SpaceX's big launch aims to send NASA astronauts to the International Space Station — a 21-year-old laboratory orbiting 250 miles above the Earth

• SpaceX is preparing to launch two NASA astronauts on its Crew Dragon spaceship for the first time.
• The launch was scheduled for Wednesday afternoon but delayed until Saturday due to bad weather.
• The two astronauts are headed to the International Space Station (ISS), which has orbited Earth for more than 21 years and hosted people from 19 countries.
• The two astronauts will stay on the ISS for one to four months before returning to Earth on the Crew Dragon.

SpaceX stands to make history this weekend.

If weather, hardware, and other factors cooperate, the company's Crew Dragon spaceship will launch NASA astronauts Bob Behnken and Doug Hurley into space on a mission called Demo-2.

Their destination: the International Space Station (ISS), a floating laboratory that orbits 250 miles above the Earth.

SpaceX had planned the launch for Wednesday afternoon, but bad weather forced mission commanders to postpone it to Saturday.

The mission plan calls for Crew Dragon to reach orbit in 8 to 12 minutes. But catching up to and docking with the ISS — which whizzes around the planet at 17,150 miles per hour — will take another 19 hours or so.

So Behnken and Hurley won't step aboard the ISS until midday ET on Sunday.

The two will then spend between one and four months there, depending on the Crew Dragon's performance during the mission, before returning to Earth in the same spaceship.

NASA's Commercial Crew Program and the ISS

Saturday's launch is part of a years-long NASA effort to restore the US's ability to launch and ferry its own astronauts to the ISS and back. Since 2011, the agency has been buying astronauts seats on Russian Soyuz rockets.

But that has become increasingly expensive — one seat costs upwards of $90 million — and has limited US access to the station. A seat on SpaceX's Crew Dragon, meanwhile, is projected to cost $55 million, according to NASA's inspector general. The Crew Dragon ship can also carry more than twice as many astronauts to the ISS at a time than the Soyuz.

Having unfettered access to the ISS is critical: The space station houses labs in which astronauts can do research that cannot be done on Earth.

That's why NASA began funding SpaceX and its competitor, Boeing, to develop human-ready spacecraft in 2010. The larger project, called the Commercial Crew Program, is three years past its original deadline, but Demo-2 could finally mark the first crewed commercial spaceflight ever.

Once NASA can launch more astronauts into space at a lower cost, it will be able to use the space station's microgravity environment to conduct more science experiments — in pharmaceuticals, materials science, astronomy, medicine, and more.

"The International Space Station is a critical capability for the United States of America. Having access to it is also critical," Jim Bridenstine, NASA's administrator, said during a televised briefing on May 1.

The ISS is more than 20 years old

The first module of the ISS launched into orbit on November 20, 1998, and the first crew of astronauts arrived two years later. Humans have lived onboard the station continuously ever since — it's the longest-lasting human presence in space. To date, 239 astronauts from 19 countries have visited the ISS.

It took astronauts from five space agencies representing 15 countries a decade and more than 30 missions to fully assemble the station. The final piece was added in 2011.

Astronaut's time and research focus aboard the ISS is divvied up based on how much money or equipment respective space agencies contribute.

Although SpaceX's upcoming launch will be its first with a crew, its spaceships have been sending cargo to the space station since 2012. NASA previously awarded SpaceX a multi-billion contract to fly 20 supply missions, which have used a capsule called Dragon 1.

As big as a five-bedroom house

The entire space station weighs almost 1 million pounds, and it's big enough to cover a football field, including the end zones. It circles the Earth every 90 minutes.

Inside, the ISS is as big as a five-bedroom house, according NASA. It has two bathrooms, a gymnasium, and a big bay window. Typically, six astronauts live there at a time, though that number can double during the overlap period between missions as crew members change over.