The third long-duration team of astronauts launched by SpaceX to the International Space Station (ISS) for NASA departed the orbiting outpost early this morning to begin their descent back to Earth, capping a six-month science mission.

The SpaceX Crew Dragon capsule carrying three US NASA astronauts and a German astronaut from the European Space Agency undocked from the ISS shortly after 1am EDT (5am Irish time) to embark on a return flight expected to last less than 24 hours.

Live video showing the capsule moving away from the station was shown on a NASA webcast.

Wearing helmeted white-and-black spacesuits, the four astronauts were seen strapped into the crew cabin shortly before the spacecraft separated from the space station, orbiting some 400km above the Earth.

A few brief rocket thrusts then autonomously pushed the capsule safely clear of the ISS.

If all goes smoothly, the Crew Dragon craft, dubbed Endurance, will parachute into the sea off the coast of Florida in the early hours of tomorrow morning.

The Endurance crew, consisting of American astronauts Tom Marshburn, 61, Raja Chari, 44 and Kayla Barron, 34, along with ESA crewmate Matthias Maurer, 52, arrived at the space station on 11 November last.

Their departure came about a week after they welcomed their replacement team aboard the station, also currently home to three Russian cosmonauts on a long-term mission.

One of those cosmonauts, Oleg Artemyev, assumed command of the ISS from Dr Marshburn in a handover before today's undocking, NASA said.

Earlier in April, a separate all-private astronaut crew launched by SpaceX to the space station under contract for the Houston-based company Axiom Space left the orbiting laboratory, concluding two weeks in orbit.

The NASA-ESA team flying home today was officially designated "Crew 3," the third full-fledged long-duration group of astronauts that SpaceX has launched to the space station for the US space agency.

'Crew 3' pictured ahead of their launch in November

They will be carrying some 550 pounds of cargo with them on their flight back to Earth.

SpaceX, the California-based company founded in 2002 by Elon Musk, the billionaire CEO of electric carmaker Tesla who recently clinched a deal to buy social media platform Twitter, has launched a total of seven human spaceflights over the past two years.