NASA calls the equation linking a rocket's change in velocity to its exhaust velocity and mass ratio the ideal rocket equation. From the ideal rocket equation, 90% of the weight of a rocket going to orbit is propellant weight; big rockets have stages so that they can drop used-up sections and stay as light as possible.

A rocket engine turns its fuel into hot gas and shoots the gas out the back; when a rocket pushes its exhaust out, the exhaust pushes the rocket the other way. 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.

Apollo 11 carried Neil Armstrong, Michael Collins and Buzz Aldrin, and launched on July 16, 1969; on July 20, 1969, Neil Armstrong became the first person to walk on the Moon; Buzz Aldrin joined him. While Armstrong and Aldrin were on the Moon, Michael Collins stayed in orbit in the command module Columbia; Armstrong and Aldrin spent about 21 hours on the Moon's surface, with about two and a half hours of it outside. Neil Armstrong had to manually pilot the Apollo 11 lunar module in the final minutes before landing.

As he stepped onto the Moon, Armstrong said, "That's one small step for man, one giant leap for mankind." An estimated 650 million viewers were watching the Moon landing broadcast; the three Apollo 11 astronauts splashed down in Hawaii on July 24, 1969. Apollo missions brought back more than 800 pounds of Moon rock and soil.

Astronauts wear spacesuits for spacewalks, outside the station, because space is airless and dangerous without one; a lunar spacesuit had to provide a pressurized enclosure, supply oxygen, and protect from solar radiation, large temperature variation, and tiny high-speed meteorites. In the suit's cooling garment, water flows through tubes to keep the astronaut cool; 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. Spacewalk gloves must let astronauts pick up objects, and fingers are the part of the body that gets coldest in space.

On April 12, 1961, Yuri Gagarin became the first person to orbit Earth, in Vostok 1, and his flight around Earth lasted 108 minutes; Valentina Tereshkova became the first woman in space on June 16, 1963. Her Vostok 6 flight lasted 70.8 hours and made 48 orbits of Earth, and she was an experienced skydiver before she flew in space; 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.

Alan Shepard became the first American in space on May 5, 1961, in a 15-minute flight that did not go all the way around Earth; Sally Ride became the first American woman in space on June 18, 1983. She studied physics and English literature at Stanford University; Ride and five other women were chosen in 1978 for NASA's Astronaut Group 8.

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. New NASA astronauts spend about two years learning basic astronaut skills at Johnson Space Center, and astronaut training is long and intensive. As of November 2026, NASA has selected 370 astronaut candidates.

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. NASA says health risks from space radiation are mainly driven by long-term impacts.

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. Researchers are still working out the right combination of diet, exercise and medication to prevent bone and muscle loss in space.

Arriving at Mars safely takes more than launch energy: a spacecraft also needs propulsive energy to slow down so it can stop there; 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. Because help and supplies would be far away, a Mars crew has to plan for self-sufficiency.

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. Artemis II carried NASA astronauts Reid Wiseman, Victor Glover and Christina Koch, and it launched on April 1, 2026, from Kennedy Space Center in Florida.

As of November 2026, Artemis III is planned as a 2027 test flight in low Earth orbit that checks systems needed for future lunar landings. 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.

Words to know

• propellant: rocket fuel
• meteorites: space rocks
• synodic: repeating cycle
• exhaust: hot gas