NASA astronauts aboard SpaceX capsule heading to a splashdown in the Gulf of Mexico

Now they are coming home.

NASA astronauts Bob Behnken and Doug Hurley boarded their Endeavour spacecraft and undocked from the station at 7:35 p.m. Eastern time Saturday as the space station flew 267 miles above Johannesburg.

“It’s been a great two months and we appreciate all you’ve done to help us prove Dragon for its maiden flight,” Hurley radioed to SpaceX mission control as the Dragon capsule left the station’s immediate vicinity. “We look forward to splashdown tomorrow.”

“Safe travels and have a successful landing. Endeavour’s a great ship. Godspeed,” said NASA astronaut Chris Cassidy, the space station’s commanding officer.

Even though Hurricane Isaias is projected to hit the east coast of Florida just as Dragon would be returning, NASA and SpaceX, which owns and operates the spacecraft, could still proceed with a landing attempt, aiming for a site in the Gulf of Mexico near Pensacola in the Florida Panhandle, where waves are expected to be between one and two feet.

“Not intuitive, but Isaias may actually help make nice weather on landing a few hundred miles west,” Zebulon Scoville, NASA’s flight director, wrote on Twitter on Saturday morning.

It will be the first landing ever by astronauts in the gulf, according to Jonathan McDowell, astronomer at the Harvard-Smithsonian Center for Astrophysics.

The crew undocked from the station on schedule, first retracting cables that supplied power from the space station, then unlatching 12 hooks that had held the two together for more than two months. As it floated away, Endeavour fired small booster engines to push away. Splashdown Sunday is scheduled for 2:48 p.m.

Even without a menacing hurricane, the return journey is a treacherous one. The spacecraft will have to withstand temperatures as high as 3,500 degrees Fahrenheit as it plummets through the atmosphere. A quartet of parachutes will have to slow the 21,200-pound capsule for a soft landing at sea. Then rescue crews will have to quickly recover the vehicle from the Atlantic Ocean or the Gulf of Mexico in what would be the first water landing for United States astronauts since a joint U. S.-Soviet mission in 1975.

If all that weren’t challenging enough, NASA and SpaceX are attempting to bring the crew home in the midst of an unusually active hurricane season. And the possibility of strong winds from Tropical Storm Isaias kicking up an unruly churn has put NASA and SpaceX officials on alert.

But if SpaceX is able to bring Hurley and Behnken home safely in the first test flight with humans on board, it would be the triumphant culmination of years of work and the opening of a new era in human spaceflight in which corporations play a starring role alongside NASA.

Last year, SpaceX successfully completed a test run of the mission without astronauts that went smoothly and paved the way for Hurley and Behnken’s mission. It’s also flown its cargo Dragon spacecraft back to Earth in water landings many times successfully, so it has lots of practice.

Still, no one is ready to celebrate until they are safely home.

“The hardest part was getting us launched, but the most important part is bringing us home,” Behnken said Saturday morning during a farewell ceremony on the space station.

At the ceremony, Hurley and Behnken gathered with their fellow station crewmates, NASA astronaut Chris Cassidy and Russian Cosmonauts Anatoly Ivanishin and Ivan Vagner. Cassidy handed Hurley an American flag that was brought up to the station on the very last space shuttle mission in 2011. Hurley, a member of that flight, now gets to bring it home, marking the restoration of human spaceflight from American soil.

“This flag has spent some time up here, on the order of nine years,” Hurley said. “I’m very proud to return this flag home and see what’s next for it on its journey to the Moon.”

Just after the successful launch of the spacecraft to orbit, Elon Musk, SpaceX’s founder, said “there’s an argument that the return is more dangerous in some ways than the ascent. So we don’t want to declare victory yet. We need to bring them home safely, make sure that we’re doing everything we can to minimize the risk of reentry and return.”

Thinking about the astronauts and their families, he got emotional, unable to speak. “I’m getting choked up. … We’re going to do everything we can to make sure they get home safely.”

SpaceX started putting pictures of Hurley and Behnken on work orders to remind employees that lives were at stake. Recently, they had another reminder. Behnken’s wife, Megan McArthur, also a NASA astronaut, was recently chosen to fly on a SpaceX flight in the spring of 2021. To prepare, she spent a few days this week at the company’s headquarters.

