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 …read more
Source:: Time – Science