Astronaut candidates need a strong science or technical education (NASA asks for a master's degree in a STEM field) and must be able to pass the astronaut physical. As of November 2026, NASA astronaut applicants must be U.S. citizens and have at least three years of related work experience after their degree. ESA astronauts are chosen only from countries that are ESA Member States and Associated Member States. As of November 2026, NASA has selected 370 astronaut candidates.

New NASA astronauts spend about two years learning basic astronaut skills at Johnson Space Center, and astronaut training is long and intensive. Astronauts practice spacewalks in a huge pool of water, the closest you can get to microgravity on Earth. In water, a neutrally buoyant object neither floats nor sinks, which is how a pool imitates weightlessness. NASA's Neutral Buoyancy Laboratory pool holds 6.2 million gallons of water.

A centrifuge is a spinning machine that presses on astronauts with the same g-forces they feel during launch.

In microgravity the fluids in the body shift upward toward the head. Weight-bearing bones lose on average 1% to 1.5% of their mineral density per month during spaceflight. Astronauts lose muscle mass in microgravity faster than they would on Earth. A station exercise machine called ARED lets astronauts mimic weightlifting in microgravity. As of November 2026, crews on the space station exercise about two hours a day, and NASA schedules two and a half hours.

Pieces of the International Space Station have been assembled in orbit since November 1998. The station's mass is 925,335 pounds (419,725 kilograms). As of November 2026, the station is operated by an international partnership of five space agencies from 15 countries.

The space station cleans and recycles water from sweat and breath, and astronauts drink it. The station recycles about 90% of its water. Water is heavy, and it takes a lot of energy to send it up into space. A space toilet uses moving air, not gravity, to pull urine down into a hose.

The Deep Space Network has three complexes: Goldstone near Barstow in California, one near Madrid in Spain, and one near Canberra in Australia. When one Deep Space Network site loses sight of a spacecraft, another site can pick up the signal. Radio signals take roughly 3 to 25 minutes to travel between Earth and Mars, depending on where the planets are.

The Outer Space Treaty says outer space cannot be claimed by any country: it is not subject to national appropriation by claim of sovereignty. Under the treaty, the Moon and other celestial bodies are to be used exclusively for peaceful purposes.

As of November 2026, NASA says Artemis II launched in April 2026 on a nearly 10-day voyage around the Moon. As of November 2026, NASA says its first crewed Moon landing under Artemis will take place in 2028. As of November 2026, NASA's goal is to send humans to Mars as early as the 2030s.

Words to know

• microgravity: tiny gravity
• centrifuge: spinning machine
• buoyant: able to float
• appropriation: taking as owned