Nasa astronauts Butch Wilmore and Suni Williams returned to Earth in a SpaceX capsule on Tuesday with a soft splashdown off Florida's coast, nine months after their faulty Boeing Starliner craft upended what was to be a week-long stay on the International Space Station.

Their return caps a protracted space mission that was fraught with uncertainty and technical troubles and turned a rare instance of Nasa's contingency planning - and the latest failures of Boeing's Starliner - into a global spectacle.

Wilmore and Williams, two veteran Nasa astronauts and retired US Navy test pilots, had strapped inside their Crew Dragon spacecraft along with two other astronauts and undocked from the orbiting laboratory of the ISS at 1.05 a.m. ET (0505 GMT) to embark on a 17-hour trip to Earth.

The four-person crew, formally part of Nasa's Crew-9 astronaut rotation mission, re-entered Earth's atmosphere around 5:45 p.m. ET. Using Earth's atmosphere and two sets of parachutes, the craft slowed its orbital speed of roughly 17,000 miles per hour to a soft 17 miles per hour at splashdown.

Dressed in re-entry suits, boots and helmets, the astronauts were seen earlier on Nasa's live footage laughing, hugging and posing for photos with their colleagues from the station shortly before they were shut into the capsule for two hours of final pressure, communications and seal tests.

The astronaut pair had launched into space as Starliner's first crew in June for what was expected to be an eight-day test mission. But issues with Starliner's propulsion system led to cascading delays to their return home, culminating in a Nasa decision last year to have them take a SpaceX craft back this year as part of the agency's crew rotation schedule.

The mission has captured the attention of US President Donald Trump, who upon taking office in January called for a quicker return of Wilmore and Williams and alleged without evidence that former President Joe Biden "abandoned" them on the ISS for political reasons.

SpaceX CEO Elon Musk, a close adviser to Trump, echoed his call for an earlier return. SpaceX's Crew Dragon is the United States' only orbital-class crew spacecraft, which Boeing had hoped its Starliner would compete with before the mission with Wilmore and Williams threw its development future into uncertainty.

After the splashdown, the astronauts will be flown to their crew quarters at the space agency's Johnson Space Center in Houston for several days of health checks, per routine for astronaut returns, before Nasa flight surgeons approve they can go home to their families.

Living in space for months can affect the human body in multiple ways, from muscle atrophy to possible vision impairment.

Wilmore and Williams have logged 286 days in space on the mission - longer than the average six-month ISS mission length, but far short of US record holder Frank Rubio. His continuous 371 days in space, ending in 2023, were the unexpected result of a coolant leak on a Russian spacecraft.

Williams, capping her third spaceflight, has tallied 608 cumulative days in space, the second most for any US astronaut after Peggy Whitson's 675 days. Russian cosmonaut Oleg Kononenko set the world record last year at 878 cumulative days.

Swept up in Nasa's routine astronaut rotation schedule, Wilmore and Williams could not begin their return to Earth until their replacement crew arrived, in order to maintain adequate US staffing levels, according to Nasa.

Their replacements arrived on Friday night - four astronauts as part of Nasa's Crew-10 mission that briefly put the station's headcount at 11.

"We came prepared to stay long, even though we planned to stay short," Wilmore told reporters from space earlier this month, adding that he did not believe Nasa's decision to keep them on the ISS until Crew-10's arrival had been affected by politics.

"That's what your nation's human spaceflight programme's all about," he said. "Planning for unknown, unexpected contingencies. And we did that."

Wilmore and Williams had been doing scientific research and conducting routine maintenance with the station's other five astronauts. Williams had performed two six-hour spacewalks for maintenance outside the ISS, including one with Wilmore.

The ISS, about 254 miles (409 km) in altitude, is a football field-sized research lab that has been housed continuously by international crews of astronauts for nearly 25 years, a key platform of science diplomacy managed primarily by the US and Russia.

Williams told reporters earlier this month that she was looking forward to returning home to see her two dogs and family. "It's been a roller coaster for them, probably a little bit more so than for us," she said. (Reuters)