Nasa has announced a rare health-related delay in its SpaceX rocket launch of four astronauts to the international space station, the second postponement of the mission in a week, citing an unspecified medical issue with one of the crew. The space agency on Monday described it as a “minor medical issue” that was “not a medical emergency and not related to Covid-19”, but it declined to elaborate on the nature of the problem or say which astronaut was involved. The launch, originally set for Sunday but then postponed until Wednesday because of unsuitable weather conditions, had been rescheduled for Saturday night, Nasa said. 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, according to Nasa. That delay was followed by two additional weather-related postponements. The SpaceX-built vehicle set to fly this weekend, consisting of a Crew Dragon capsule and a two-stage Falcon 9 rocket, is now set for liftoff at 11.36pm on Saturday from Nasa’s Kennedy space centre 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 400km (250 miles) above the Earth to begin a six-month science mission. For the time being, the four crew members would remain under routine quarantine at the Cape as they continued launch preparations, Nasa said. 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. Chari, a US air force combat jet and test pilot, Barron, a US navy submarine officer and nuclear engineer, and Maurer, a materials science engineer, are all making their debut spaceflights aboard the Dragon vehicle, called Endurance. Marshburn, a physician and former Nasa flight surgeon, is the most experienced astronaut of the crew, having logged two previous spaceflights and four spacewalks. Saturday’s liftoff, if successful, would count as the fifth human spaceflight SpaceX has achieved to date, following its inaugural launch in September of a space tourism service that sent the first ever all-civilian crew into orbit.