The pair were due to be on the ISS for only eight days, but because of technical issues with the experimental spacecraft they arrived on, they have been there for more than nine months.

The astronauts are due to begin their journey back to Earth later this week. Steve Stich, manager of Nasa’s commercial crew programme said he was delighted at the prospect.

“Butch and Suni have done a great job and we are excited to bring them back,” he said.

Live footage showed the SpaceX Crew Dragon docking with the ISS and opening a hatch. Shortly after 0545 GMT, the astronauts embraced and hugged their counterparts in zero gravity.

The astronauts, along with their ISS workmates, Nasa’s Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov, will be relieved by four astronauts, from Russia, Japan and two from the US.

There will be a two-day handover after which the old crew are due to begin their journey back to Earth. But there could be a small further delay, as they wait for conditions on Earth to be right for a safe re-entry of the returning capsule, according to Dana Weigel, manager, of the ISS programme.

“Weather always has to cooperate, so we’ll take our time over that if it is not favourable,” she told reporters.

Ms Weigel explained that the astronauts had begun getting ready for the handover last week.

“Butch rang a ceremonial bell as Suni handed over command to cosmonaut Alexei Ovchinin,” she said.

The astronauts have consistently said that they have been happy to be on board the space station, with Suni Williams describing it as her “happy place”. But Dr Simeon Barber, of the Open University, told BBC News that there would likely have been a personal cost.

“When you are sent on a work trip that is supposed to last a week, you are not expecting it to take the best part of a year,” he said.

“This extended stay in space will have disrupted family life, things will have happened back home that they will have missed out on, so there will have been a period of upheaval.”

Butch and Suni arrived at the ISS at the beginning of June 2024 to test an experimental spacecraft called Starliner, which was built by the aerospace firm Boeing, a rival to SpaceX.

Nasa decided that it would not take even a small risk in bringing back Butch and Suni on Starliner, when they had the option of returning them on SpaceX’s Dragon capsule. Nasa decided the best option was to do this during a scheduled crew rotation, even though it would mean keeping the astronauts on the space station for several months.

Boeing has consistently argued that it would have been safe to bring Butch and Suni back on Starliner, and were unhappy about the decision to use a rival’s capsule instead, which will be “embarrassing” for Boeing, according to Dr Barber.

“It’s not a good look for Boeing to see astronauts they took into space come back in a competitor’s craft.”

Both President Trump and SpaceX CEO Elon Musk have said that Butch and Suni could have been brought home sooner, most recently in a joint interview with Fox News in February.

President Trump states: “They got left in space.”

When the interviewer, Sean Hannity, elaborates, saying “They were supposed to be there eight days. They’re there almost 300,” Mr Trump responds with one word: “Biden.” Mr Musk follows up asserting: “They were left up there for political reasons.”

The assertion is denied by Nasa’s Steve Stitch.

“We looked at a wide range of options and worked hand-in-hand with SpaceX to look at what was the best thing to do overall and when we laid all that out the best option was to have the one we are embarking upon,” he said.

That decision was supported by Dr Libby Jackson, who is head of space at the Science Museum in London and worked at Europe’s control centre for the ISS.

“Butch and Suni’s wellbeing would always have been at the very forefront of everybody’s minds as the decisions were being made for how best to deal with the circumstances that they were presented,” she said.

“Nasa made those decisions based on good technical reasons, on programmatic reasons, and found the right solution that has kept Butch and Suni safe.

“I really look forward to seeing them return to Earth, safe and sound, along with the rest of their crewmates.”

TheFalcon 9 launch from Florida’s Cape Canaveral Space Force Station was put back due to a hydraulic ground issue. The next possible launch opportunity is on Friday evening.

The rocket aimed to fly four new crew members to the International Space Station (ISS) and pave the way for the return of Suni Williams and Butch Wilmore.

The two Nasa astronauts flew to space in June but were not able to return on a Boeing spacecraft after it was deemed unsafe. The pair should be able to return to Earth within days of the SpaceX mission reaching the ISS.

The Nasa astronauts left earth on 5 June 2024 for an eight-day mission and have been stuck at the ISS for more than nine months, instead helping ongoing missions at the station with maintenance and experiments.

The Wednesday launch would have made it possible for them to return to Earth as early as Sunday.

SpaceX said the issue that forced the launch to scrub related to the hydraulics on one of the clamp arms, which engineers were trying to fix. There were concerns the arm would not have been able to open fully during launch.

