NASA-SpaceX mission: Astronauts prepare to board Crew Dragon from the ISS

(CNN) -- NASA astronauts Robert Behnken and Douglas Hurley are preparing to leave the International Space Station this evening and begin their 19-hour journey home, capping off their historic mission aboard SpaceX's Crew Dragon spacecraft.

Later this afternoon, Behnken and Hurley are expected to climb aboard their Crew Dragon spacecraft, named Dragon Endeavour, which has been docked at the space station since the astronauts arrived in May. If the weather forecast looks acceptable, the capsule will leave the space station around 7:30 pm ET and begin its 19-hour trek to a splashdown off the coast of Florida. If the Crew Dragon is cleared to leave this evening, it's expected to reach its water landing at 2:42 pm ET Sunday.

The biggest question is if the weather will hold.

NASA and SpaceX have been keeping a close eye on Hurricane Isaias, which is expected to approach Florida's east coast this weekend.

That's why NASA is hoping Crew Dragon can land in the Gulf of Mexico, off Florida's western coast, where forecasters are expecting calmer waters.

A NASA spokesperson said Friday that officials are currently aiming to land the capsule near Panama City — one of four potential splashdown sites in the Gulf that were preselected by NASA and SpaceX.

Officials will continue to keep an eye on the forecast leading up to Dragon Endeavour's departure this evening. If all goes well, Hurley and Behnken will strap into Crew Dragon. Immediately after undocking at 7:32 pm ET, the Crew Dragon's engines will light up twice in quick succession to thrust the capsule away from the space station.

Then, Dragon Endeavour will slowly descend from the ISS, which orbits about 250 miles above Earth using brief engine burns to lower its altitude overnight. Crew Dragon's maneuvers will all be executed by on-board computers, and Behnken and Hurley will have plenty of food and water on the vehicle. They'll also have a chance to sleep before splashdown.

By Sunday afternoon, the Crew Dragon will be orbiting just above the thick halo of atmosphere that surrounds Earth. The vehicle will ignite its engine once again as it slices back into the air, traveling at 17,500 miles per hour. Rapid air compression and the friction between the air and spacecraft will heat the outside of the spacecraft to about 3,500 degrees Fahrenheit — a dangerous part of the journey. But a thick heat shield will keep Behnken and Hurley safe inside.

The capsule will then deploy a series of parachutes to slow its descent. By the time it hits the water, it should be traveling less than 20 miles per hour, according to a NASA spokesperson.

"I don't think we're nervous," Hurley said from the space station during an interview with CNN Business' Rachel Crane last month. "We have full confidence that the vehicle will perform just like it's supposed to. That being said, it's a completely different entry profile than what we are used to or had been used to in the Space Shuttle."

CNN will be posting live updates of the final stretch of Hurley and Behnken's journey on Sunday. NASA is also hosting continuous webcast coverage of their return beginning at 5:15 pm ET Saturday.

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