Watch continuing coverage of SpaceX crewed launch to the International Space Station Darrell Etherington

SpaceX  is once again preparing to make history – the private spaceflight company is set to launch its Crew Dragon Demo-2 mission in collaboration with NASA today. This is the second time they’ve prepared to launch this mission, after an attempt on Wednesday last week was scrubbed due to bad weather. Today’s attempt is set for 3:22 PM EDT (12:22 PM PDT) and preparations, along with the launch itself, will be streamed above starting at 11 AM EDT (8 AM PDT).

The launch will take off from Cape Canaveral in Florida, and once again weather is a concern for today’s launch window. SpaceX  and NASA have an instantaneous launch window today, which means they only have the one shot to take off – if the weather isn’t cooperating at 3:22 PM EDT, they’ll have to re-attempt the launch again, with the next possible window set for tomorrow, Sunday May 31.

This is the first time ever that SpaceX will be launching humans aboard one of its spacecraft – NASA astronauts Bob Behnken and Doug Hurley have the honor of being those first passengers. The mission itself is actually technically still a test, the final demonstration mission in the multi-year development of Crew Dragon, SpaceX’s first human-rated spacecraft. This launch will serve as the proof that Crew Dragon and the Falcon 9 rocket that carries it, is ready for human-rating, after which it will be ready for regular operational service, flying U.S. and allied astronauts to and from the International Space Station (ISS) in low Earth orbit.

That will mean the U.S. once again has domestic human launch capabilities, something it hasn’t been able to claim since it ended the Space Shuttle program in 2011. That’s a big deal for a number of reasons, but primarily because it means that NASA won’t rely on buying berths on Russian Soyuz spacecraft to get to the ISS, which will help it save money and ultimately control its own access to Earth’s orbital lab.

If successful, SpaceX will be the first of NASA’s two Commercial Crew partners to achieve this milestone. The other, Boeing,  is still in the process of working out the kinks of its CST-100 Starliner human crew capsule, which encountered errors during its first uncrewed demonstration mission, resulting in the need to run that launch again sometime later this year, and then, depending on how that goes, fly its first human flight hopefully in 2021. SpaceX, meanwhile, is set to begin operational missions with Crew Dragon later this year, if all goes well with Demo-2.

Provided the launch occurs today, Behnken and Hurley will then spend 19 hours on orbit as they make their way to rendezvous with the Space Station for docking. They’ll then staff the station for a period of between a few weeks and a few months, depending on NASA’s decision regarding their ultimate mission length. That will involve helping with station maintenance and conducting experiments, and then they’ll re-enter Crew Dragon and make the trip back to Earth for an Atlantic Ocean splashdown and recovery once their time at the station is over.