The company said the next launch window would be at 1903 Eastern Time (2303GMT) on Friday.

Ms Williams, 58, and Mr Wilmore, 61, have taken their peculiar situation in their stride, saying in a news conference from the ISS in September that they have been trained to “expect the unexpected”.

“This is my happy place,” Ms Williams said at the time, though admitting she missed her family and two dogs.

Mr Wilmore noted the issues with the Starliner spacecraft they arrived on made them not “comfortable” to fly back home on.

He added, however, that 90% of their training was “preparing for the unexpected”.

He said they would stay up there for “eight months, nine months, 10 months” if necessary.

Ms Williams said being in space makes her think more about planet Earth.

“It opens up the door to making you think a bit differently,” she explained. “It’s the one planet we have and we should be taking care of it.”

Ms Williams and fellow astronaut Butch Wilmore were due to return to Earth after a week-long mission in June 2024 but their return has been delayed because of a technical issue.

They now won’t be back until late March or possibly April.

Ms Williams – an experienced astronaut who has conducted many spacewalks during previous stays aboard the ISS – teamed up with astronaut Nick Hague on Thursday to perform maintenance on the craft.

Their tasks include repairing equipment that governs station orientation, patch light filters on the NICER X-ray telescope, and replacing a reflector device on an international docking adapter.

Additionally, the pair will check access areas and connector tools that will be used for future maintenance work on the Alpha Magnetic Spectrometer, a particle physics experiment module mounted on the ISS.

Ms Williams will conduct a second spacewalk on 23 January with Mr Wilmore.

Together, they will remove a radio frequency group antenna assembly (a collection of components designed to transmit and receive radio waves) and collect surface samples for microorganism analysis.

They will also prepare a backup elbow joint for the Canadarm2 robotic arm by positioning the joint in the optimal configuration for a quick replacement if needed.

Each spacewalk is expected to last about six and a half hours.

Nasa is providing live coverage of both spacewalks that can be viewed here.

Nicole McElroy, a Flight Director at Nasa, said in a press briefing before the spacewalk that “The road to our preparations for all of these EVAs (extra-vehicular activity) has been going well.

“The crew on board has been studying their procedures, getting familiar with all of the tasks, and are really excited and looking forward to going out the door.”

Ms Williams and Mr Wilmore went on what was meant to be an eight day mission to the ISS aboard Boeing’s Starliner in June 2024.

However, technical issues including helium leaks and thruster malfunctions meant that the Starliner was unsafe for their return.

Nasa plans to bring them back to Earth in late March aboard a spaceship built by SpaceX, a rival company of Boeing.

Despite these setbacks, the astronauts have continued their work aboard the ISS while awaiting a safe journey home.

Butch Wilmore and Suni Williams were due to be back after just a week when they blasted off in June.

Their stay was extended to February next year because of technical issues with the experimental spacecraft, Starliner, built by Boeing.

Now – following a delay in launching a new capsule to the ISS – the pair won’t be back until late March or possibly April.

Nasa said the delay posed no risk to the astronauts.

In a statement Nasa stated: “The International Space Station recently received two resupply flights in November and is well-stocked with everything the crew needs, including food, water, clothing, and oxygen.

The resupply spacecraft also carried special items for the crew to celebrate the holidays aboard the orbital platform.”

Most space station missions last six months, with a few reaching a full year. So the extension to Butch and Suni’s already overdue stay in space should not be a problem, according to Dr Simeon Barber, from the Open University.

“I’m sure that they are already disappointed that they were going to miss Christmas back home with the folks. But this is only another two months on an already quite long mission, and I’m sure if you ask them, I’m sure they would tell you that the space station is where they love to be,” he said.

A new crew needs to launch before Wilmore and Williams can return and the next mission has been delayed by more than a month, according to the space agency.

Nasa’s next crew of four for the ISS was supposed to have been launched in February 2025. The capsule carrying that crew was due to be the one bringing Butch and Sunni home, as well as NASA astronauts Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov as part of the normal crew rotation.

But there has been a delay by the private sector firm SpaceX in preparing a brand-new Dragon capsule for the mission. That is now scheduled for flight readiness no earlier than late March.

Nasa said it considered using a different SpaceX capsule to fly up the replacement crew to keep the flights on schedule.

But it has now decided the best option is to wait for the new capsule to transport the next crew.