NASA plans to bring astronauts back to Earth in SpaceX's Crew Dragon spaceship on August 2. The process is Elon Musk's biggest worry.

• After launching two NASA astronauts into orbit in May, SpaceX is now set to bring them back from the International Space Station on August 2 in the same spaceship.
• Elon Musk has said this stage of the mission worries him most.
• SpaceX's Crew Dragon ship will need to undock from the space station, survive a fiery fall through Earth's atmosphere, and deploy parachutes to safely land in the Atlantic Ocean.

Two astronauts aboard the International Space Station are in the middle of a historic mission: the first-ever commercial human spaceflight.

The high-stakes test of SpaceX's human-grade spaceship, the Crew Dragon, sent NASA's Bob Behnken and Doug Hurley to the station atop the company's Falcon 9 rocket on May 30. The spaceship successfully docked to the space station the next day.

But now comes the hard part: bringing the crew 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 this is the part of the mission that worries him most.

"The return is more dangerous in some ways than the ascent, so we don't want to declare victory yet," Musk told reporters after the launch.

The astronauts' return timeline was initially unclear. The spaceship — currently sitting attached to the ISS — has up to 110 days from arrival until a corrosive form of oxygen high above Earth degrades its solar panels.

But NASA set a date for the return on Friday: Administrator Jim Bridenstine tweeted that the mission is targeting August 1 to undock Crew Dragon from the space station, and August 2 to land it in the Atlantic Ocean.

"Weather will drive the actual date," Bridenstine said.

If the mission, called Demo-2, finishes according to plan, it will set the stage for SpaceX to regularly ferry astronauts to and from the ISS. Full end-to-end success would restore human spaceflight in the US for the first time since the Space Shuttle program ended in 2011.

Here's what to expect during the astronauts' return.

The Crew Dragon must survive a fiery fall through Earth's atmosphere

Returning Behnken and Hurley from the space station to a safe landing point in the ocean could take anywhere from six to 30 hours, NASA spokesperson Stephanie Schierholz told Business Insider. The length of the journey depends on when NASA chooses to begin it.

When the time comes, Behnken and Hurley will climb back into the Crew Dragon and the spaceship will retract the hooks that hold it onto the ISS.

Crew Dragon will then gently fire its thrusters to propel itself away from the orbiting laboratory. 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 of Florida's Atlantic coast.

From there, the spaceship will shed its tube-like trunk — a lower section outfitted with fuel tanks, solar panels, and other hardware that the astronauts will no longer need. The trunk should fall into Earth's atmosphere and burn up.

This will expose the capsule's heat shield. After another six minutes or so of firing thrusters to push it into Earth's atmosphere, the ship will begin to fall. Its heat shield will deflect and absorb the energy of superheated plasma, protecting the hardware and astronauts as they plow through Earth's atmosphere at 25 times the speed of sound.

"The part that I would worry most about would be reentry," Musk told Aviation Week's Irene Klotz in May, a few days before the launch.

Musk added that his "biggest concern" about the new spaceship is the capsule's asymmetric design. The shape was necessary for the emergency escape system, which can jettison the capsule away from the launching rocket if it fails in mid-air. Though Musk said the asymmetry is unlikely to cause a problem, he worries 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 said, adding this could overheat parts of the ship or cause it to lose control due to wobbling. "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."

Catching the spaceship's fall is its own challenge

Assuming the Crew Dragon survives the fall through Earth's upper atmosphere, the capsule's parachutes must deploy just minutes later to slow the ship as it falls through thicker parts of the atmosphere. 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.

"The parachutes are new. Will the parachutes deploy correctly? And then will the system guide Dragon 2 to the right location and splash down safely?" Musk said in March 2019, after the company's Demo-1 mission — an uncrewed test flight of its spaceship — lifted off. The parachutes worked well at the end of that mission, but they failed a test just one month later.

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

If all goes well, however, the capsule should splash down in the ocean 22 to 175 nautical miles off the Florida coast.

Behnken and Hurley will then 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. The teams will retrieve the astronauts and give them a preliminary medical checkout.

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

At that point, the astronauts will have safely completed a mission with a 1-in-276 chance of killing them.

"I'm breathing a sigh of relief," Bridenstine said after the Crew Dragon reached orbit on May 30. "But I will also tell you I'm not going to celebrate until Bob and Doug are home safely."

Dave Mosher contributed reporting.