On the space station, absorbent beds containing a material called zeolite pull carbon dioxide out of the cabin air; oxygen for the station cabin comes from electrolysis: electricity splits water into hydrogen and oxygen. 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.

In microgravity the fluids in the body shift upward toward the head; fluid moving toward the head may put pressure on the eyes and cause vision problems. Weight-bearing bones lose on average 1% to 1.5% of their mineral density per month during spaceflight; on a spaceflight, bone-building cells become less active while bone-breakdown cells keep their usual pace, so breakdown outpaces growth and bones get weaker and more brittle. Astronauts also lose muscle mass in microgravity faster than they would on Earth, and the loss of bone and muscle in space is called atrophy.

A machine called ARED lets astronauts lift weights in space. As of November 2026, crews on the space station exercise about two hours a day, and NASA schedules two and a half hours. Exercise in space puts less stress on the joints than exercise on Earth.

Because food floats even inside an astronaut's stomach, astronauts feel full faster. Fresh plants can give astronauts food, lift their spirits and improve their space home, and to go farther and stay longer in space, crews will need sustainable sources of food.

Part of the radiation risk beyond Earth comes from high-energy galactic cosmic rays, which are remnants from supernovas. NASA says space radiation exposure increases cancer risk, and it is a serious risk for astronauts traveling to Mars. Spacesuits protect astronauts from radiation and from bits of space dust.

Astronauts wear spacesuits for spacewalks, outside the station, because space is airless and dangerous without one. On March 18, 1965, Alexei Leonov became the first person to float in open space; the first American spacewalk, by Ed White, came about three months later.

In the near vacuum of space there is no air to carry heat away, so suits cool astronauts with water in tubes. The flexible parts of a spacewalk suit are made from as many as 16 layers, with about 300 feet of cooling tubes closest to the skin; a spacesuit backpack holds oxygen for astronauts to breathe and removes the carbon dioxide they breathe out. Spacewalk gloves must let astronauts pick up objects, and fingers are the part of the body that gets coldest 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, 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. The Orion spacecraft carries astronauts, and the SLS rocket gives the power to lift it off Earth and send it into deep space.

Mars launch opportunities are limited by the Earth-Mars synodic cycle, which is about 26 months; a round trip to Mars using the usual efficient paths takes an average of 3 to 3.5 years in total, partly waiting at Mars for the planets to realign. As of November 2026, NASA's goal is to send humans to Mars as early as the 2030s.

Radio signals take roughly 3 to 25 minutes to travel between Earth and Mars, depending on where the planets are; nothing can travel faster than light, so a Mars crew's messages will always arrive late. Because help and supplies would be far away, a Mars crew has to plan for self-sufficiency.

Words to know

• zeolite: gas-absorbing mineral
• electrolysis: splitting by electricity
• synodic: repeating cycle
• brittle: easily broken