Nasa SpaceX launch: Who are the astronauts? By Paul Rincon

Doug Hurley and Bob Behnken have broken a nine-year hiatus for Nasa, becoming the first astronauts to launch from US soil since the retirement of the space shuttle in 2011.

In the intervening years, Nasa bought seats for its astronauts - at a cost of tens of millions of dollars per flight - on the Russian Soyuz.

But officials have also worked with Elon Musk's company SpaceX and aerospace giant Boeing to develop new, American spacecraft capable of ferrying humans to and from the ISS - under the space agency's Commercial Crew Program.

Musk's vehicle was first to fly; Hurley and Behnken travelled to the ISS in the sleek Crew Dragon spacecraft.

"It's well past time to be launching an American rocket from the Florida coast to the International Space Station and I am certainly honoured to be a part of it," Hurley, 53, said at the beginning of May, before the flight.

Behnken, 49, added: "On my first flight... I didn't have a son, so I'm really excited to share the mission with him."

Nasa chose two of its most experienced astronauts to help California-based SpaceX ready the Crew Dragon for launch. The two are also longstanding friends.

"Being lucky enough to fly with your best friend... I think there's a lot of people who wish they could do that," says Hurley.

When they launched atop a Falcon 9 rocket, their spouses knew exactly what they were going through. That's because they're astronauts too.

Marine Colonel Hurley's wife Karen Nyberg flew into space twice - aboard the shuttle and the Soyuz - retiring from Nasa this year. They have a 10-year-old son, Jack, whose formative years were shaped by space travel.

Nyberg began training for a six-month space station mission just a few months after Jack's birth. In the meantime, Hurley was preparing for his own flight - piloting the last ever shuttle mission. Sometimes, Nyberg took Jack to Russia, at other times he stayed at home in Texas.

"Literally from the time Jack was old enough to comprehend things, he was either going to Russia or Skyping with mommy. That's just the way it was," Hurley told the Houston Chronicle in 2013.

Air Force Colonel Behnken is married to Megan McArthur, who flew on the last mission to service the Hubble Space Telescope in 2009.

As an active member of the astronaut corps, she is a potential candidate to be the first woman on the Moon when Nasa returns in 2024. Their son, Theo, is six.

Hurley, Behnken, Nyberg and McArthur all graduated from the same astronaut class (2000) and attended each other's weddings. The men are so used to each other's company, they now have a spooky rapport.

They "can predict - almost by body language - what the person's opinion is or what their next action is going to be," Behnken told CNN before the launch. "We've just been doing this so long that it's kind of like having a second set of hands."

Hurley says of Behnken: "I know instantaneously when I've not done something correctly - just put it that way. He doesn't have a good poker face."

But Behnken admits Hurley is the more organised of the two.

The older of the two men was raised in the hamlet of Apalachin, in upstate New York. "It was just a great small town existence... we didn't get a stoplight until I was, I think, in college," Hurley said in 2009.

Behnken hails from St Ann, a suburb of St Louis, Missouri. In 2010, he described it as "a blue collar kind of a neighbourhood", adding: "I guess, in my bag of tricks... I'm more of a working class sort of a person."

He took jobs in construction before deciding that working outdoors in the summer heat wasn't for him.

Both men went to college on military scholarships and gained undergraduate degrees in engineering. While Behnken went on to complete a PhD at Caltech - the elite institute featured in the Big Bang Theory television show - Hurley became an officer in the Marines.

They both subsequently trained as military test pilots - at different training schools. It's been the archetypal background for Nasa's astronauts since the days of its first intake - the Mercury Seven.

"When I showed up [at Nasa] it was like: 'Well, I have a PhD and an MD,' and I'm sitting there going, 'Mmm, wow! Maybe I was a bit of a slacker'," said Hurley.

"But you know, your professional development as a pilot and the thousands of hours you get as a pilot... brings something to the table."

Hurley and Behnken were selected as astronaut candidates three years before the shuttle Columbia broke up on re-entry, killing seven crew members. After the disaster, Nasa decided it would retire the shuttle, handing over space station transport to private firms.

Thus, when the two men were finally assigned their flights, the shuttle programme was in its final phase. Their missions focused on fulfilling Nasa's prior commitments to complete construction of the ISS, including the delivery of crew compartments designed to fit in the shuttle's payload bay.

When the spaceplane was retired, the pair were assigned to the Commercial Crew Program. In August 2018, Hurley and Behnken were announced as the prime crew for Demo-2, the first flight of SpaceX's vehicle with humans onboard.

"Bob and I, the last two years, have essentially been living in California, working hand-in-hand with the folks at SpaceX to get us to this point," Hurley said this month.

They've had to get used to the Crew Dragon's touchscreen controls after previously working with the chunky buttons on shuttle instrument panels.

Hurley says a background in evaluating military aircraft as test pilots proved crucial in their work with SpaceX.

"That, in and of itself, helped both of us tremendously, because all along the process that you see in the military, there are delays, there are technical challenges, there are things that you don't expect and you have to work through them," he explained.

Setbacks - including two spectacular explosions that destroyed a rocket and one of the Crew Dragon capsules - have seen the mission slip by nearly four years from its original date of October 2016.

"We were well-prepared for that part of it, when I think it caused some frustration within Nasa as launch dates weren't made," explained Hurley.

Despite bumps in the road, Behnken's enthusiasm remained undimmed: "It's probably the dream of every test pilot school student to have the opportunity to fly on a brand new spaceship."

Hurley told CNN: "From a first flight standpoint, certainly, there might be some greater quantifiable risk to some degree.

"But probably no different to any other spaceflight we've flown humans on before."