SpaceX's first crewed mission is headed back to Earth. Here's every step that must go perfectly for 2 NASA astronauts to come home safely.

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Two NASA astronauts aboard the space station are about to embark on a fiery fall through the atmosphere and into the ocean.

Bob Behnken and Doug Hurley became the first people to fly SpaceX's brand-new spaceship, the Crew Dragon, on May 30. It was the first crewed launch from American soil since July 2011 and the first-ever launch of a commercial spacecraft with humans inside. The ship docked to the International Space Station the next day, and Behnken and Hurley have since been conducting science experiments and space walks there.

But now comes the hard part: bringing them back to Earth.

Behnken and Hurley must board the Crew Dragon again and hurtle back through the atmosphere — a voyage that will require the spacecraft to weather temperatures up to 3,500 degrees Fahrenheit.

SpaceX CEO Elon Musk has said the fall to Earth is what worries him most about the Demo-2 mission.

Here's how each step of the return trip must play out to bring the astronauts home safely.

The Demo-2 return trip is set to begin on Saturday, when NASA astronauts Bob Behnken and Doug Hurley suit up and climb back into the Crew Dragon capsule, which they've named Endeavour.

Behnken and Hurley wait for launch inside the Crew Dragon spaceship on May 30.

Screenshot/NASA TV

That's only if Hurricane Isaias, which is headed for Florida, doesn't whip up winds and waters where the capsule might land. NASA and SpaceX will watch the weather closely to make sure it's safe for the capsule to splash down in at least two preselected sites.

If the wind is too strong, or the waves are too high, the mission has about 50 days to try again before the capsule begins to deteriorate in the harsh environment of space.

If the Crew Dragon safely brings the men back to Earth, the culmination of the mission will officially kick off a new era of commercial spaceflight.

An illustration showing SpaceX's Crew Dragon spaceship returning to Earth and withstanding intense heat from atmospheric reentry.

SpaceX/YouTube

Behnken and Hurley have been in space since May 30, when a Falcon 9 rocket carried them into Earth's orbit in the Crew Dragon spaceship.

SpaceX's Demo-2 mission, launched with a Falcon 9 rocket, lifts off with Behnken and Hurley inside on May 30.

Tony Gray and Tim Powers/NASA

The spaceship docked to the space station on May 31, and the astronauts then crawled through its hatch to join their colleagues.

Behnken and Hurley float into the International Space Station on May 31 after riding SpaceX's Crew Dragon spaceship to the orbiting laboratory.

NASA

The spaceship is still sitting attached to the ISS. When it's ready to leave, Crew Dragon will retract the hooks that hold it to the station's dock.

SpaceX's Crew Dragon Endeavour spaceship photographed by astronauts Behnken and Chris Cassidy while performing a space walk on July 1.

NASA

The undocking is scheduled for 7:34 p.m. ET on Saturday.

Crew Dragon will then gently fire its thrusters to propel itself away from the orbiting laboratory.

An illustration of Crew Dragon docking with the International Space Station.

SpaceX

Once it's far enough from the ISS, the capsule will fire more aggressively to put itself on the right path to its splashdown location off the Florida coast.

From there, the spaceship will shed its tubelike trunk — a lower section outfitted with fuel tanks, solar panels, and other hardware, which the astronauts will no longer need.

An illustration of SpaceX's Crew Dragon spaceship shedding its trunk before returning to Earth.

SpaceX via YouTube

The trunk should fall into Earth's atmosphere and burn up.

This will expose the capsule's heat shield. After it fires thrusters for another six minutes or so to push it into Earth's atmosphere, the ship will begin to fall.

An illustration of Crew Dragon returning to Earth with a blaze of plasma ahead of its heat shield.

SpaceX via YouTube

The heat shield will deflect and absorb the energy of superheated plasma, enduring temperatures up to 3,500 degrees Fahrenheit. The shield should protect the hardware and astronauts as they plow through Earth's atmosphere at 25 times the speed of sound.

Elon Musk has said this stage of the mission is the part he worries about most, because of the ship's asymmetric design.

Elon Musk on March 9.

Photo by Yasin Ozturk/Anadolu Agency via Getty Images

The shape was necessary for the emergency-escape system, which can jettison the capsule away if a launching rocket fails in midair. Though Musk said the asymmetry was unlikely to cause a problem, he said he worried it could complicate the plunge back to Earth.

"If you rotate too much, then you could potentially catch the plasma in the super Draco escape thruster pods," Musk told Aviation Week's Irene Klotz in May, a few days before the launch. "We've looked at this six ways to Sunday, so it's not that I think this will fail. It's just that I worry a bit that it is asymmetric on the backshell."

