Shamil Zhumatov / Reuters

A police helicopter flies next to the Soyuz TMA-08M spacecraft as it is transported to its launch pad at the Baikonur Cosmodrome in Kazakhstan on March 26. The Soyuz will carry NASA astronaut Chris Cassidy along with Russian cosmonauts Pavel Vinogradov and Alexander Misurkin to the International Space Station.

By James Oberg, NBC News Space Analyst

For more than 30 years, Russian spaceships have taken two days to dock with their target but on Thursday, the travel time for a Soyuz capsule carrying three spacefliers to the International Space Station is being trimmed to six hours.

Has the Soyuz suddenly become speedier? Not really.

The Soyuz itself won't fly any faster when it's sent into space at 4:43 p.m. ET from Russia's Baikonur Cosmodrome in Kazakhstan. It won't have any fundamentally new or improved guidance and navigation system. "All the systems of the vehicle are the same, but the work is more intense," Russian cosmonaut Pavel Vinogradov, the Soyuz's commander, said last week during a news briefing. "There are no new systems or modes in the vehicle, but the coordination work of the crew should be better."

This faster flight plan is possible only because someone else is doing the real work. The space station itself has shifted its position to be nearer to the Soyuz when that spacecraft goes into orbit. It is quite literally moving itself right in front of the speeding Soyuz.

The rapid rendezvous procedure has already been tested twice with robotic supply flights, but this is the first time it's been used with a crewed spacecraft. If it works, the crew should be docking with the station at 10:31 p.m. ET Thursday, taking the fastest ride to an orbital destination since NASA's Skylab missions, 40 years ago.

Hunter and hunted Chasing down a target in the trackless void of space is not as simple as merely catching sight of it and thrusting towards it. The inflexible rules of orbital mechanics motion along orbital paths demand precise timing of critical course changes on the part of the vehicle that's doing the chasing.

For any space rendezvous, the first critical time is the moment when the chasers launch pad passes below the targets circular orbit. If the chaser is launched during this moment and heads in a direction parallel to the target's orbital course, it winds up more or less in the same orbital plane as the target. That's the "planar window" for a launch.

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Space station shifts its orbit to make speedy crew rendezvous possible