After being delayed a day by bad weather along the flight route, the Crew-2 spacecraft lifts off from Florida's Kennedy Space Center at 5:49 a.m. Eastern Time (0949 GMT).
SpaceX is scheduled to send its third crew to the International Space Station early Friday (local time), marking the first time a rocket and crew capsule have been reused in a human flight.
After being postponed a day by bad weather along the flight line, the Crew-2 spacecraft lifts off from pad 39A at the Kennedy Space Center in Florida at 5:49 a.m. Eastern Time (0949 GMT).
"Looks like we'll attempt to launch tomorrow!!!" tweeted French astronaut Thomas Pesquet, who will be the first European to ride on a SpaceX Crew Dragon.
Our @Space Station friends are awaiting us to arrive, and we don't want to be late. My bedroom was newly completed, and they practically made my bunk. "What wonderful hosts!"
The extra "bed" is required to fit an extraordinarily large number of people onboard the ISS: 11 in all, including three Russian cosmonauts, as the Crew-2 team overlaps with Crew-1 astronauts for a few days.
Pesquet will be joined by Shane Kimbrough and Megan McArthur from the United States, as well as Akihiko Hoshide from Japan.
On April 28, Crew-1 will splash down off the coast of Florida.
Following the completion of the Space Shuttle programme, the United States relied on Russian rockets for rides to the International Space Station for nine years. The first mission, Demo-2, took place last year and marked the end of nine years of American reliance on Russian rockets for rides to the ISS.
"It's definitely better the third time you do it in terms of making the operations ready," NASA launch integration manager Daniel Forrestel told AFP.
"I will never characterise spaceflight as normal," he said, "but'more familiar' is a fair way to say it."
The Crew-2 mission will reuse the Demo-2 capsule as well as the Falcon 9 booster that was previously used for the uncrewed Demo-1 mission, which is a primary cost-cutting aim of NASA's private-sector collaborations.
Europe has taken a significant move forward.
ESA astronaut Andreas Mogensen of Denmark told AFP ahead of the launch that the flight was also a big move forward for Europe, which has named it "Alpha" after its own naming convention.
"Seeing an astronaut go to the International Space Station, on the one hand, means a lot."
Matthias Maurer of Germany will be the next European on a SpaceX mission in the fall, followed by Samantha Cristoforetti of Italy in the spring.
ESA will also be a central participant in the US's Artemis programme to return to the Moon, supplying the Orion spacecraft with power and propulsion as well as crucial components of a proposed lunar orbital station named Gateway.
During their six-month mission, the Crew-2 team would conduct about 100 experiments.
This involve studies into "tissue chips," which are miniature versions of human organs made up of various types of cells and used to investigate topics like immune system ageing, kidney function, and muscle weakness.
