An astronaut expressed her determination as she and three other explorers blasted off in a rocket for last night.

Anne McClain is one of four astronauts deployed to replace Butch Wilmore, 62 and 59-year-old Suni Williams - stuck there since June - in a long-awaited mission. on Friday night at Kennedy Space Centre in Florida, US.

Minutes into the flight, Ms McClain, 45, said: "Spaceflight is tough, but humans are tougher." She referred to the bravery she shares with the three other astronauts, including Ms Ayers, also from US and Japan's Mr Onishi.

Arrival is set for late Saturday night. NASA wants overlap between the two crews so Mr Wilmore and Ms Williams can fill in the newcomers on happenings aboard the orbiting lab. That would put them on course for an undocking next week and a splashdown off the Florida coast, permitting.

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The launch was supposed to happen on Wednesday - but Concerns over a critical hydraulic system were raised just as the four astronauts had been strapped into their seats.

But the quartet is finally on its way now, so it looks likely Mr Wilmore and Ms Williams will finally return home. They expected to be gone just a week or so when they launched from Cape Canaveral in Florida on June 5, but .

NASA and Boeing have been investigating in the months since to try to establish how to best proceed. The saga took a political twist when President and SpaceX’s Elon Musk vowed earlier this year to accelerate the astronauts’ return and blamed the former administration for stalling it.

Retired Navy captains who have lived at the space station before, Mr Wilmore and Ms Williams have repeatedly stressed that they support the decisions made by their NASA bosses since last summer. The two helped keep the station running — fixing a broken toilet, watering plants and conducting experiments — and even went out on a spacewalk together. With nine spacewalks, Ms Williams set a new record for women - the most time spent spacewalking over a career.

The duo's extended stay has been hardest, they said, on their families — Mr Wilmore’s wife and two daughters, and Ms Williams’ husband and mother. Besides reuniting with them, Mr Wilmore, a church elder, is looking forward to getting back to face-to-face ministering and Ms Williams can’t wait to walk her two Labrador retrievers.

Mr Williams said in an interview earlier this week: "We appreciate all the love and support from everybody. This mission has brought a little attention. There’s goods and bads to that. But I think the good part is more and more people have been interested in what we’re doing” with space exploration."