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Space station shifts its orbit to make speedy crew rendezvous possible

The Soyuz TMA-08M spacecraft is transported from its assembly hangar to the launch pad at the Baikonur Cosmodrome on March 26.
The Soyuz TMA-08M spacecraft is transported from its assembly hangar to the launch pad at the Baikonur Cosmodrome on March 26.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.Shamil Zhumatov / Reuters

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 chaser’s launch pad passes below the target’s 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.

But there's another critical timing requirement, having to do with how far ahead the target is when the chaser enters orbit. The target could be at any point in the circular path it follows around Earth, but it's important to choose the right point for launching the chaser.

The Soyuz TMA-08M spacecraft is transported from its assembly hangar to the launch pad at the Baikonur Cosmodrome on March 26.Shamil Zhumatov / Reuters

The numbers give you an idea of the scale of the problem: The space station travels in a circular orbit that averages around 224 miles (360 kilometers) in altitude, and the chaser spacecraft are usually launched into initial orbits averaging around 143 miles (230 kilometers). That lower orbit is faster, both because gravity is slightly stronger there, and because the radius is smaller, which makes each circuit shorter.

For that difference in average altitude, a typical chaser spacecraft will catch up with the station at a rate of 560 mph (900 kilometers per hour). So if the chaser starts out 5,600 miles (9,000 kilometers) behind the station in its orbit, it will take about 10 hours to overtake the station. If it’s 16,800 miles (27,000 kilometers) behind, it would take 30 hours. And it might be even farther.

Flexibility is key

If you have a long period of time available for making your rendezvous — say, one or two days — you have more flexibility for launch opportunities, even if your chaser spacecraft starts out lagging far behind the station. Mission designers prefer to pick launch days on which the lag falls within a certain range. If it’s relatively far away, the chaser stays lower and faster for a longer period, to make up the lag. If the target is not so far away, the crew flies their ship higher sooner, to slow down the approach rate and arrive at the target at the same desired time.

The fast-rendezvous scenario, in contrast, has very little flexibility. The Soyuz has only a few hours to vary its altitude in order to accommodate a range of possible target distances. The range of acceptable distances between the chaser spacecraft and the space station is known as the "phase window." For a fast rendezvous, the phase window shrinks from what's typically about half of each orbit to as little as 5 percent of each orbit.

There are only a few launch opportunities when the precise time of the planar window also falls within the narrow slot of the phase window. That makes it harder to select an appropriate launch date for a fast rendezvous.

The job was easier back in the '60s, for the early rendezvous missions conducted by NASA and the Soviets. That's because those missions involved launching the target satellite first, and then launching the chaser no more than a few hours later. In such cases, the lag distance for the chaser's launch could be customized to fit the short range for a quick docking.

These days, the only way to approximate that required narrow slot in the sky is to have the International Space Station do an engine burn. This can push the station ahead or behind in its orbit, so that it happens to be at the proper distance at precisely the time when the Soyuz is launched.

That critical orbital maneuver took place a week ago: On March 21, a Progress cargo craft attached to the station fired its thrusters for 11 minutes and 13 seconds, pushing its orbital altitude from 253.5 to 255 miles (408 to 410.5 kilometers). It's just a mile and a half, but it's enough to ensure that the station will be in the right place, assuming that the Soyuz launches at the right time.

For all the virtuosity of the cosmonauts in their steering, the factor that makes the briefer trip at all possible is the target generously maneuvering itself right into the chaser’s sights. And for every quick rendezvous in the future, by Russian or American or other orbital vehicles, the same elaborate target line-up will be required.

More about orbital hookups:

NBC News space analyst James Oberg spent 22 years at NASA Mission Control, where he carried the title of Rendezvous Guidance and Procedures Officer — RGPO, pronounced "Arr-Jeep-O." In that capacity he sat in the center of Mission Control's front row, down in the legendary "trench" of space maneuvering specialists.