US astronauts have been flying to the International Space Station (ISS) on Russian Soyuz rockets since the shuttle program ended in 2011 -- a dependence they are keen to break.

"It has been a long road," said Douglas Hurley, who will be one of the astronauts and was also on the last shuttle flight.

He and astronaut Robert Behnken will be the first humans to fly on SpaceX's Crew Dragon capsule, which was tested with a dummy last year.

The Crew Dragon will take off from Kennedy with help from SpaceX's Falcon 9 rocket and dock at the ISS, which is currently housing two Russians and one other American.

"This is an awesome time to be an astronaut, with a new spacecraft," Behnken said during a press conference in Florida.

The two arrived in Florida on a NASA jet after being in quarantine since May 13 in Houston in an effort to protect themselves and those aboard the ISS from the novel coronavirus.

NASA administrator Jim Bridenstine -- who refrained from shaking hands with the pair -- reiterated that it was only the fifth time in history that the United States would launch a new space flight program.

It is the first program to be carried out as a public-private partnership -- with SpaceX producing the Crew Dragon spacecraft and Boeing producing the Starliner.

To limit public spending, NASA financed development of the spacecrafts but has signed contracts with the companies to ensure six round-trip flights to the ISS.

In another difference from the previous programs, the May 27 launch will occur without the usual crowds of spectators due to the COVID-19 pandemic.

NASA is already under pressure from President Donald Trump who has instructed the space agency to return to the moon by 2024, accelerating an already risky undertaking.

The head of NASA's human spaceflight program, Doug Loverro, abruptly resigned Tuesday after only six months on the job, in a move possibly related to procurement of spacecraft for the Artemis lunar mission.