Live SpaceX Launch: Latest Updates

However, weather forecasts for the afternoon looked grim with rain likely around Kennedy Space Center.

But because people are flying to space, the weather has to be good in two places: at the launch site and in the long swath of ocean — along the East Coast and then across the North Atlantic to nearly Ireland — where the astronauts might splash down in an emergency. A tropical storm, Bertha, formed off South Carolina, which might churn up choppy waves in the Atlantic where the capsule would need to be recovered in an emergency.

If the launch is postponed, SpaceX and NASA can try again on Saturday at 3:22 p.m. and Sunday at 3 p.m., with a 60 percent chance of being able to begin the mission on either of those days.

They both have backgrounds as military test pilots and have each flown twice previously on space shuttle missions, although this is the first time they have worked together on a mission. Mr. Hurley flew on the space shuttle’s final mission in 2011.

In 2015, they were among the astronauts chosen to work with Boeing and SpaceX on the commercial space vehicles that the companies were developing. In 2018, they were assigned to the first SpaceX flight.

Around 1 p.m., Mr. Behnken and Mr. Hurley put on their spacesuits. Mr. Musk and NASA’s administrator, Jim Bridenstine, each wearing surgical masks and standing a socially distanced six feet from the astronauts, shared some final words with the men, who then began their trip to the launchpad.

What are they flying in?

SpaceX has never taken people to space before. Its Crew Dragon is a gumdrop-shaped capsule — an upgraded version of SpaceX’s original Dragon capsule, which has been used many times to carry cargo, but not people, to the space station.

Crew Dragon has space for up to seven people but will have only four seats for NASA missions. If this launch succeeds, it will ferry four astronauts to the space station later in the year.

When will the astronauts arrive at the space station?

The Crew Dragon is scheduled to arrive at the International Space Station 19 hours after launch, at about 11:39 a.m. Eastern time. That time will allow the astronauts to test how the spacecraft flies and verify that the systems are performing as designed. Unless something goes wrong, the Crew Dragon’s computers usually handle all of the maneuvering and docking procedures.

How long will they stay and what will they do?

Originally, Mr. Behnken and Mr. Hurley were scheduled to stay at the space station for only two weeks. But those plans were made when NASA thought the mission would fly in 2019. With delays in the development of Crew Dragon and another capsule, Boeing’s Starliner spacecraft, NASA ran out of available seats aboard Russia’s Soyuz capsule to the space station. It now finds itself short-handed there, with only one NASA astronaut, Christopher J. Cassidy, currently on the station with two Russian counterparts.

Thus, Mr. Behnken and Mr. Hurley are now expected to stay at the station at least a month to help Mr. Cassidy. Mr. Behnken has trained to perform spacewalks, and Mr. Hurley took refresher classes on how to operate the station’s Canadian-built robotic arm.

Why is NASA working with SpaceX on this?

To replace the shuttles, NASA decided to turn to two private companies — SpaceX and Boeing — in essence to produce the rental-car equivalent of spacecraft. NASA would then buy tickets aboard its capsules for the rides to space.

This program has turned out much less expensive than if NASA had developed its own replacement spacecraft, although the capsules have faced many delays on the way to being ready to launch.

NASA under the Trump administration is also hoping to spur more commercial use of the space station, for purposes including tourism. Although the tickets would be expensive, passengers can buy rides to orbit aboard SpaceX’s capsule and may purchase seats on the Boeing capsule once it is ready to fly.

Why did NASA retire the space shuttles?

The decision to retire the space shuttles was made in 2004 during the administration of President George W. Bush after the loss of the Columbia shuttle a year earlier. The shuttles were needed to complete construction of the space station. But their engines, heat tiles and aerodynamics made them complex to fly and maintain. Those factors, and the expense of continuing to operate them, led the Bush administration to decide that the money should be directed instead to sending astronauts back to the moon in a program called Constellation.

The space station was completed in 2011, and the shuttles were retired. The Obama administration, however, decided that Constellation was too expensive and canceled it. It then started the commercial crew program that led to the Crew Dragon and Boeing’s Starliner.