Controllers in Russia and at Johnson Spaces Centers ISS Mission Control center have been going through the paces for some time now.

"Theres just a ton of activity going on. Theyre (Moscow) doing simulations every day in the month of June," said Mark Ferring, lead flight director at Johnson Space Center (JSC). "We are doing a bunch of simulations ourselves."

Controllers in Houston are also practicing operation of the station after Russian controllers dock the two units together. Once docked, the modules computer systems must be linked together, followed by the handing of electronic control of the station from the Zarya to the newly orbiting Zvezda.

Before handing off to the U.S. side of the operation days later, controllers based near Moscow are in the hot seat as they merge the two independent spacecraft into one.

About two hours before docking, the Zvezdas computers will command it to make the first of two final engine burns to close in on the 50-foot- (15-meter-) long ISS. The thrusters also will be fired about 45 minutes before docking.

Then, the largely automated service module simply waits. Its solar arrays are pointed away from the ISS to prevent any engine propellant from damaging them.

"The ISS is the active vehicle during the final rendezvous and docking phase," Ferring said. "The Zvezda is basically sitting there waiting for something to come into contact with it."

At about 3 miles (5 kilometers) out, the KURS radar docking system aboard the ISS will begin communicating with Zvezda to determine the closing rates, distance and the orientation of each craft.

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The ISS has two KURS systems aboard. The first unit has showed some erratic behavior and will be saved as a backup. Engineers have developed a software patch and will turn off unused electronic equipment that is suspected of causing the problem.

The KURS continues to exchange information as the ISS maneuvers to a position slightly above and about 220 yards (200 meters) from Zvezda.

Russian ground stations will be in range by now and will analyze the data sent by the vehicles. If all is well, controllers will command the KURS aboard the ISS to close in at about 6 feet (2 meters) per second and dock with Zvezda.

Once docked, the Zvezdas solar arrays will begin tracking the sun again to provide power to the module.

Should the two craft be unable to dock on the first attempt, ground controllers will abort the effort and spend some time troubleshooting what went wrong.

Ferring said the ISS has enough propellant to make at least two docking attempts. He suspects there may possibly be enough fuel for several more.

"All our calculations are worse-on-worse assumptions," he said. "Well know for sure after a couple of attempts if theres more fuel available.

If all attempts to dock the two craft fail, then Crew Zero swings into action about two weeks later.

A Soyuz with two cosmonauts aboard will lift off from the Baikonur Cosmodrome to manually dock the ISS with Zvezda by using the Russian TORU manual docking system.

"It will be possible to position the two space objects and provide the necessary parameters for docking," said veteran cosmonaut Yuri Malenchenko through a translator about the TORU system.

The cosmonauts would dock the Soyuz to the rear of the Zvezda and spend two days setting up the docking system that uses a video feed and joystick system, much like a video game. They would then use the system to guide the ISS to the service module.

Malenchenko used the same system in 1994 while aboard Mir. A Progress supply vehicle failed to dock automatically and Malenchenko manually docked the craft. He said he is confident the cosmonauts are more than up to task should the need arise.

"Theyre well trained and have trained for a long time," he said. "We can be quite sure they will be able to perform the task with quite [a] high probability of success."

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As long as the cosmonauts are visiting the station, in effect as the unofficial first crew, theyll spend a few weeks there after docking. The duo will prepare the station for the arrival of a Progress supply ship and Space Shuttle Atlantis upcoming mission to outfit the newest addition to ISS.

Once the three ISS pieces are joined, controllers at JSC will begin the process of turning them into one spacecraft, physically and electronically.

During the two to three days following docking, controllers will pressurize the vestibule linking the Zarya and Zvezda and check for leaks between the two craft. If leaks are detected, controllers will undo the latches and pull the craft together to reseat them.

Following the leak checks, about four days after docking, controllers will unify the computer systems of the three modules, effectively turning them into a space station.

The next visitors will arrive aboard Atlantis in September as the drive to finish outfitting the station for the first resident crews arrival in November continues.