Two Nasa astronauts “stranded” aboard the International Space Station (ISS) since last summer are finally on their way back to Earth on Tuesday, more than nine months after the failure of Boeing’s pioneering Starliner capsule scuppered their originally scheduled week-long mission.

A SpaceX Dragon capsule containing four astronauts, including Starliner’s test pilots Sunita Williams and Barry “Butch” Wilmore, undocked from the orbiting outpost at 1.05am ET (5.05am GMT). The spacecraft is scheduled for a splashdown somewhere off the Florida coast at about 6pm ET following a 17-hour descent, with mission managers determining the precise location after assessing weather conditions.

Williams and Wilmore arrived at the ISS on 6 June last year, intending to stay long enough to evaluate Starliner’s docking and operational capabilities during its first crewed flight and return home no more than 10 days later.

But a series of technical issues and safety fears led Nasa and Boeing to send the capsule back to Earth empty in September, and extend the pair’s stay by making them crew members aboard the space station in place of two other astronauts still on the ground who were reassigned to other future missions.

In the months since, their odyssey has become something of a political football, with SpaceX founder and Donald Trump acolyte Elon Musk insisting without evidence they were “abandoned” by the Biden administration, and Trump attempting to paint last week’s long-scheduled routine crew rotation flight as a special rescue mission ordered by the White House.

Astronauts Sunita Williams and Butch Wilmore. Photograph: NASA

The fallout left the astronauts in an awkward position, with Wilmore telling reporters from space earlier this month that Musk’s claim he offered to bring them home last year, but was rebuffed by Joe Biden, was “absolutely factual”, while admitting: “We have no information on … what was offered, what was not offered, who it was offered to, how that process went.”

Yet in February he told CNN: “We don’t feel abandoned, we don’t feel stuck, we don’t feel stranded. I understand why others may think that … if you’ll help us change the rhetoric, help us change the narrative, let’s change it to ‘prepared and committed’, that’s what we prefer.”

Musk, meanwhile, became embroiled in a public dispute with Danish astronaut and space station veteran Andreas Mogensen, who accused him of lying, and pointed out that Tuesday’s return of Williams and Wilmore, alongside their ISS Crew 9 colleagues American Nicholas Hague and Russian cosmonaut Aleksandr Gorbunov, was scheduled as long ago as September.

In response, Musk posted to the X platform he owns that Mogensen was “fully retarded”, drawing him deeper into conflict with retired astronauts and ISS veterans and brothers Scott and Mark Kelly, who defended their European colleague.

The bad blood has continued, with Musk calling Mark Kelly, Democratic senator for Arizona, “a traitor” for visiting Ukraine and urging US military and humanitarian support for the country in its war against Russia; and the politician retorting that Musk was “not a serious guy”. Kelly has also ditched his Tesla car, manufactured by another of Musk’s companies, in protest at the billionaire’s role in slashing federal budgets and staffing.

Tuesday’s splashdown is scheduled for 5.57pm ET, but is likely to change depending on an assessment of weather and sea conditions before Dragon’s final de-orbit burn and re-entry to Earth’s atmosphere.

According to a schedule published on Nasa’s website, the four Crew 9 astronauts were set to enter the capsule at 10.45pm ET Monday, a little more than two hours ahead of their undocking. Williams and Wilmore will be accompanied by the Nasa astronaut Nick Hague and Roscosmos cosmonaut Aleksandr Gorbunov.

Live coverage of the final stages of the descent, including the splashdown and crew recovery, will be broadcast on Nasa TV and new streaming service Nasa+, followed by a later press conference.

The future of Boeing’s Starliner capsule, developed as part of Nasa’s commercial crew program, remains uncertain. Engineers have examined the spacecraft at its White Sands, New Mexico, base attempting to find the cause of problems that arose during its maiden crewed flight, including thruster issues and a series of small helium leaks.

Aviation Week reported that Starliner is unlikely to fly again in 2025, but that the company retained confidence in its product and was working towards earning a new flight readiness certification.