American astronauts Doug Hurley and Bob Behnken have detached their Dragon Endeavor capsule from the space station to begin their return to Earth.
The pair are expected to splash off the coast of Florida just after 2:40 p.m. local time (7:40 p.m. BST) on Sunday.
A successful landing would once again mean that America has a fully functional, fully certified means to put its own people in and out of orbit.
This capacity was lost when the country withdrew its shuttles in 2011.
- Astronauts begin historic mission on a private boat
- What is the SpaceX Crew Dragon?
The US space agency Nasa and its business partner, SpaceX, have chosen a projection location well away from Hurricane Isaias, which appears to follow the east coast of Florida.
The pending recovery vessels are therefore directed to the Gulf of Mexico, to the waters off Pensacola and Panama City, in western Florida.
Mission Controllers follow strict guidelines on permitted wind and wave conditions, and will review the latest forecast before giving a final “go” for re-entry.
When this happens, Hurley and Behnken’s capsule will ignite its thrusters to begin the fall out of orbit.
It’s a high-speed descent, initially at several miles per second, and will see Endeavor heat up to 2000C on its armored underside as it pushes down through the atmosphere.
Two sets of parachutes are programmed to deploy – a stabilization system at about 5,500 m (18,000 feet) altitude when the capsule is still moving at about 560 km / h; then four main falls, 1,800 m high, which should gently drop the vehicle to the surface of the ocean.
As is always the case with a reentry, there will be a few minutes of radio silence while the hot gases (plasma) temporarily envelop the machine.
It’s been 45 years since the last American crewed capsule plunged into the ocean. It was an Apollo vehicle that returned to the Pacific after encountering a Soviet Soyuz vessel above Earth.
Doug Hurley said he read the reports at the time and found astronauts could experience nausea when they rocked in the water while awaiting recovery.
“There are bags if you need them, and we’ll have them on hand,” he told reporters on Friday. “We’ll probably have towels on hand as well. If that were to happen, it certainly wouldn’t be the first time. People who fly in space know that sometimes the climbs can have an effect on your system and sometimes the descents are the same. “
- Who are Doug Hurley and Bob Behnken?
- How does NASA use business partners?
The astronauts launched for the space station in late May. Their ascent on a Falcon-9 rocket, again supplied by SpaceX, ushered in a new era in American spaceflight.
NASA has decided that it will no longer own and operate the low Earth orbit crew transport equipment, preferring instead to purchase this service from business partners.
California-based SpaceX is the primary supplier. Much of its material, including parts of the Falcon rocket, is reusable.
This approach has cut costs, said NASA administrator Jim Bridenstine.
“We basically established the high-level criteria, the requirements, in terms of payload and safety, but we didn’t get involved in designing everything downstream. We let private companies go and innovate. That eventually got us to a point where we “We are now reusing these rockets, reusing the capsules, and of course we want to apply that to what we’re doing with the Moon and eventually with Mars,” explained the head of the agency.
The Boeing company is also developing a “taxi service” to the space station, but had to delay its introduction after encountering software problems on its Starliner capsule.
Assuming this demo mission is successfully completed with a notebook splashdown, NASA will move forward with routine, “operational” SpaceX flights, possibly by the end of September.
Endeavor will go for the renovation with the hope that it will be put back on a rocket next year.
The crew of this flight will include, by chance, astronaut Megan McArthur, wife of Bob Behnken.
The husband said he would have some advice on the best place to pack personal items in the capsule.
“Like any trip, if you pack your bags properly, it can be fun,” he joked.
“But if you tuck away in the back of the big van that you take on vacation, and you have to take it all out one item at a time at different times, it can be tiring and eaten away at your fun.
Hurley and Behnken bring back a commemorative American flag that was left on the space station by the crew of the last shuttle mission (which included Doug Hurley). The Stars and Stripes also flew on the very first shuttle mission in 1981. It is expected to return to space when America brings astronauts back to the Moon later this decade.
and follow me on Twitter: @BBCAmos