NASA's two stranded astronauts, Butch Wilmore and Suni Williams, are finally set to return home after spending nine months aboard the International Space Station (ISS). Their replacements -- NASA's Anne McClain and Nichole Ayers, Japan's Takuya Onishi, and Russia's Kirill Peskov -- launched aboard a SpaceX capsule from Kennedy Space Center on Friday night and are expected to arrive late Saturday.

NASA scheduled an overlap between the crews to ensure a smooth transition before Wilmore and Williams depart next week, weather permitting. They will return with the rescue mission crew that arrived last September, with two empty seats reserved for them on the SpaceX capsule.

What we don't know:

While NASA has planned for a mid-March return, it remains uncertain if weather conditions will allow for a timely splashdown off the Florida coast. It is also unclear if additional technical issues could arise, given the past delays in securing their return. Furthermore, the full extent of any long-term physical or psychological effects on Wilmore and Williams from their unexpectedly extended stay in space is unknown.

The backstory:

Wilmore and Williams launched aboard Boeing's new Starliner capsule on June 5, expecting to stay for just a week. However, their mission took an unexpected turn when helium leaks and thruster failures made it unsafe for Starliner to return them to Earth. After months of investigation, NASA decided to send the capsule back empty and arranged for them to return on a SpaceX flight.

Their homecoming, initially planned for February, was postponed again due to battery issues on SpaceX's new capsule. To expedite the process, NASA opted for a used SpaceX capsule, moving their return up to mid-March.

Crew-10 was expected to launch earlier this week, but Wednesday night's mission was scrubbed due to a hydraulic issue with a clamp arm attached to the transporter erector system, officials said.

Big picture view:

The extended stay of Wilmore and Williams has highlighted the risks and challenges associated with space travel, particularly with new spacecraft like Boeing's Starliner. Their situation has drawn political attention, with figures like President Donald Trump and SpaceX's Elon Musk blaming delays on the previous administration.

However, NASA has maintained that the decisions were made with safety as the top priority. Meanwhile, the incident underscores the ongoing rivalry between SpaceX and Boeing in the commercial spaceflight industry and the reliability concerns surrounding new spacecraft development.

Timeline:

The replacements for NASA's two stuck astronauts launched to the International Space Station on Friday night, paving the way for the pair's return after nine long months. Butch Wilmore and Suni Williams need SpaceX to get this relief team to the space station before they can check out. Below is their timeline of events:

Timeline:

After lifting off from Launch Complex 39A at NASA's Kennedy Space Center in Florida, Crew-10 will fly through space until it reaches the International Space Station.

Crew-10 is expected to reach the ISS at 11:30 p.m. on Saturday, March 15. It takes a little more than 26 hours to reach the space station.

Dig deeper:

During their time in the orbiting laboratory, SpaceX said the crew will conduct new research to prepare for human exploration beyond low-Earth orbit and to benefit humanity on Earth.

They will test spacecraft materials, engage with students via ham radio, test a lunar navigation system, and study how space affects the body.

The four new astronauts heading to space on the Crew-10 mission are Kirill Peskov, Nichole Ayers, Anne McClain and Takuya Onishi.