- SpaceX has entered the final stage of its historic first flight of people: NASA astronauts Bob Behnken and Doug Hurley are on their way home.
- Behnken and Hurley boarded the company's new Crew Dragon spaceship on Saturday, undocked from the International Space Station, and flew away.
- The crew is now targeting a landing of their ship, which they named "Endeavour," in the Gulf of Mexico on around 2:41 p.m. ET on Sunday.
- Elon Musk, the founder of SpaceX, has described the landing phase as his "biggest concern" for the mission.
- Visit Business Insider's homepage for more stories.
Bob Behnken and Doug Hurley are on their way home.
The two NASA astronauts departed the International Space Station on Saturday evening, beginning a roughly day-long voyage back to their families on Earth.
Behnken and Hurley left in a vehicle called Crew Dragon "Endeavour," a spaceship designed, built, and operated by SpaceX with about $2.7 billion in government funding. It's SpaceX's second experimental flight of the vehicle, a mission called Demo-2, and the aerospace company's first time putting people on board.
The Demo-2 mission's departure makes SpaceX, founded by Elon Musk in 2002, the first company in NASA's Commercial Crew Program to successfully pull off a visit to the $150 billion, football field-size laboratory.
But Behnken and Hurley's return trip has just begun. Now that they've flown away from the ISS, they must perform a risky descent through Earth's atmosphere — the phase of the flight that Musk said is his "biggest concern."
If the flight successfully lands on Sunday afternoon, it would be the first crewed American spaceship to leave and return to Earth since July 2011, when NASA retired its space shuttle program.
The beginning of the end to an historic 63-day mission
Behnken and Hurley began squirreling supplies, experiments, and personal effects — including an historic American flag — into Endeavour early Saturday morning.
Once they were packed up, they said goodbye to the ISS crew members they worked with for 63 days: NASA astronaut Chris Cassidy, the space station's commander, and Russian cosmonauts Anatoly Ivanishin and Ivan Vagner.
Behnken and Hurley then climbed into Endeavour, donned their spacesuits, closed the hatch, and began preparing to depart. At 7:35 p.m. ET, the spaceship gently shoved off the ISS with a series of small burps of propellant, right on schedule.
"Dragon departing," Hurley said as Endeavour glided away.
"Dragon, SpaceX: Separation confirmed," a mission controller responded on the ground.
The crew then fired departure burns to fly farther away from the ISS, leaving a protective "keep-out sphere" around the space station to begin their journey in earnest.
"Safe travels, and have a successful landing. Endeavour's a great ship. Godspeed," a mission controller told the crew.
Hurley later tweeted his thanks to the space station crew, called Expedition 63.
"It was an honor and privilege to be part of Expedition 63," Hurley said. "Now it's time to finish our DM-2 test flight in order to pave the way for future Dragon crews. Go Endeavour!"
If all goes well — Hurricane Isaias is now unlikely to interfere with current landing plans — the astronauts will splash down in the Gulf of Mexico on Sunday at 2:41 p.m. ET.
To return to Earth, the astronauts must perform a series of critical maneuvers with Endeavour over the next 19 hours.
From 17,5000 mph to splash down
The first key steps for Behnken and Hurley to perform, which are occurring now, include firing the spaceship's thrusters to slip into the correct orbit.
Next they'll shed Endeavour's cylindrical trunk — a heavy piece of hardware that powered and helped navigate the ship in orbit, but is no longer needed. The maneuver is essential to exposing the heat shield of the aerodynamic crew capsule.
After sleeping overnight, Behnken and Hurley will wake up on Sunday and prepare for landing. They'll need to fire Endeavour's built-in thrusters for about 11 minutes to dramatically slow down the capsule, helping it fall from orbit and back to Earth. At this point, the heat shield must absorb blistering 3,500-degree-Fahrenheit heat that's generated by plowing through the planet's atmosphere at about 25 times the speed of sound.
When it's about 18,000 feet above the water, Endeavour should automatically deploy a set of drogue parachutes and slow down to a speed of about 350 mph. Around 6,500 feet, four large main parachutes should pop out and slow Behnken and Hurley's capsule further — to less than 120 mph — before they splash into the ocean.
NASA and SpaceX are hoping to land Endeavour at a site off the coast of Pensacola, Florida, and immediately green them with recovery boats and a helicoptor to whisk them back to land. If weather conditions turn poor near Pensacola, astronauts will try to land at a backup site about 100 miles east, near Panama City.
Behnken and Hurley's is SpaceX's first flight of people and an essential step to proving that Crew Dragon is safe to fly people in the future, including private citizens. NASA and SpaceX plan to finish reviewing data from the experimental mission over the following six weeks — likely resulting in its crucial human-rated certification from the space agency.
In a future mission that NASA recently announced, astronaut Megan McArthur (who is married to Behnken) will ride the same Crew Dragon ship to orbit for a roughly six-month stay.
Watch the ongoing Demo-2 mission live on NASA TV:
This is a developing story.
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