Saudi Arabia is hoping to make its mark in space exploration tonight, after it sends two of its citizens to the International Space Station for an eight-day science mission.
Rayyanah Barnawi and Ali Al Qarni are set to become the first Saudi citizens in space in nearly 40 years, when the kingdom launched Prince Sultan bin Salman on Nasa's Space Shuttle.
They are launching as part of a private mission, called Axiom 2, alongside Americans Peggy Whitson, the commander of the trip, and pilot John Shoffner.
A SpaceX Falcon 9 rocket will carry the crew of four to space at 1.37am GST from the Kennedy Space Centre in Florida, with arrival at the orbiting outpost expected 16 hours later, at about 6pm.
They will join the UAE's Sultan Al Neyadi, who is currently on the ISS for a six-month mission - the Arab world's first long duration space mission.
Saudi Arabia is also hoping to make history tonight, with Ms Barnawi as the first Arab woman to go on an orbital mission.
How to watch
Nasa, SpaceX and Axiom Space, the Houston company that has helped arrange the trip, will broadcast the launch live on its website and social media channels.
Nasa's live stream will begin at 12.30am.
Live scenes are expected to show the suited-up astronauts getting into their Teslas and making their way to the launch pad.
It is customary for astronauts launching on SpaceX rockets to ride Teslas, as both companies are owned by billionaire Elon Musk.
The live stream will also show the crew arriving at the launch pad, walking across the launch tower and boarding the Dragon spacecraft, placed on top of the Falcon 9 rocket.
Lift-off will take place once all checks have been completed and Nasa and SpaceX have given the final 'go'.
The crew's docking and arrival into the space station will also be broadcasted.
There is a back-up launch date of May 23, around midnight. But if both launch opportunities are missed, the mission will stay grounded for several weeks at least, as Nasa and SpaceX will have to prioritise other missions.
Weather around launch time
There is a 40 per cent chance that the launch will violate weather rules, as skies over the launch site will have cumulonimbus and cumulus clouds, according to the US Space Force 45th Weather Squadron.
If the launch is pushed to the back-up date, weather conditions will be even more unfavourable, with an 80 per cent probability of violation.
Brian Cizek, weather officer at the US Space Force, said in a pre-launch media briefing last night that they are monitoring the weather.
"Overall, weather is looking pretty good for tomorrow (launch day), just a 40 per cent probability of violation again, the main concern being the anvil clouds coming back eastward from those thunderstorms that will be to our west," he said.
"But then the pattern becomes a lot messier as we head into the beginning of the work week.
"So, unfortunately, the probability of violation bumps up to 80 per cent as we had to that first backup day on Monday, early evening,"
SpaceX carried out a successful static fire test, a ground test, of the Falcon 9 rocket last night to determine whether it was launch ready.
Who are the two Saudi astronauts?
Ms Barnawi, 33, is a research laboratory specialist, with nine years of experience in cancer stem cell research. She has a master's degree in biomedical science from Alfaisal University.
Mr Al Qarni, 31, is an air force captain with 2,387 flying hours logged. He has 12 years of experience on fighter aircraft. He has a bachelor's degree in aeronautical sciences from the King Faisal Air Academy.
The duo have been assigned 14 experiments by Saudi researchers, including one that will test cloud-seeding techniques in a reaction chamber.
Another experiment is for the King Faisal Specialist Hospital and Research Centre.
It involves studying the inflammatory response of human immune cells in microgravity.
They will investigate changes in the mRNA – a genetic material that tells the body how to make proteins.
