A ROCKET carrying the replacement crew for two NASA astronauts who have been stranded in space for nine months has finally launched. SpaceX's Falcon 9 rocket lifted off at 7.03pm ET from NASA's Kennedy Space Center in Florida carrying the four replacement astronauts. US astronauts Butch Wilmore and Suni Williams have been trapped on the International Space Station (ISS) for nine months and have had their journey home delayed a number of times. The two veteran astronauts were the first to fly Boeing's Starliner capsule to the ISS in June but issues with its propulsion system during the flight forced an extension to their trip. Originally planned as an eight-day stay, NASA deemed it too risky from them to fly home on the craft, which returned empty to Earth in September. Mission Crew-10 was initially scheduled to launch on Wednesday with its four astronauts, but a last-minute issue with the rocket's ground systems forced a delay. Now on the way, the Crew-10 mission is a long-awaited key step to bringing the stranded astronaut duo back to Earth. Williams and Wilmore are set to depart the station on March 19 after the replacement crew arrive on Saturday night. The mission has become entangled in politics, with President Donald Trump and his key adviser Elon Musk, who is also SpaceX's CEO, claiming former President Joe Biden left the pair on the ISS for political reasons. There is no evidence for these claims, and Wilmore himself has said he did not believe NASA's decision to keep them on the ISS until Crew-10's arrival had been affected by politics. The veteran astronaut said: "We came prepared to stay long, even though we planned to stay short. "That's what your nation's human spaceflight program's all about, planning for unknown, unexpected contingencies - and we did that." NASA says the two astronauts were kept onboard as the ISS has to maintain its minimum staffing level. Having seen their short mission turn into a regular NASA rotation to the ISS, Wilmore and Williams have been undertaking scientific research. The pair have also been conducting routine maintenance with other astronauts. NASA's regular procedures changed slightly for this mission, following intervention from Trump and Musk who both demanded an earlier return of the stranded astronauts. The space agency brought forward the Crew-10 mission from March 26, swapping a delayed SpaceX capsule for one that would be ready sooner. This pressure from the two powerful politicians has hung over NASA's preparations and safety process, which normally follows a well-defined course. NASA's Commercial Crew Program manager, Steve Stich, said SpaceX's "rapid pace of operations" had required NASA to change some of the ways it verifies flight safety. The agency had to address some "late-breaking" issues, NASA space operations chief Ken Bowersox told reporters. This included investigating a fuel leak on a recent SpaceX Falcon 9 launch and deterioration of a coating on some of the Dragon crew capsule's thrusters. Bowersox said it was hard for NASA to keep up with SpaceX: "We're not quite as agile as they are, but we're working well together." When the new crew arrives aboard the station, Wilmore, Williams and two others - NASA astronaut Nick Hague and Russian cosmonaut Aleksandr Gorbunov - can return to Earth in a capsule that has been attached to the station since September, as part of the prior Crew-9 mission. Crew-10 is expected to dock with the ISS at 11.30pm ET on Saturday, and this will be followed by a traditional handover ceremony that will allow for the Crew-9 crew's departure on March 19.