The demonstration of a new US system to get astronauts into orbit is under way.
The SpaceX company has launched a capsule from the Kennedy Space Center in Florida designed to carry people.
The mission is uncrewed for this flight, but if it goes well the American space agency is likely to approve the system for regular astronaut use from later this year.
Not since the retirement of the shuttles in 2011 has the US been able to put humans in orbit.
It’s had to pay to use Russian Soyuz vehicles instead.
SpaceX’s Falcon 9 rocket and Dragon crew capsule lifted off from Kennedy’s
historic Pad 39A at the precise planned time of 02:49 EST (07:49 GMT).
The 11-minute ascent will put the Dragon on a path to rendezvous with the International Space Station on Sunday.
Who is this character Ripley?
Because this is just a demonstration, there are no astronauts aboard – but there is a “test dummy”.
Dressed in a spacesuit and sitting next to a window, this anthropomorphic simulator is fitted with sensors around the head, neck, and spine.
It will gather data on the type of forces that humans will experience when they get to ride in the spacecraft.
SpaceX has nicknamed the dummy “Ripley” – after the Sigourney Weaver character in the Alien movies.
For the California company, this flight is a key milestone in its 17-year history. It was set up by entrepreneur Elon Musk with the specific intention of taking people beyond Earth.
“Human spaceflight is basically the core mission of SpaceX,” explained Hans Koenigsmann, the company’s vice president of build and flight reliability.
“There is nothing more important for us than this endeavour, and we really appreciate the opportunity from Nasa to do this.”
The Dragon crew capsule is a variant on the ISS cargo freighter flown by SpaceX.
Upgrades include life-support systems, obviously; and more powerful thrusters to push the vessel to safety if something goes wrong with a rocket during an ascent to orbit.
It also has four parachutes instead of the freighter’s three to control the return to Earth.
Dragon crew capsules will splashdown in the Atlantic not far from Kennedy.
How has Nasa changed since the shuttle?
The American space agency is essentially now contracting out crew transport to SpaceX.
Whereas in the past, Nasa engineers would have top-down control of all aspects of vehicle design and the agency would own and operate the hardware – the relationship with industry has been put on a completely new footing.
Today, Nasa sets broad requirements and industry is given plenty of latitude in how it meets those demands.
Agency officials still check off every step, but the approach is regarded as faster, more efficient and less costly.
“I fully expect we’re going to learn something on this flight,” said Bill Gerstenmaier, Nasa’s head of human spaceflight.
“I guarantee that not everything will work exactly right. That’s cool. That’s exactly what we want to do.
“We want to maximise our learning so we can get this stuff ready so that when we put crew on, we’re ready to go do a real crew mission, and it’ll be the right safety for our crews.”
How should this mission play out?
After being taken to orbit, the Dragon makes its own way to the station using onboard thrusters.
One of the big differences between this mission and standard cargo flights is the mode of approach and attachment to the ISS. Freighters come up under the orbiting lab and are grappled by a robotic arm and pulled into a berthing position.
On this occasion, we will see the crew version of Dragon approach the station at the bow and dock automatically, using a new design of connection ring. Arrival is set for 11:00 GMT on Sunday.
ISS astronauts will be watching closely to see that the capsule behaves as it should.
The Dragon is expected to stay at the station until Friday. The current plan has it undocking, firing its thrusters to come out of orbit, and splashing down at roughly 13:45 GMT.
Kirk Shireman, the manager of Nasa’s International Space Station programme, said: “You’ll hear us talk about this being a flight test; it absolutely is. Although, we view it also as a real mission, a very critical mission.
“The ISS still has three people onboard so this mission coming up to the ISS for the first time has to work; it has to work.”
Nasa is also working with Boeing on crew transport. The company has developed a capsule of its own called the Starliner. This will have its equivalent demo flight in the next couple of months.
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