Who are the NASA SpaceX astronauts in tonight's historic rocket launch

Two brave NASA astronauts on track to make world history tonight are such close friends that they finish each other's sentences.

If tonight's rocket launch goes ahead as planned, NASA veterans Doug Hurley, 53, and Bob Behnken, 49, will be the first astronauts to man a private spacecraft to the International Space Station.

A successful mission will bring the world one step closer to space tourism, in a crucial moment for SpaceX founder Elon Musk, NASA, and the US space program.

As they blast off in the Crew Dragon capsule atop a Falcon 9 rocket tonight, the two men will be taking a daring test that could change the course of space travel history.

It's not only the first manned mission for US astronauts since the US Space Shuttle was retired over a decade ago.

It's also SpaceX's first-ever crewed mission in its 18-year-history.

It is a dangerous and unprecedented mission, but these are no novices - and they set off tonight with an unbreakable bond of trust.

Hurley and Behnken were at test pilot school together, graduated together, trained together, and they attended each other's weddings - to their astronaut wives.

In fact, the pair about to blast off into space together are said to be so close they can finish each other's sentences.

Who are the astronauts?

Today's spaceflight will mark the first time humans have been launched from the US since 2011, and NASA has chosen two of its top astronauts to crew the mission.

Doug Hurley and Bob Behnken have been working on the historic Demo-2 launch plan for five years.

Both flew on the Russian Soyuz, after Nasa bought the astronauts seats on the space flight following the retirement of the US Space Shuttle in 2011.

The pair are off to the International Space Station (ISS), and have between them, logged hundreds of hours aboard the satellite orbiting the Earth.

Both are veteran space travellers- and they keep it in the family, with wives who also have had careers as top astronauts.

Both came from simple beginnings, went on to achieve military scholarships and undergraduate degrees in engineering and took the traditional route into the US space program of becoming military test pilots.

They are both acutely aware of the danger of their journey.

The two men were both selected as astronaut candidates just three years before the shuttle Columbia broke up on re-entry, in a tragedy that cost the lives of seven crew members.

So it is fitting that the pair placing their lives in each other's hands and their support crews back at ground control are said to be incredibly close friends.

The two men and their top astronaut wives also all graduated from the same astronaut class in 2000.

The four attended each other's weddings - and their kids are no strangers to waving farewell to their parents soar off on epic space missions.

The astronauts can even finish each other's sentences, according to a CNN profile of the duo earlier this month.

"(We) can predict - almost by body language - what the person's opinion is or what their next action is going to be," Behnken said.

"We've just been doing this so long that it's kind of like having a second set of hands."

Doug Hurley

Colonel Hurley hails from rural upstate New York in the small town of Apalachin, and was just two when the US sent man to the moon for the first time.

In a 2009 NASA profile in which he speaks of his hopes space explorers could someday journey to Mars, he recalls becoming fascinated by space travel by watching TV as young boy.

"Sometimes in between the cartoons they would show these little clips of the Sky Lab missions and these guys running around in circles inside Sky Lab and, you know, as a young kid obviously [you] think, “Wow, that would be pretty neat to get a chance to do.”

The interest in space endured so he became a military pilot and Marine, then focused his ambitions on joining the US space program.

He recalled how he was out hiking in mountains with his dad with only a satellite phone when he received the news he had gotten the call-up in 2000.

Hurley said: "It was just an incredible experience to be out in the wilderness with my dad. It was probably one of the more special moments in my life and something I’ll never forget." 

He went on to wed astronaut Karen Nyberg, who has flown into space twice - once aboard  the US space shuttle and the second time aboard the Soyuz.

The couple have a 10-year-old son together, named Jack.

Nyberg began training for her six-month space station mission just a few months after she gave birth to Jack.

She retired from Nasa just this year.

Jack has spent his early years between Texas and Russia as he followed his mother around before her retirement.

"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.

Tonight he gets to see his dad lift-off too.

Bob Behnken

Air Force Colonel Behnken comes from humble beginnings in suburban St Louis, Missouri, but had his heart firmly set on shooting for the stars.

Before he was an astronaut he followed his dad's foot-steps and worked in construction.

In a 2010 NASA profile, he described his hometown 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."

It took some time for his dreams to be realised, but in a 2010 NASA profile, Behnken recalled his father taking him to see a shuttle making its ferry mission across the US for the first time prior to launching into space.

After a couple summers of hard work in the Missouri heat, he decided to go to university and was able to obtain an engineering degree supported by the US Air Force.

It took him some years before his astronaut ambitions began to really take shape, and he went on to get a PhD at elite institute, Caltech.

He said: "There was a time when I was young, when I was interested in being an astronaut but at that time I was also interested in being a race car driver, a rock star and all the standard jobs that were out there. A fireman, things like that, all those exciting things that kids want to do."

He was inspired by his pilot classmates to enter the astronaut program, and got in.

In 2008, he flew on the Space Shuttle and was able to perform three space-walks.

He recalled: "That first time going out the door as, just the experience and the rush of going off to do it is something that I’ll remember forever."

He would go on to marry fellow astronaut Megan McArthur, who flew on the last mission to service the Hubble Space Telescope in 2009.

McArthur is a potential candidate to be the first woman on the Moon as part of NASA's 2024 lunar mission plan.

The couple have one son, Theo, who is six.

Behnken said he didn't have a child yet when he took his first space flight, and he is excited about sharing the mission with his boy.

[My son] has only seen the pictures and video from before he was born, from my previous missions...I do want him to know that it's a mission that I feel strongly about," Behnken told CNN.

"It's going forward in a way that I think is important for us, as humans — as Americans as well. And I want him to be proud of his father."

Where you can watch their rocket in the UK

The rocket is set to lift-off at around 4.33pm EDT (9.33pm BST) and is set to be visible over the UK skies around 9.45-9.50pm BST.

There is a chance of cancellation due to unfavourable weather conditions across the Atlantic, but US Weather Squadron forecasters appeared optimistic last night.

The astronauts will fly to the ISS, where the Crew Dragon will dock with the station to test whether it’s capable on future missions of remaining connected for up to 210 days.

The duo are feeling confident and ready to go, they told reporters.

"Bob and I have been working on this program for five years, day in and day out," Hurley  told Reuters as he and Behnken arrived at the Kennedy Space Center from Houston last week.

"It's been a marathon in many ways, and that's what you'd expect to develop a human-rated space vehicle that can go to and from the International Space Station."

Kirk Shireman, manager of NASA’s International Space Station Program, said the two astronauts were likely to receive a warm welcome at the IIS.

He said: “I think the on-orbit crew is definitely ready for some company, and very much looking forward to the launch this Wednesday."