Here's what to expect - from liftoff on Friday to an ocean splashdown six months later.
SpaceX is launching four astronauts into Earth's orbit on Friday.
The company's Crew Dragon spaceship is the first and only commercial vehicle to carry astronauts into space. It's now a routine part of NASA's human spaceflight program.
Friday's mission, called Crew-2, is the second of six crewed space-station flights that the agency has contracted from SpaceX. NASA officials gave SpaceX the green light for launch on Tuesday after two in-depth reviews of the rocket, spaceship, and launch preparations.
"We've completed thousands and thousands of tests to get to this day, just like we always have in the past and will continue to do," Benji Reed, senior director of SpaceX's Human Spaceflight Programs, said in a Tuesday briefing.
"We want to be paranoid, right?" he added. "We want to make sure that we're going to fly these people safely and be able to bring them home safely when it's time. So we check. We check under every rock and we double check and triple check."
The four Crew-2 astronauts - Shane Kimbrough and Megan McArthur of NASA, Akihiko Hoshide of the Japan Aerospace Exploration Agency, and Thomas Pesquet of the European Space Agency - are set to climb aboard the Crew Dragon capsule early Friday morning, then rocket into space at 5:49 a.m. ET.
The spaceship must reach orbit, dock to the International Space Station (ISS), and stay there for about six months while the astronauts work on the orbiting laboratory. Then it has to bring them back safely.
The astronauts will suit up early Friday morning
Launch day begins early for the Crew-2 astronauts, who will don their SpaceX spacesuits around 2 a.m. ET. Then they'll say goodbye to their families, climb into a pair of custom Teslas, and drive out to Launch Complex 39A.
"We ask ourselves all the time: Would we be willing to fly our families on these vehicles?" Reed said.
They'll strap into a recycled spaceship around 3:15 a.m. ET
Early Friday, the Crew-2 astronauts will ascend the launch tower and climb aboard the Crew Dragon capsule, named Endeavour.
McArthur's husband, Bob Behnken, piloted the first crewed SpaceX flight in that same capsule last year. A demonstration mission called Demo-2 took him and astronaut Doug Hurley to the ISS for three months.
Like her husband, McArthur will pilot Endeavour for her mission.
"I'm going to launch in the same seat. So that is kind of a fun thing that we can share, you know, I can tease him and say, 'Hey, Can you hand over the keys? I'm ready now to go,'" McArthur recently said in a press call.
The rocket is loaded with propellant just before liftoff
Thirty-five minutes before liftoff, at 5:14 a.m. ET, technicians will begin remotely loading the Falcon 9 rocket with kerosene and cryogenically chilled liquid-oxygen propellant.
The rocket booster that will push Crew-2 into space has also been recycled. It's the same one that launched the Crew-1 mission in November.
"Flying on reused vehicles, on flight-proven vehicles, is key towards greater flight reliability and lowering the cost of access to space, which is ultimately what helps us make life multiplanetary," Reed said.
Reusability, he added, is "the holy grail of spaceflight."
Go time is exactly two seconds after 5:49 a.m.
When the countdown clock hits zero - precisely two seconds after 5:49 a.m. ET - the Falcon 9 engines will roar to life, heaving the rocket past the launchpad.
The astronauts will be pressed into their seats for about 10 minutes as the vehicle screams toward space. Then the rocket booster should fall away - to land back on Earth and launch another day - giving the Crew Dragon one final push into Earth's orbit.
Liftoff is instantaneous, meaning it must occur at the precise second at which it's scheduled. Waiting would allow the ultracold propellant to warm up, expand, and boil off - reducing the engines' thrust and inviting risks NASA doesn't want to take with humans on board.
But for liftoff to happen, skies must be clear and winds must be low around the launch site. Friday's launch was originally scheduled for Thursday, but NASA pushed it back due to an unfavorable forecast.
NASA is also monitoring weather conditions for a band of splashdown locations across the Atlantic Ocean. If something goes wrong during launch, the Crew Dragon should automatically jettison away from the Falcon 9 rocket and parachute into the sea. So winds and waves must be gentle at those locations, and SpaceX recovery teams must be ready to speed to wherever the Endeavour capsule lands.
If weather is bad at the launchpad or splashdown spots, NASA and SpaceX will scrub the launch and try again on Monday.
The Crew Dragon will dock to the space station on Saturday
Once the Crew Dragon slips into orbit, it will stay there for nearly 24 hours. The astronauts will likely change out of their spacesuits, eat, get a full night's sleep, have breakfast, organize their belongings, and, eventually, put their spacesuits back on to prepare for arrival at the ISS.
SpaceX and NASA expect the Crew Dragon to perform a series of automated maneuvers to dock to the ISS around 5:10 a.m. on Saturday. The astronauts have to be suited up in case something goes wrong and the Crew Dragon has to prematurely return to Earth.
Crew-1 will return to Earth shortly after Crew-2 arrives
The ISS will be crowded with 11 astronauts for at least four days, as the Crew-2 mission overlaps with Crew-1, the first operational SpaceX mission.
As early as April 28, the Crew-1 astronauts - Shannon Walker, Victor Glover, Mike Hopkins, and Soichi Noguchi - will climb back aboard their own Crew Dragon capsule.
That capsule, called Resilience, will then undock from the ISS, push itself toward Earth, and plummet through the atmosphere. Parachutes should release, allowing the spaceship to drift to a splashdown off the coast of Florida.
A recovery crew will be waiting to retrieve the charred Resilience capsule and carry the astronauts to shore. SpaceX, NASA, and the Coast Guard plan to secure a 10-mile no-boat perimeter around the splashdown site in order to prevent the crowd of dangerously close onlookers that surrounded Demo-2 when that capsule landed.
Crew-2 will stay on the ISS for 6 months
The Crew-2 astronauts will stay on the ISS for about six months, maintaining the station, doing repairs, and conducting scientific research. They'll return to Earth with their own splashdown in autumn.
Crew Dragon missions are the fruit of NASA's Commercial Crew Program, which the Obama administration funded in 2010 to restore the agency's human spaceflight capabilities. Through this program, NASA worked with SpaceX to make the Dragon spaceship and Falcon 9 capable of safely transporting humans. Their missions have restored US human spaceflight for the first time since the Space Shuttle program ended in 2011.
Boeing has also developed a spaceship through the Commercial Crew Program, but it has to re-do an uncrewed test flight to the ISS before it can carry anybody into space.
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