Astronauts on the space station float because they are in free fall, not because gravity has switched off, since the crew and everything around them are falling together; an orbiting spacecraft keeps falling toward Earth but never lands, because it moves sideways at just the speed that makes the curve of its fall match the curve of Earth.
At the station's height, Earth's gravity is only about 10 percent weaker than on the ground; the station circles Earth at about 28,800 kilometers an hour, 400 kilometers up, and one circuit takes about 92 minutes. The crew sees 16 sunrises and 16 sunsets every day.
On 4 October 1957 the Soviet Union launched Sputnik, the world's first artificial satellite, which NASA marks as the start of the Space Age; Sputnik weighed 184 pounds.
On 12 April 1961 Yuri Gagarin became the first person to orbit Earth, in Vostok 1, and his single orbit took only 108 minutes from ignition to landing. Alan Shepard became the first American in space on 5 May 1961. His flight was suborbital: a 15-minute flight reaching about 490 kilometers, not a trip around Earth.
A rocket engine turns its fuel into hot gas and shoots the gas out the back; when the rocket pushes its exhaust out, the exhaust pushes the rocket the other way, which is Newton's third law. A rocket engine carries everything it needs to burn its fuel, so it can work in space where there is no air. A jet engine needs oxygen from the air to work; big rockets have stages so that they can drop used-up sections and stay as light as possible.
The International Space Station is a home and laboratory where astronauts live and work while it orbits Earth. As of November 2026, it is operated by an international partnership of five space agencies from 15 countries. The principal partners are the space agencies of the United States, Russia, Europe, Japan and Canada. The station is 356 feet (109 meters) long from end to end.
As of November 2026, the station weighs about 925,000 pounds (about 420,000 kilograms). As of November 2026, people have lived on the station without a break since November 2000. As of November 2026, more than 290 people from 26 countries have visited it. As of November 2026, seven people are living on the station (Expedition 75).
Space makes astronauts weaker, so they exercise to keep their muscles and bones strong. Without gravity pressing down on them, astronauts' bones lose about 1 to 1.5 percent of their mineral density each month. They also lose muscle mass in space faster than they would on Earth. The loss of bone and muscle in space is called atrophy; crew members on the station are scheduled for about two and a half hours of exercise every day, and a machine called ARED lets them lift weights in space.
As of November 2026, NASA's Artemis program aims to build a place where astronauts will live, work and explore near the Moon's South Pole. As of November 2026, Artemis II carried astronauts on a nearly 10-day trip around the Moon and back to Earth in April 2026. It launched on April 1, 2026, from Kennedy Space Center in Florida. The Orion spacecraft carries astronauts, and the SLS rocket gives the power to lift it off Earth and send it into deep space.
Mars is, on average, 140 million miles from Earth. As of November 2026, NASA says Mars remains its horizon goal for human exploration. As of November 2026, getting to Mars could take up to 6 to 9 months, and a round trip with a long wait for the planets to line up could take 3 to 3.5 years. Radio signals take roughly 3 to 25 minutes to travel between Earth and Mars, depending on where the planets are.
Words to know
- orbit: path around
- suborbital: short of orbit
- exhaust: hot gas
- atrophy: wasting away
