SpaceX is about to launch its longest and most ambitious NASA mission to date. Here's what to expect from liftoff to landing.

SpaceX is about to launch its most important NASA mission yet: On Saturday, Elon Musk's rocket company is slated to send four astronauts to the International Space Station on its Crew Dragon spaceship.

Crew-1, as the flight is called, will be SpaceX's first full-length mission for NASA. It's also the company's second time launching people and the longest-duration human space mission ever launched from US soil. The current record, 84 days, has held since the longest Skylab mission more than 45 years ago.

Once the crew members dock to the ISS, they're expected to stay aboard the floating laboratory for about half a year. 

"We are ready for this launch. We are ready for the six months of work that is waiting for us on board the International Space Station, and we are ready for the return," NASA astronaut Mike Hopkins, who will command the mission, told reporters during a pre-mission news conference.

NASA said Tuesday that after analyzing flight data from SpaceX's Demo-2 mission earlier this year, the company's launch system had finally ticked off the last requirements to be fully certify for routine use by NASA astronauts. That approval was the culmination of nearly a decade of funding and work in NASA's partnership with SpaceX.

Hopkins and his fellow crew members — NASA astronauts Victor Glover and Shannon Walker, as well as Japanese Aerospace Agency astronaut Soichi Noguchi — are scheduled to lift off at 7:49 p.m. ET on Saturday.

Here's how their flight should play out and what to expect at each stage of the mission.