SpaceX's launch marks the first time in almost a decade that astronauts from the United States will be sent into space - and the first time ever that a private company has teamed with NASA on a space mission.
Elon Musk's SpaceX has built a spacecraft that will send Robert Behnken and Douglas Hurley to the International Space Station.
Today (Saturday) SpaceX and Nasa will make history as they launch the astronauts inside a capsule from the US making history.
The launch was originally set for Wednesday but was cancelled over fears there would be a lightning strike.
Libby Jackson, human exploration programme manager at the UK Space Agency has said the work done by NASA and SpaceX is a "major milestone for the global space sector."
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Here's what we know - and how you can watch the SpaceX launch Saturday 30th May.
Where can I watch the SpaceX launch live?
The launch was due to take place on Wednesday but was postponed due to the weather conditions.
It was rescheduled to Saturday, 30th May with liftoff at 3:22pm ET - that's 8:22pm UK time. NASA TV will begin coverage at 11am.
The relaunch is being broadcast live on NASA's YouTube channel - with the spacecraft set to take off at 8:22pm UK time.
If you want to see the SpaceX launch in the UK you can also follow Tim Peake's advice. He's said the rocket will be visible if you look south-west.
You can also register on NASA's website. There's more info there too on the mission and a tour of the launchpad.
Just remember the live broadcast takes place on NASA TV on Saturday, 30th May at 8:22pm. Then you can watch up to the docking on Thursday.
What is the purpose of the mission?
NASA says that the purpose of the mission, which has been referred to as Demo-2, is to test the capabilities of SpaceX to safely take astronauts to and from the International Space Station - with Elon Musk's company hoping to eventually get certification from NASA's Commercial Crew Programme.
The mission is the first major step by the carrier, the Crew Dragon, to get certified for more long-term manned missions to space.
What happens after SpaceX's launch?
SpaceX's Falcon 9 rocket will take off from launchpad 39A at the Kennedy Space Centre in Florida. The SpaceX craft will carry the Crew Dragon spacecraft where the astronauts will be strapped in. The journey will take about 10 minutes, after two minutes the rock will separate into first stage and second stage.
The first stage will return SpaceX's landing ship in the Atlantic Ocean off Florida's coast. The second stage will carry on with the Crew Dragon.
Once the capsule is in orbit it will separate from the second stage and travel at about 17kmph to get into position and dock at the space station about 24 hours later.
Who are the astronauts?
Both men, Robert Behnken and Douglas Hurley, are experienced NASA astronauts who have previously travelled to space - Behnken has spent 29 days in space while Hurley was on the final flight of the space shuttle Atlantis in 2011 before it was discontinued
Both men have separate roles - joint operations commander Behnken's duties include the docking and undocking of the capsule, whereas spacecraft commander Hurley is responsible for the launch, landing and recovery of the vehicle.
Once both are in the Space Station they have roles to play.
Mr Behnken and Mr Hurley will test the Crew Dragon's environmental control system, the controls and so on.
They are also monitoring the docking system during their approach to the space station.
The pair will become the Expedition 63 crew and carry out more tests on the Crew Dragon. The main aim is the certification.
When will the astronauts return?
The Demo-2 mission could last between one to four months. The time frame will be decided by when the next crew can head out to the Space Station.
The spacecraft can stay in orbit for at least 210 days.
If you're looking for more to watch, check out our TV guide.