China is launching three astronauts to the Tiangong space station Sunday in a mission that will relieve a crew stuck in orbit a month longer than originally scheduled and potentially is China's first year-long spaceflight.

The Long March 2F rocket, hauling the Shenzhou 23 mission, is sitting at the launch pad in the Gobi Desert, while the airspace closures say liftoff is meant to be around 11:10 pm Beijing time. 

The crew of three will be aboard Tiangong for roughly six months, but the way the mission is put together adds this oddly different timeline twist, like, you know, not what you’d expect right away.

Because Shenzhou 24, set to launch later this year, will have a Pakistani astronaut for a brief stopover, one Shenzhou 23 crew member will basically stay circling in orbit when the Pakistani visitor heads back on the outgoing spacecraft. 

The astronaut will finish a complete year in space, and that would be a historic first for China's human spaceflight program.

The China Manned Space Agency had kept the crew names under wraps until a press conference just one day before launch at the Jiuquan Satellite Launch Center. Wednesday’s pre-launch rehearsal showed everything looks operational. Astronauts went through medical reviews and did final preparations.

The agency has not pinned down an official launch time. Instead it says Shenzhou 23 will lift off "at an appropriate time in the near future," though airspace notices suggest Sunday morning.

The Shenzhou 23 crew will take over from the Shenzhou 21 astronauts, whose mission ran past its original timetable due to emergency circumstances.