May 28, 2020 12:22 PM EDT

When you’ve got a 230-ft. tall rocket filled with 76,000 gallons of explosive fuel sitting on the launch pad, the President in the viewing stands and millions worldwide waiting to watch the great machine fly, you’d figure you wouldn’t schedule the event for a spring afternoon in Florida, when bad weather stands to wreck the whole party. Those are exactly the conditions in which the SpaceX Falcon 9 rocket carrying astronauts Bob Behnken and Doug Hurley tried—and failed—to get off the launch pad on May 27, for the historic first crewed launch from American soil since 2011.

The scrubbed flight left a lot of people asking, Why don’t NASA and SpaceX just pick a day and time to launch when the forecast is clear? The answer: It’s not up to them. It’s up to physics.

If you were trying to launch any old spacecraft into any old orbit you could, indeed, pick pretty much any old time to fly. But things are almost never as simple as that, especially when you’re trying to rendezvous with another object already in Earth orbit — in this case, the International Space Station (ISS). Pulling off so delicate a pas de deux typically requires precise timing, which means launching in a fixed time frame on a fixed day within what’s known as a “launch window.”

The most conspicuous orbiting object with which astronauts have attempted to rendezvous is the moon. Back in the days of the Apollo program, the trick was not to aim for where the moon was in the sky at the moment of launch, but for where it would be three days later, when the spacecraft had covered the Earth-to-moon distance. With the moon orbiting the Earth at 3,683 km per hour (2,288 mph), that took some careful planning. Things were made even tougher by the fact that just reaching the vicinity of the moon wasn’t sufficient; after traveling 386,000 km (240,000 mi.), the crews were aiming to enter a lunar orbit just 97 km (60 mi) above the surface of the moon. That’s not just like standing in one end zone of a football field and taking aim at an apple in the other end zone—it’s like trying to skin the apple with your bullet.

The Apollo crews were also trying to arrive at the moon during optimal lighting conditions for landing, when the sun was at the correct angle in the lunar sky to cast shadows that would highlight but not exaggerate the terrain. That narrowed the launch window even further. If all of those constraints meant trying to launch in the rain and hoping for a break in the clouds, that’s what you’d have to do.

Merely getting into Earth orbit is simpler, but presents launch window headaches of its own. Spacecraft typically do not circle the planet in perfectly tidy trajectories that take them around and around the equator. Rather, they are launched into orbits that are tilted at inclinations relative to the equator. Cape Canaveral’s latitude is approximately 28º, so an orbiting spacecraft launched from the Kennedy Space Center flies similarly inclined, like a crooked hula hoop, moving 28º above and below the equator as it travels around the world.

Things get more complicated still. Earth itself is tilted 23º, and while the planet takes 24 hours to make a single rotation, a spacecraft needs only 90 minutes to complete an orbit. The result: on a flat map, the spacecraft would appear to be inscribing a sort of sine wave pattern, moving above and below the equator, never passing over precisely the same point on the ground on two consecutive orbits. The ISS orbits at a higher inclination of 51.6º—a concession the U.S. made to Russia, which launches its spacecraft further north, from the Baikonur Cosmodrome in Kazakhstan.

Flight planners sending astronauts to the ISS, where Behnken and Hurley were headed, must time their launches for exactly when the station is in an optimal position to make the rendezvous happen at all and, if possible, to make the pursuit time as short as possible. Uncrewed cargo missions sent from Baikonur to the space station may have comparatively wide launch windows requiring as much as a two-day flight to the station. But cargo doesn’t kvetch about discomfort. Crewed missions often have narrower windows that allow astronauts and cosmonauts to catch up with the ISS in as little as four orbits—or six hours—freeing them from the confinement of their spacecraft as quickly as possible. For Behnken and Hurley, launching from Cape Canaveral, at its 28º latitude, into the ISS’s 51.6º inclination means a relatively long 19-hour chase. But it would be much longer still if they picked the wrong launch window.

With the weather having slammed that window closed on May 27, SpaceX’s historic mission is looking at a new one that would permit liftoff at 3:22 PM EDT on Saturday, May 30. If that window is closed shut, too, the next one opens the following day at 3:00 PM EDT. You can watch the next attempts right here.

Space—as people have said and said and said again—is hard. Sometimes, the hardest part of all is just getting off the ground.

Write to Jeffrey Kluger at jeffrey.kluger@time.com.

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