Two Nasa astronauts have returned to Earth in a dramatic, retro-style splashdown, their capsule parachuting into the Gulf of Mexico to complete an unprecedented test flight by Elon Musk's SpaceX company.

It was the first splashdown by US astronauts in 45 years, with the first commercially built and operated spacecraft to carry people to and from orbit. The return clears the way for another SpaceX crew launch as early as next month and possible tourist flights next year.

Test pilots Doug Hurley and Bob Behnken's final descent was slowed by drogue parachutes before they landed in the Gulf of Mexico just before 6.50am (NZT) this morning.

Their Dragon capsule was then hauled on board a recovery vessel and the hatch opened about 8am (NZT).

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Behnken was the first to come out of their capsule, just after 8am (NZT). He was put on a stretcher and taken to a medical area. This was a normal procedure to adjust to gravity.

Behnken gave the thumbs up as he was lifted out into the fresh air for the first time in 64 days.

Before Hurley was lifted out, he commented that the voyage was an incredible feat given current developments in world events.

Hurley was then taken out of the capsule and also gave the thumbs-up signal.

Those back at mission control gave the astronauts a standing ovation.

Behnken praised the team for bringing them home safely before he left the capsule.

The pair will soon be taken by helicopter to the mainland where they will catch another flight to Houston to be reunited with their families.

The SpaceX capsule splashes down in the Gulf of Mexico. Photo / AP
The SpaceX capsule splashes down in the Gulf of Mexico. Photo / AP

The spacecraft went from a screaming orbital speed of 28,000km/h to 560km/h during atmospheric reentry, and finally to 24km/h at splashdown. Peak heating during descent was 1900C. The anticipated top G forces felt by the crew were four to five times the force of Earth's gravity.

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The control room broke into applause at the successful splashdown.

"Welcome back to planet Earth and thanks for flying SpaceX," said Mission Control from SpaceX headquarters.

Hurley replied: "It's an honour and privilege to be part of the SpaceX crew."

The astronauts said they were "feeling good".

A team member climbed on top of the Dragon spacecraft to hook up a rigging for the crew to get out of the vessel.

Dragon was lifted onto the Go Navigator recovery ship about 7.17am (NZT).

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The astronauts had been expected to be out of the craft about 7.30am (NZT) but there was a delay to the hatch being opened due toxic vapour detection.

The recovery team worked to purge the craft near the side hatch. They were initially concerned about a fuel leak as the tanks were near this part of the capsule.

It was the space between the exterior of the capsule and the sealed interior below where the astronauts were seated in the capsule.

The astronauts were asked by engineers to provide an air sample from inside their cabin.

The spacecraft's commander said two readings were perfectly clear and there were no traces of rocket fuel vapour in their sealed section of the capsule.

He added there was "no problem" remaining inside until the purge was complete.

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"Let's just keep everybody safe. There's no reason to rush," he told engineers.

The astronauts had been inside their capsule for more than 19 hours after leaving the International Space Station.

Before the two astronauts exited the capsule a surgeon entered the craft to assess their health.

Live pictures showed the scorched spacecraft bobbing in the water before it was lifted out.

A hydraulic lift on board the recovery vessel hoisted the spacecraft out of the water.

After being rehearsed and carefully choreographed prior to today's operation, the amount of time taken to lift the capsule had halved since the first time.

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Recovery personnel - after checking for toxic vapours - worked alongside Dragon soon after it splashed down. They gathered the four parachutes that helped bring the craft safely to Earth.

As the astronauts plunged to Earth they would have been almost lying on their backs, according to a specialist. Back on Earth, their seat would have been in an upright position.

They would stay strapped in their seats until the recovery process was complete and it was time to get them out of the craft.

An eyewitness on the water said there was a sonic boom when Dragon re-entered the atmosphere.

At 6.30am (NZT), Nasa tweeted the SpaceX Crew Dragon's nosecone had closed in preparation for re-entry and "the spacecraft is on its way toward a splashdown in the Gulf of Mexico off the coast of Pensacola, Florida".

And at 6.32am (NZT), SpaceX's Elon Musk tweeted the somewhat intriguing message: "4 mins to Dragon loss of signal due to atmospheric entry plasma."

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At 6.39am (NZT), SpaceX said it was "T-10 minutes until Crew Dragon's splashdown" and shortly afterwards they had regained communication with the spacecraft following the blackout period during re-entry to the Earth's atmosphere.

Despite Tropical Storm Isaias' surge toward Florida's Atlantic shore, Nasa said the weather looked favourable off the coast of Pensacola on the extreme opposite side of the state.

It was the first splashdown for astronauts in 45 years.

The last time was following the joint US-Soviet mission in 1975 known as Apollo-Soyuz.

Earlier, space station commander Chris Cassidy rang the ship's bell as Dragon pulled away, 267 miles (430 kilometres) above Johannesburg, South Africa.

Within a few minutes, all that could be seen of the capsule was a pair of flashing lights against the black void of space.

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"It's been a great two months, and we appreciate all you've done as a crew to help us prove out Dragon on its maiden flight," Hurley radioed to the space station.

The SpaceX Crew Dragon capsule separates from the International Space Station. Photo / AP
The SpaceX Crew Dragon capsule separates from the International Space Station. Photo / AP

"Safe travels," Cassidy replied, "and have a successful landing." The astronauts' homecoming capped a mission that ended a prolonged launch drought in the US, which has relied on Russian rockets to ferry astronauts to the space station since the end of the shuttle era.

In launching Hurley and Behnken from Nasa's Kennedy Space Center on May 30, SpaceX became the first private company to send people into orbit.

SpaceX support teams are deployed on fast boats from the SpaceX GO Navigator recovery ship ahead of the landing of the SpaceX Crew Dragon Endeavour spacecraft. Photo / AP
SpaceX support teams are deployed on fast boats from the SpaceX GO Navigator recovery ship ahead of the landing of the SpaceX Crew Dragon Endeavour spacecraft. Photo / AP

- AP