The men have been on board the International Space Station for two months, after launching from Kennedy Space Center in Florida in a SpaceX Crew Dragon capsule.
Their journey began with an historic May launch that marked the first crewed mission to take off from US soil in nearly a decade, and it could be the first of many if the capsule safely splashes down off the coast of Florida this weekend.
On Saturday, Hurley and Behnken are scheduled to board their Crew Dragon spacecraft, nicknamed Endeavour, and leave the space station around 7:30 pm ET to begin their 19-hour journey home. Splashdown is scheduled for 2:42 pm ET on Sunday.
But all eyes are on the weather: Hurricane Isaias is headed for Florida, possibly causing winds and high waves at all of the possible Crew Dragon landing sites.
As of Thursday night, NASA said it was still planning to move forward with splashdown, but "teams will continue to monitor weather before undocking Saturday night," the space agency said in a tweet.
A safe homecoming is crucial. Though SpaceX previously launched a Crew Dragon on an uncrewed demonstration mission, Hurley and Behnken's mission is still considered a test. Both men are veteran NASA astronauts and test pilots specifically trained to respond to any technical issues that may arise on the new vehicle, and NASA won't officially certify Crew Dragon as a human-rated spacecraft until it makes a safe return.
And the return trip is, in some respects, an even riskier journey than the launch. Crew Dragon will need to slice back through the Earth's atmosphere 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, according to NASA.
Behnken described his experience reentering the atmosphere on previous NASA missions last year: "You actually see the light from the atmosphere as it heats up the external portions of the spacecraft. You see some orange lights flickering the plasma as it kinda goes past the windows," he said. "The vehicle's going through something pretty severe — and we'll be hoping it takes care of us as it takes us through entry."
Then, as the Crew Dragon approaches Earth, it'll deploy a small set of parachutes, called "drogue parachutes," to begin slowing its descent before a large plume of four parachutes fans out to slow the vehicle down even further. If all goes well, Crew Dragon will be traveling less than 20 miles per hour when it hits the water.
"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."
The astronauts will experience much higher G-forces on the Crew Dragon, Hurley said. And it will mark the first time astronauts have landed in water since 1
