The thrusters on Zvezda are fired to boost the craft's orbit following launch. Photo: NASA

The command module Zvezda, Russian's centerpiece contribution to the international space station, is poised to dock with the outpost high above Earth tonight.

Launched July 12 atop a Proton rocket, Russian flight controllers have spent two full weeks checking out and activating Zvezda's systems and maneuvering the spacecraft into the proper position for docking. The final phases of the rendezvous, however, will be carried out by the Zarya module and the international space station with Zvezda playing a passive role.

Docking is expected around 8:44:30 p.m. EDT (0044:30 GMT Wednesday). Spaceflight Now will running updates in our Mission Status Center and a live QuickTime streaming video broadcast starting at 8 p.m. EDT (0000 GMT).

Two orbits prior to the docking, as the Zvezda and the space station pass over Russian ground stations, Zarya's thrusters will be fired twice to fine tune the approach to the service module.

"Then one orbit prior to docking is when all the action starts happening," Ferring said. "The ISS is the active vehicle during the final rendezvous and docking phase, it does all of the control. The Zvezda is basically sitting there waiting for something to come into contact with it."

The Russians will activate Zarya's "regime nine" rendezvous software and make sure it is executing properly. Then they will test the KURS antenna system used to communicate with and track the target Zvezda module. Long-range guidance will be initiated with Zarya's computers measuring relative motion between the two spacecraft.

"Then when they get to a range of about five kilometers (3.1 miles), when the relative rates between each other is around one-and-a-half meters per second (3.4 mph), they transition into a short-range guidance mode and this mode will perform a range adjustment between the two vehicles down to about 300 meters (1,000 feet)," Ferring said.

"At that point, the ISS is flying above the service module in a relative sense. It flies over the service module with the target being to stop behind the service module on the velocity vector."

After a final calibration of the KURS rendezvous system, range, velocity and angular alignment data will be generated using a set of three high accuracy antennas. Then, about 10 minutes before the spacecraft sail back into contact with Russian ground stations, the station will be poised about 200 meters (656 feet) directly behind Zvezda.

"And then it waits and doesn't do anything until it receives a command from the Russian ground sites," Ferring said. "When they acquire the signal, there's about five minutes where the Russian flight control team monitors all the data and they decide whether everything looks good. And if it does, they command it to proceed to docking and that final phase will occur completely autonomously."

The two spacecraft will come together at a relative velocity of about 0.2 meters per second (0.45 mph). Small thrusters will fire to ensure capture and Zarya's motion control system will be shut down.

The space station after Zvezda's arrival. The Zarya module is located in the center and Unity is on the right. Photo: NASA

The KURS automatic rendezvous system is critical to the success of the docking. The KURS system has two redundant "strings," or sets of equipment. One of them has shown a higher-than-expected sensitivity that has caused it, on occasion, to fail its start-up health test.

Ferring said software patches and filters have been written that will force that equipment to pass its self test in the unlikely event the primary KURS string suffers a failure.

The Zarya module has enough on-board fuel to make at least two docking attempts and possibly a third. As a last resort, a manually operated TORU rendezvous system is available if Zvezda is unable to dock on its own.

To protect against that possibility, however remote, two cosmonauts will be standing by at Baikonur, ready to blast off on a Soyuz rocket to carry out a manual docking.

It was the TORU system that was in use in 1997 when a Progress supply ship crashed into the Mir space station's Spektr module during a test that later was determined to have been especially challenging. If a manual docking is required for Zvezda, the conditions would be much more benign for the service module's crew and Malenchenko said he would not expect any problems.

Assuming the automated rendezvous and docking go smoothly, however, a probe in the docking system will retract and 12 hooks on the Zarya side of the interface will drive closed. After that has been confirmed, the solar arrays on both modules will begin tracking the sun once again to recharge on-board batteries.

"They'll take one orbit in free drift to make sure everything looks OK with all the mechanisms," Ferring said. "At the next daily orbit after they dock, they'll manually command 12 hooks on the service module side to engage so now there's 24 hooks holding the surfaces of the two vehicles together. And at that point, they'll start doing active attitude control with the service module."

After three days of leak testing, computers in the Zvezda module will assume central control of the international space station.

"At that point, all the computers in the three different modules are communicating with one another," Ferring said. "We control our Unity from Houston, they control the Russian segment from Moscow. They also monitor their data from Houston, but they do their control primarily via Moscow."