Minutes later, the capsule's parachutes must deploy to slow the ship as it falls through thicker parts of the atmosphere.

A view of Crew Dragon returning to Earth after a six-day Demo-1 mission in space. It splashed down in the Atlantic Ocean off the coast of Florida.

NASA TV

The first chute should release at 18,000 feet as Crew Dragon rockets toward the ground at 350 mph. It should slow the capsule's fall to about 119 mph by the time it reaches 6,000 feet, when more parachutes will deploy.

During a press briefing before the mission's launch, Hans Koenigsmann, SpaceX's vice president of mission assurance, was asked what kept him up at night in regard to the mission. He pointed to the parachutes, since their packing can't be tested until they're deployed.

If all goes well, the capsule should splash down in the Atlantic Ocean or the Gulf of Mexico — 22 to 175 nautical miles off the Florida coast — at 2:42 p.m. ET on Sunday.

US Air Force personnel train to rescue astronauts from Crew Dragon.

NASA Langley Research Center/Ron Beard

At that point, the astronauts will be nearly done with a mission that NASA estimated had a 1 in 276 chance of killing them.

Hurley, left, and Behnken work with teams from NASA and SpaceX to rehearse crew extraction from Crew Dragon on August 13, 2019.

Bill Ingalls/NASA/AP

They are well aware of those odds: "I think we're really comfortable with it," Behnken told Business Insider ahead of the launch.

After splashdown, Behnken and Hurley will wait inside the capsule for 30 minutes to an hour, depending on the weather and the state of the spacecraft, as recovery teams in boats approach.

Crew Dragon is loaded onto the SpaceX's recovery ship, Go Searcher, in the Atlantic Ocean about 200 miles off Florida's east coast on March 8.

SpaceX

The recovery teams will retrieve the astronauts and give them a preliminary medical checkout.

Hurley rehearses crew extraction from Crew Dragon on August 13, 2019, at the Trident Basin in Cape Canaveral, Florida.

NASA/Bill Ingalls

A helicopter will then carry Behnken and Hurley to shore. From there, they will take a plane to Houston.

Teams from NASA and SpaceX practice procedures for medical-emergency evacuation on board the Go Searcher ship on August 15, 2019.

Bill Ingalls/NASA

All in all, the return journey should take 21 hours and 27 minutes.

NASA astronauts Bob Behnken and Doug Hurley practicing a full simulation of launch and docking of the Crew Dragon spacecraft on March 19.

SpaceX

After undocking, the spaceship has up to 60 hours before it must land.

Behnken and Hurley will bring a trophy back to Earth with them: the coveted prize in a nine-year game of capture the flag.

Hurley, center, holds up the flag during a press call with Behnken, left, and Cassidy aboard the International Space Station on June 1.

NASA TV

The American flag flew on the first space shuttle and has stayed on the International Space Station since the shuttles stopped launching in 2011, waiting for the first commercial-spaceship crew to claim it.

SpaceX and Boeing have both been developing astronaut-ready spaceships through a public-private partnership program that the Obama administration started. Musk's company got to a crewed mission first.

Hurley and Behnken are the first humans SpaceX rocketed into orbit.

SpaceX; NASA; Business Insider

That means Behnken and Hurley get to bring back the flag.

Musk and NASA officials have been anxiously waiting for the moment the astronauts return to Earth: "I'm not going to celebrate until Bob and Doug are home safely," NASA Administrator Jim Bridenstine said after the Crew Dragon reached orbit on May 30.

Musk celebrates after the launch of the Demo-2 mission.

Steve Nesius/Reuters

Musk has said he feels responsible for the men's lives while they're in his company's spaceship.

"I felt it most strongly when I saw their families just before coming here," Musk told reporters ahead of the mission's launch.

Hurley and Behnken say goodbye to their families and give distant "hugs" on May 27.

NASA TV

He paused for a few seconds and appeared to choke up before continuing: "I said, 'We've done everything we can to make sure your dads come back OK.'"

If all goes well, NASA will use Crew Dragon to ferry astronauts to and from the ISS regularly — a capability that will free the US of its dependence on expensive Russian Soyuz rockets.

A SpaceX Falcon 9 rocket with Crew Dragon attached rolls out of the hangar at NASA Kennedy Space Center's Launch Complex 39A on January 3, 2019.

SpaceX

After Demo-2, NASA has contracted six round trips on Crew Dragon. The first one is scheduled to launch in late September.

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