NASA and SpaceX announced early on Monday they were postponing the launch of four astronauts to the International Space Station (ISS) due to technical issues.
The countdown was halted minutes before liftoff from the Kennedy Space Center in Florida. Both NASA and SpaceX said it was due to an issue with "ground systems."
The launch team could not be sure ground equipment fully loaded the engine ignition fluid, officials told the AP news agency. One SpaceX engineer compared the critical system to cars' spark plugs, which ignite the combustion needed to start the vehicle.
A new date for the launch of the six-month mission has yet to be announced.
What do we know about the launch?
Besides two NASA astronauts, the launch would've also included one Russian cosmonaut and one Emirati astronaut. The latter is the second to travel to space from the United Arab Emirates, as well as being the fourth Arab.
SpaceX announced it would start draining fuel from the rocket.
"Both Crew-6 and the vehicles are healthy and propellant offload has begun ahead of the crew disembarking Dragon," it said.
The crew would be the sixth to be transported to the ISS by a SpaceX rocket.
Politics in space
The space sector witnesses rare US-Russian cooperation, amid deteriorating relations between the two countries since Russia invaded Ukraine last year.
"We're all professionals. We keep focused on the mission itself," the AFP news agency quoted crew commander Stephen Bowen. "It's always been a great relationship we've had with cosmonauts once we get to space."
The ISS, a laboratory orbiting about 250 miles (420 km) above Earth, was launched in 1998, amid increased US-Russian cooperation after the end of the Cold War.
It involves five space agencies: NASA, Russia's Roscosmos, Japan Aerospace Exploration Agency, the European Space Agency and Canada's Canadian Space Agency.
NASA has in recent years started commissioning SpaceX to carry astronauts to the ISS roughly every six months.
Meanwhile Russia, which has since the 1960s relied on its aging but reliable Soyuz capsules to make the trip, has suffered the loss of satellites and vehicles.
