Trevor Mahlmann for Ars Technica.

Trevor Mahlmann for Ars Technica.

Trevor Mahlmann for Ars Technica.

Trevor Mahlmann for Ars Technica.

NASA/Joel Kowsky

NASA/Joel Kowsky

NASA/Joel Kowsky

NASA/Joel Kowsky

3:05am ET Saturday Update: SpaceX's Falcon 9 rocket hit its instantaneous launch window, and the booster blasted off at 2:49am ET Saturday morning from Kennedy Space Center in Florida. The rocket's second stage safely delivered the Crew Dragon into orbit, where it will now spend about 24 hours raising its orbit and catching the International Space Station. The spacecraft will undergo tests during that time, but the next major milestone is docking with the station, set for Sunday morning.

As for the Falcon 9 rocket's first stage, it returned to Earth in the middle of the night, and landed on a droneship in the Atlantic Ocean. SpaceX has now recovered 35 of 69 Falcon 9 rockets it has launched—more than half of them.

Ars will have continuing coverage of this historic mission throughout the weekend, and next week.

Original post: It's finally go time. For the first time since 2011 and the space shuttle's retirement, a rocket and spacecraft stand on a launch pad in Florida capable of blasting humans into space. Launch time is set for 2:49am ET (07:49 UTC) Saturday from Kennedy Space Center. NASA and SpaceX have worked toward this goal for nine years. It hasn't always been easy, but now here we are.

This particular Dragon won't carry humans—just a single mannequin named Ripley as an homage to Sigourney Weaver's iconic character in the movie Alien. Ripley will wear a flight suit and be well instrumented in order to determine conditions inside the spacecraft. "The idea is to get an idea of how humans would feel in her place basically," Hans Koenigsmann, the vice president of build and flight reliability at SpaceX, said at a news briefing Thursday.

Critical flight

NASA is taking few chances for this critical flight. Although SpaceX has flown a cargo version of the Dragon spacecraft to the International Space Station since 2012, the crew variant of the spacecraft is entirely made over—from the exterior solar panels to the interior life support system. When Dragon arrives at the space station, which it will on Sunday morning if the launch proceeds as scheduled, crew there will enter the vehicle with full oxygen masks. This is because Dragon's cooling system relies on freon, and there's presently no robust way to check for leaks.

This uncrewed flight must go off smoothly before NASA and SpaceX can proceed to crewed missions and end the space agency's dependence on Russian transportation to the station. So much must go right from start to finish. It will begin about an hour before Saturday morning's launch, when NASA will be closely watching the load-and-go fueling operations of the Falcon 9 rocket, which will occur with astronauts on board during crewed flights.

NASA engineers also want to see how Dragon performs in orbit, how smoothly it docks with the space station, and the condition of the vehicle's interior once the hatch opens. (Hopefully there will be no free freon.) Then, perhaps the most critical phase of the mission will come during the return to Earth, when Dragon re-enters Earth's atmosphere and lands in the Atlantic Ocean, under parachutes. This is presently expected to occur on Friday, March 8, at around 8:45am ET.

"It’s a flight test, but we view it also as a real mission, a really critical mission," said Kirk Shireman, manager of the International Space Station program for NASA, during a briefing last week. "This vehicle has to work. We have a lot of things to be learned, built and tested."

Following this flight, SpaceX must still perform an in-flight abort test to ensure that Dragon can rapidly pull away from the Falcon 9 rocket in case of an emergency during launch. Although, officially, NASA says the first crewed flight of Dragon—carrying astronauts Doug Hurley and Bob Behnken—is possible in July, that is highly unlikely. Even with successful flights this month and of the abort test later this spring, SpaceX and NASA have much work to do to configure the final, crew-ready variant of Dragon. Multiple sources said NASA and SpaceX would be quite happy to get the first crew flight off during fall or early winter, and those sources rated the odds of a 2019 crewed launch at 50 percent or less.

Must see delta-V

This Dragon spacecraft weighs 12 tons, and it will deliver about 200kg of supplies and equipment to the station. NASA says it will return about 150kg of cargo and experiments from the space station, which astronauts in orbit will load during the five days that Dragon is docked to the station.

SpaceX has put cameras inside the vehicle, and we can probably expect to see some amazing views from inside the spacecraft during ascent. For this reason, although the webcast will take place during the wee hours of Saturday morning, it is most definitely worth staying up for. It should begin around 2:00am ET. SpaceX will also attempt a recovery of the rocket's first stage on the Of Course I Still Love You droneship.