. SpaceX’s Starship, still in development, aims to eventually push the price below $100 per kilogram, though that target remains unproven.
Commercial Space Tourism
Space is no longer reserved for government astronauts. Several companies now sell seats on suborbital and orbital flights, though prices vary wildly depending on how high you go and how long you stay. Virgin Galactic offers a two-hour suborbital flight reaching about 80 kilometers altitude for $250,000. Blue Origin’s New Shepard capsule crosses the 100-kilometer Kármán line (the internationally recognized boundary of space) during a 12-minute flight that costs approximately $300,000. For a gentler experience, Space Perspective sells six-hour balloon rides to 32 kilometers for $125,000, floating passengers to the edge of space without rocket engines.
Orbital tourism costs far more. SpaceX has flown private crews to the ISS and on free-flying missions in its Dragon capsule, with reported per-seat prices in the tens of millions of dollars. These flights last several days and involve genuine orbital spaceflight, not just a brief hop above the atmosphere.
What Comes Next
NASA’s Artemis program is the most significant crewed exploration effort currently underway. Artemis II will send astronauts around the Moon, with the Artemis III mission now targeting 2027 to test systems and operational capabilities in low Earth orbit. Artemis IV, planned for 2028, aims to land astronauts on the lunar surface for the first time since Apollo 17 in 1972. The long-term goal is to establish a sustained human presence on and around the Moon as a stepping stone toward crewed Mars missions.
Mars remains the ultimate near-term destination. A one-way trip takes roughly six to nine months with current propulsion, meaning a round trip with surface time could last two to three years. Solving the radiation exposure problem, carrying enough supplies, and keeping a crew physically and psychologically healthy over that duration are the central challenges. No crewed Mars mission has a firm launch date, but both NASA and SpaceX have outlined plans that target the 2030s.