NASA and Elon Musk's SpaceX are preparing for a historic crewed rocket launch from Florida today.
It will be the first manned mission to launch with a US craft from American soil in nearly a decade – ending Nasa's reliance on Russia.
Nasa SpaceX launch today – what is it and why is it important?
Nasa currently sends astronauts into space by piggybacking on launches of Russian Soyuz rockets from an air base in Kazakhstan.
The US space agency last fired one of its own astronauts into space in 2011.
Nasa retired its astronaut-carrying space shuttles that year to make way for a new space exploration program aimed at sending man to asteroids and other deep-space targets.
However, multiple delays to its development schedule have left the space agency without a way to carry out manned space flights for years.
Nasa hopes to fill the gap with spacecraft launched by private companies such as SpaceX, owned by Musk, and Blue Origin, run by Amazon boss Jeff Bezos.
Nasa astronauts Robert Behnken and Douglas Hurley will make their way to a SpaceX Falcon 9 rocket at the Kennedy Space Centre on May 27.
The ultimate aim of the mission is to dock a SpaceX craft containing the astronauts on the International Space Station.
They will be ferried to the spacecraft on its launchpad in Florida inside a Tesla Model X electric car sporting the Nasa logo.
That's because billionaire SpaceX boss Elon Musk is also CEO of Tesla.
Hurley and Behnken will take a special elevator up 230ft to a SpaceX Crew Dragon capsule atop the awaiting rocket.
When the countdown hits zero, the rocket will blast into space – carrying astronauts into orbit from US soil for the first time since 2011.
Once in orbit, the Crew Dragon capsule carrying Hurley and Behnken will separate from the rocket booster.
As is customary for SpaceX flights, the booster will turn around and return to Earth so it can be refurbished and used on a future mission.
"Crew Dragon will accelerate its two passengers to approximately 17,000 mph and put it on an intercept course with the International Space Station," Nasa said.
"Once in orbit, the crew and SpaceX mission control will verify the spacecraft is performing as intended by testing the environmental control system, the displays and control system and the maneuvering thrusters, among other things."
About 24 hours after launch, Crew Dragon will be in position to dock with the space station.
It can do this automatically but astronauts have the option to take control themselves if something goes wrong.
"After successfully docking, Behnken and Hurley will be welcomed aboard station and will become members of the Expedition 63 crew," Nasa continued.
"They will perform tests on Crew Dragon in addition to conducting research and other tasks with the space station crew."
The Crew Dragon capsule will remain docked on the ISS until it's needed to take astronauts back to Earth.
Nasa has not yet selected a date for the return flight.
When is the Nasa/SpaceX flight and how can I watch it?
Demo-2, will liftoff from the Kennedy Space Centre in Cape Canaveral, Florida, at 9:33pm BST (4:33 pm ET) on May 27.
A Falcon 9 rocket will blast into space from Launch Complex 39a – the same launchpad used during the historic Apollo 11 Moon landings.
The Air Force's 45th Weather Squadron will analyse the forecast and give either a red or green light roughly four hours and 30 minutes before liftoff.
If the event is cancelled due to bad weather, the mission will be pushed back to May 29.
Demo-2 will be live-streamed for free on NasaTV.
Space fans will also be able to watch the launch via the SpaceX websiteand the firm's official YouTube channel.
We'll also be hosting a stream right here at The Sun.
Could Nasa cancel the launch?
SpaceX crew mission chief Benji Reed has warned that the mission could be cancelled at the last minute.
"I would expect there to be a very high chance of scrub due to the weather," Reed told Click Orlando last week.
"And given the time of year, it wouldn't surprise me as well."
Human spaceflights are far riskier than cargo-only trips, so weather conditions need to be perfect.
Clear skies and low winds are optimal for a successful launch – and even an emergency "mission abort" requires good weather for a safe splashdown in the Atlantic Ocean.
Nasa keeps track of more than 50 locations across the ocean to ensure a splashdown can be safely performed.
SpaceX said on Tuesday that the weather forecast for launch "is 60 per cent favourable."
In other news, a tropical storm grounded a key SpaceX launch twice last week.
Nasa recently unveiled the Tesla car that will be ferrying astronauts to tonight's historic launch.
And, incredible photos of eerie Martian landscapes have been released online by scientists.
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