NASA has delayed its Space X rocket launch to the International Space Station for the second time in a week following an unspecified medical issue with one of the four astronauts.
The US space agency says the issue is "not a medical emergency and not related to COVID-19", but has decline to elaborate on the nature of the problem or say which astronaut is involved.
The launch, which was set for Sunday but postponed until Wednesday because of unsuitable weather, has been rescheduled for Saturday night.
The last time NASA delayed a scheduled launch over a medical issue involving the crew was for a Space Shuttle Atlantis flight in 1990, when mission commander John Creighton fell ill.
The countdown was halted for three days until he was cleared to fly.
The SpaceX-built vehicle set to fly this weekend, consisting of a Crew Dragon capsule atop a two-stage Falcon 9 rocket, is now set for liftoff at 11.36pm on Saturday (1436 AEDT on Sunday) from NASA's Kennedy Space Center in Cape Canaveral, Florida.
If all goes smoothly, the three US astronauts and their European Space Agency (ESA) crewmate will arrive 22 hours later and dock with the space station 400 kilometres above the earth to begin a six-month science mission.
For the time being, the four crew members will remain under routine quarantine as they continue launch preparations.
Joining the mission's three NASA astronauts - flight commander Raja Chari, 44, mission pilot Tom Marshburn, 61, and mission specialist Kayla Barron, 34 - is German astronaut Matthias Maurer, 51, an ESA mission specialist.
Saturday's liftoff, if successful, would count as the fifth human spaceflight SpaceX has achieved following its inaugural launch in September of a space tourism service that sent the first ever all-civilian crew into orbit.