SpaceX can't wait even one second for bad weather to pass for a rocket launch with people because it could endanger their entire mission

• SpaceX and NASA launched their Crew-1 mission on Sunday: the longest and most ambitious US human spaceflight to date.
• Poor weather near the launchpad, or any other delays, at the moment of planned launch would have caused mission managers to call off an attempt; SpaceX's Falcon 9 rocket can't wait another second.
• Falcon 9's ultracold fuel warms up and vents out as it sits on the launchpad. This causes the machine to have a smaller margin for error in sending people to orbit.
• When a Crew Dragon spaceship is on top with passengers inside, the rocket must lift off 35 minutes after fueling begins. It also has to align with the International Space Station flying overhead.

SpaceX on Sunday night launched NASA's most ambitious crewed mission yet: flying four astronauts to the International Space Station.

Called Crew-1, the mission carries Mike Hopkins, Victor Glover, and Shannon Walker of NASA, as well as Soichi Noguchi of the Japan Aerospace Exploration Agency, inside a Crew Dragon spaceship. The tight-knit group hopes to stay at the space station for six months. If the crew members succeed, their mission will break the US record for the longest human spaceflight.

All day Sunday, rain and clouds closed in on the launchpad and threatened to force SpaceX to delay liftoff to Wednesday. Forecasters calculated a 50% chance that weather near the launch site at Kennedy Space Center in Cape Canaveral, Florida, would violate requirements for liftoff.

"Rain in the flight path, thick clouds, and possibly static electricity at ground level might be an issue," John Insprucker, a SpaceX engineer, said during NASA's livestream of launch preparations Sunday afternoon.

If anything had interfered with plans for the exact moment of launch, which was about 15 seconds after 7:27 p.m. ET, mission managers would have scrubbed the attempt and tried again another day. Luckily, the skies cleared before the moment came and the spaceship, called Resilience, roared into orbit.

Such a sensitive launch window is called "instantaneous," and it exists because of the rocket's design, NASA safety requirements, the trajectory of the International Space Station over Earth.

During SpaceX's first attempt in May to launch people into space — a mission called Demo-2 — puffy clouds were bound to break flight rules at the moment of launch, triggering a scrub.

"Everything was looking our way except mother nature: the weather," Insprucker said in the livestream of Demo-2's first launch attempt.

With Crew-1, onshore winds on Saturday forced a delay for the first launch attempt. A SpaceX boat was unable to get in position for the Falcon 9 rocket booster to land on it after separating from the upper stage and the Crew Dragon.

Here's why a crewed SpaceX launch can't wait a single second longer after it's supposed to lift off.

The twin problems of ultracold fuel and a laboratory moving at 17,500 mph

Once SpaceX starts fueling a Falcon 9 rocket, it begins a process in which imprecise timing could mean life or death.

"Once you get into propellant loading at T minus 35 minutes, you have to go as soon as you get to zero," Insprucker said in May ahead of Demo-2's first scrubbed flight attempt. "We don't have the ability to stop the countdown, wait five minutes."

That's because liquid oxygen is pumped into the Falcon 9 at a very low temperature: minus 340 degrees Fahrenheit. That keeps it liquid and densifies the fuel, a type of kerosene called RP-1, which allows SpaceX to cram more of it into the rocket and squeeze more performance out if the machine.

In fact, it's part of what gives the Falcon 9 its unprecedented thrust — the force behind its launch that allows it to push more than 50,000 pounds of cargo into orbit. The extra reserves also provide more wiggle room to reach orbit and for SpaceX to land and recycle the rocket's booster — the biggest and most expensive part of the Falcon 9.

Once it's inside the rocket, however, the fuel begins to warm up, expand, and boil off. That fuel loss starts the launch clock.

"That changes how much performance you get carrying into orbit, and we don't want to cut into those margins," Insprucker said.

Essentially, any additional time the rocket sits fully fueled on the launchpad increases the risk of a dangerous failure — a risk that is not worth taking when there are humans on board.

So the Falcon 9 must launch at the exact scheduled second. If it misses that brief window with a satellite, Insprucker said, the rocket can be detanked of its fuel and later reloaded with fresh fuel and cold liquid oxygen. The whole process takes roughly an hour and a half.

But that is not an option for missions to the space station, a football-field-sized laboratory that flies over Earth's surface on a winding path (because of its inclination) and at 17,500 mph.

"In the case of the International Space Station, an hour and a half from now, it's nowhere where we need to be to get to orbit," Insprucker said. "In the end we can all look at Isaac Newton and Johannes Kepler for orbital dynamics telling us, 'When do we launch?' That stuck us right in the middle of a period of bad weather."

This story has been updated with new information. It was originally published at 1:51 p.m. ET on May 30.

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