In many ways, returning to Earth is more perilous than escaping it.

Getting to orbit requires an enormous amount of energy. The spacecraft goes from sitting still atop the rocket on the launchpad to chasing the space station at 17,500 mph in a matter of minutes. Coming home requires doing the reverse, shedding all that energy quickly. Friction with the thickening atmosphere will generate an enormous amount of heat that will engulf the spacecraft in a fireball.

“Out the window, it’s all orange, and it’s glowing, and it’s quite a sight,” said Garrett Reisman, a former NASA astronaut who flew two shuttle missions. “But you don’t feel anything. You know you don’t want to be out there because its thousands of degrees, but on the inside it’s pretty cool. It’s very comfortable.”

In mission control on the ground, NASA and SpaceX officials won’t be comfortable. As the fireball envelops the spacecraft, testing the heat shield, communication with the astronauts will be lost. The blackout will last approximately six minutes, but it will feel much longer.

During Apollo 13, the nearly catastrophic mission, the blackout went on for what seemed like forever, said Gerry Griffin, a legendary former flight director at NASA during the Apollo era.

The capcom, the person in mission control communicating with the astronauts, “kept calling Apollo 13. ’This is Houston,’” he recalled. “And nothing. He went on for two minutes. You could hear a pin drop in that control center.”

Unlike the shuttle, which landed on a runway, the Dragon spacecraft is something of a throwback, a capsule that will land in water under parachutes. Parachutes are an old technology but a tricky one, and SpaceX has struggled with their design. Last year, it suffered a failure during a test of the parachute system that ultimately prompted the company to upgrade the design.

The upgraded version uses a stronger material in the lines that run to the canopy and a new stitching intended to handle the loads at deployment.

“Parachutes are way harder than they look,” Musk said in an interview with The Post in the days leading up to the launch. “The Apollo program actually had a real morale issue with the parachutes because they were so damn hard. They had people quitting over how hard the parachutes were. And then you know we almost had people quit at SpaceX over how hard the parachutes were. I mean, they soldiered though, but, man, the parachutes are hard.”

If all goes well, two drogue parachutes will deploy when the spacecraft’s altitude is about 18,000 feet, traveling at some 350 mph. Then, as the drogue chutes slow the capsule to about 119 mph, four main parachutes should deploy at about 6,000 feet.

At a news conference late last year, Musk said the Mark 3 parachutes are “probably 10 times safer” than the Mark 2 version. “In my opinion they are the best parachutes ever. By a lot.”

Since the Apollo era, parachute design has come a long way, especially in the development of lightweight but stronger materials, said Kurt Hempe, the director of space business for Airborne Systems, which designs parachutes for SpaceX as well as several other space companies.

“But testing is absolutely an ordeal,” he said. “One of the big things we do today that they couldn’t do then was create computer models. We can come up with a model and simulate before we go fly. And then we go fly and we compare the data results with the model we developed.”

NASA and SpaceX have picked out seven different landing sites along the east and west coasts of Florida, ranging from approximately 22 nautical miles from shore to 175. Two recovery ships will be ready to speed to the spacecraft once it lands.

Adjusting to Earth’s gravity is always a difficult transition for astronauts, but landing in the water makes it even harder.

“You feel sick and you’re walking like a drunken sailor, if you’re walking at all,” Reisman said. “Couple that with landing in the ocean, bobbing up and down, even in relatively calm water, it’s going to be unpleasant.”

The water landing in itself presents a challenge. Mercury astronaut Gus Grissom nearly drowned after his spacecraft splashed down in 1961 and his Liberty Bell 7 spacecraft sank after the hatch blew early.

That’s why the NASA and SpaceX teams will be holding their breath until they see Hurley and Behnken safely on the deck of the boat.

“We didn’t celebrate anything in the control center until the guys stepped out on the carrier deck,” said Griffin, the Apollo era flight director. “That’s when we lit our cigars.”

In the coronavirus era, however, that won’t be likely. NASA is taking extra precautions to protect workers and the astronauts, including testing people who come in contact with the astronauts. And everyone will be wearing masks.

Read more:

Listen to Moonrise: Our newest podcast tells a tale of nuclear brinkmanship, backroom politics and science fiction.

Living in space: Read stories from 50 astronauts who describe what it’s really like to live in space.