Two NASA astronauts have made the first splashdown return in 45 years after a two-month stay at the International Space Station.

Robert Behnken and Douglas Hurley were strapped inside the Crew Dragon capsule as it hit the Atlantic Ocean on Monday morning (Australian time), marking the first water landing since 1975. 

NASA’s Mission Control dispatched a humorous welcome home to the astronauts: “On behalf of the SpaceX and NASA teams, welcome back to Planet Earth. Thanks for flying SpaceX.” 

The astronauts cheerfully responded, “let them know we’re feeling good”, referring to the flight doctors waiting to check them out.

Tweet from @NASA

Two small boats arrived at the capsule while it was slowly bobbing upright in the water.

The recovery team detected small amounts of nitrogen tetroxide, a potentially toxic propellant, outside the capsule.

SpaceX and NASA officials confirmed the levels of nitrogen tetroxide that were detected did not pose a danger to humans and that the air inside the spacecraft was clean.

Mr Behnken and Mr Hurley are returning to Earth amid the threat of Hurricane Isaias, which is edging closer to their landing destination on the south Florida coast.

Tweet from @NASA

In the morning before their fiery plunge back to Earth, each astronaut got to hear a message from their young sons.

“I’m happy you went into space, but I’m even happier that you’re coming back home,” Jack Hurley, 10, said.

“Wake up! Daddy, wake up!” Theo Behnken, 6, called out. “Don’t worry, you can sleep in tomorrow. Hurry home so we can go get my dog!”

Their departure from the ISS was announced by SpaceX, which tweeted: “Separation confirmed. Dragon performing four departure burns to move away from the Space Station.

NASA then advised the Crew Dragon was on a safe trajectory back to Earth.

The astronauts made history on May 30 when they became the first people to launch into low-Earth orbit on a commercial spacecraft that was built by SpaceX.

Their mission, named Demo-2, also marked the first time NASA launched astronauts from US soil in nine years.

The last time astronauts made an ocean landing was on July 1975 during an Apollo mission.

NASA astronauts Bob Behnken (L) and Doug Hurley. Photo: AAP

Since then, astronauts have always landed on terra firma, using NASA’s Space Shuttle or the Russian space agency’s Soyuz capsules.

If all goes to plan, the splashdown will usher in a new era for NASA, which will have at least one commercial spacecraft ready to launch astronauts into space from US soil.

On entering the Earth’s atmosphere, the Crew Dragon will face scorching temperatures of around 1900C as it deploys parachutes to slow its speed down to around 190km/h, before landing on the ocean.

The splashdown is the final step in the mission designed to test SpaceX’s human spaceflight system – including launch, docking, splashdown, and recovery operations.

-with AAP