Latest flight to ISS sets speed record. How did they do it?

The latest Soyuz launch to the International Space Station took only six hours, where flights in the past have taken closer to two days to complete. How did they accomplish this, and why wasn't this done before?

Getting up to the International Space Station takes some very careful timing and coordination, for a spacecraft that's pointing straight up into the sky to get into an orbit that's not only parallel to the planet's surface, but that also matches the orbital height and speed of the space station, and at the right time so they can dock.

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In order to do that safely, flights to the ISS have taken two days — roughly nine minutes to reach low-Earth orbit, and then 34 orbits of the planet while the spacecraft picks up speed, and increases its distance from the planet's surface. That gives the astronauts a lot of time to coordinate the flight and leeway to recover in case there's any delays or problems with any of the steps. Given how dangerous it is to fly into orbit, taking their time has always been a good idea.

Taking longer than that to rendezvous with the ISS isn't advisable, since the spacecraft only has a certain amount of fuel and supplies (and for the sanity of the astronauts, I'm sure). Getting there faster is possible (of course) — it just takes even more careful timing and coordination, and there's less leeway if they run into a problem.

This new four-orbit rendezvous that the astronauts used yesterday has only been used three times before this, with unmanned Progress spacecraft launches delivering supplies to the space station, and it was made possible by two things.

First, the Progress and Soyuz spacecrafts themselves were upgraded so that they now have digital flight control systems. Apparently, before this, the astronauts had to wait until they were over the ground control station to communicate and coordinate the manual engine burns to change their orbit. With this new digital system, ground control can now upload a whole schedule for the engine burns into the computer, and the spacecraft can perform all the maneuvers itself, even when it's out of communication with the ground.

The second was the new flight program that the Russians figured out for the spacecraft, after they did a careful study of the maneuvering capabilities of both the ISS and their launch craft. This study showed that, although it would take very careful maneuvering by both the space station and the spacecraft, and an extremely precise orbital insertion of the launch craft, it could be done. The three unmanned launches tested the new program and showed that it worked, and now it was time to try it with a manned spacecraft.

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I should also mention that it's not "game over" for the astronauts, should they miss their six-hour rendezvous. If they run into any problems, they can change to an alternate one-day rendezvous that takes them around the planet a few more times before they meet up with the ISS.

Either way, this new flight schedule is a definite improvement over the old one, and cosmonauts Pavel Vinogradov and Alexander Misurkin, and astronaut Chris Cassidy do seem pretty happy about the faster trip.

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