The two astronauts whose ISS mission was extended due to a troubled Boeing Starliner are expected to return to the SpaceX capsule in February. (Credit: NASA)

SpaceX on Saturday launched a Falcon rocket from Cape Canaveral, Florida, on Saturday with two crew members on board to deliver the Dragon capsule to the International Space Station that will bring back the astronauts stranded by their troubled Boeing Starliner next year.

In June, NASA astronauts Butch Wilmore and Suni Williams launched on the Starliner, which suffered thruster problems and helium leaks, leaving the space agency to conclude returning them to Earth in the capsule after what was supposed to be an eight-day mission was too risky.

The crewless Starliner eventually landed in New Mexico earlier this month, while Wilmore and Williams remained on the space station.

NASA astronaut Nick Hague and Russian cosmonaut Alexander Gorbunov launched from the Kennedy Space Center on Saturday's mission with two empty seats on Dragon that will bring Wilmore and Williams home in February.

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Two astronauts were cut from the SpaceX mission to make room for Wilmore and Williams.

Once Hague and Gorbunov arrive at the space station Sunday afternoon, four other astronauts who were delayed a month by Starliner's problems will return to Earth in their own SpaceX capsule.

"There's always something that is changing," Hague said before the launch of the evolving nature of space missions. "Maybe this time it's been a little more visible to the public."

NASA Deputy Administrator Pam Melroy called "human spaceflight" "complicated and dynamic" at a post-launch news conference Saturday.

"What a fabulous day it was today," she said. "We only have two crew members instead of four. A crew member change is not a small thing."

She added that "it was the right thing to do."

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Melroy said the two astronauts who were cut from the mission were there to emotionally support Hague and Gorbunov at the launch.

"The International Space Station is unbelievable, it's a really special place," she added, saying it "allows people to "look beyond gravity" and to build technologies to explore the whole solar system.

After they reach the space station, Hague and Gorbunov will be doing hundreds of "exciting experiments," including studying human cells and blood clotting.

Dana Hutcherson, deputy program manager of NASA's Commercial Crew Program, said he was "pleased to report Nick and Aleksandr are safely in orbit" after an "incredibly smooth" launch and are expected to land at the space station around 5:30 p.m. ET Sunday.

"The crew is getting out of their suits at this time," Hutcherson said, adding that Gorbunov and Hague would be getting some rest and a meal in the capsule before preparing for docking operations."

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After docking on the ISS, they're expected to have a welcome ceremony.