Definition
The International Space Station (ISS) is the largest artificial structure ever built in space, operating as a multinational scientific research outpost. Orbiting roughly 250 miles (400 kilometers) above Earth, crews of international astronauts live and work aboard in shifts. It serves as an orbital workshop running 365 days a year, conducting experiments in materials science, physics, biology, and human health that are impossible under normal Earth gravity.
Why Do Astronauts Float in Space?
When you see astronauts drifting effortlessly through the space station, it is easy to assume they are far away in deep space where gravity no longer exists. In reality, at the station's altitude of 250 miles (400 km), Earth's gravitational pull is still roughly 90 percent as strong as it is on the ground.
So why do astronauts float instead of walking on the floor? The secret lies in continuous free fall. Imagine riding in an elevator when the cable suddenly snaps. Because both the elevator car and you are plunging downward at the exact same rate, your feet lift off the floor, giving you a brief sensation of weightlessness.
The space station is in that exact same state. It is constantly falling toward Earth, but its forward horizontal speed is so high that the planet curves away beneath it just as fast as it drops. Because Earth's surface bends away at the exact rate the station falls, the station never hits the ground, remaining locked in orbital flight. Traces of the upper atmosphere create a tiny amount of drag over time, so the station periodically fires its thrusters to boost itself back into a higher orbit.
Sixteen Sunrises and Sunsets Every Day
To stay in orbit without crashing down to Earth, the space station must travel at an astonishing speed. It races along at roughly 17,500 miles per hour (28,000 kilometers per hour, or nearly 5 miles per second). That is fast enough to travel from New York to Los Angeles in about ten minutes.
At this breakneck pace, the ISS completes one orbit around Earth every 90 minutes. As a result, astronauts on board witness 16 sunrises and 16 sunsets every 24 hours, cycling between day and night roughly every 45 minutes.
Rapidly shifting daylight cycles can easily throw off the human body's circadian rhythm. To maintain their health, the crew runs on a strict schedule synchronized to Coordinated Universal Time (UTC). When bedtime comes, astronauts pull down window shades and dim the cabin lighting, artificially creating a regular night environment.
A Closer Look: The Magic of Microgravity
Scientists describe the space station's environment as microgravity rather than zero gravity. That is because slight residual forces—such as faint atmospheric drag against the hull and vibrations from onboard equipment—mean gravity never truly drops to absolute zero.
Even so, this near-weightless environment unlocks science that cannot be done on Earth. On the ground, gravity pulls heavier substances down while lighter, heated fluids rise in currents (convection). On the ISS, without gravity pulling things down, fluids do not separate into layers by density, and liquid droplets form floating spheres shaped purely by surface tension.
This unique setting allows researchers to synthesize novel high-strength materials and grow larger, more orderly protein crystals than on Earth. Scientists also study how living cells and tissues adapt to microgravity, laying the groundwork for long-duration space exploration to destinations like the Moon and Mars.
Common misconceptions
The space station floats because it is far out in deep space where Earth's gravity is zero.
Earth's gravity at the station's altitude is still around 90% of surface gravity. Astronauts float because both the station and the crew are in continuous free fall toward Earth at the exact same rate.
Where you meet it
On Earth, a candle flame stretches into a teardrop shape due to rising hot air, but on the ISS, it burns as a round, dome-shaped sphere because convection does not occur.
On Earth, shaken oil and water quickly separate into distinct layers, but on the ISS, they remain evenly dispersed as mixed droplets for hours because gravity does not pull the denser liquid down.
In one sentence
The International Space Station is humanity's orbital laboratory, falling endlessly around Earth at hyper-speed to create a unique weightless environment for science.