Stranded US astronauts to leave space station early

A SpaceX capsule due to bring home two US astronauts stranded on the International Space Station (ISS) since June is to return to earth earlier than anticipated.

US space agency NASA says the capsule will set off on Monday night, Eastern Daylight Time (Tuesday afternoon AEDT), after reaching the space outpost in the early hours of Sunday EDT.

Hatch closure preparations to leave the ISS will begin at 10.45pm on Monday (1.45pm AEDT Tuesday) and the capsule will splash down off the coast of Florida about 20 hours later, NASA said.

The space agency had initially said the capsule would not depart the ISS before Wednesday.

"The updated return target continues to allow the space station crew members time to complete handover duties while providing operational flexibility ahead of less favorable weather conditions expected for later in the week," NASA said in a statement.

The SpaceX Dragon spacecraft brought NASA astronauts Anne McClain and Nichole Ayers, Japan Aerospace Exploration Agency astronaut Takuya Onishi and Roscosmos cosmonaut Kirill Peskov to the ISS.

Its return journey is set to bring US astronauts Barry Wilmore and Sunita Williams, along with Roscosmos cosmonaut Aleksandr Gorbunov, back to earth.

Williams and Wilmore arrived at the ISS at the beginning of June on the first manned test flight of Boeing's Starliner spacecraft. They were only meant to stay in space for a week, but numerous technical problems with the craft left them stranded.

The Boeing-produced Starliner is a partially reusable spacecraft that consists of a capsule around three metres high for the crew and a service module.

Unlike the Crew Dragon vehicle built by Elon Musk's SpaceX company, the Starliner does not land on water but on dry land.