Discovery’s rendezvous and docking with the International Space Station actually begins with the precisely timed launch of the shuttle on a course for the station. During the first two days of the mission, periodic engine firings will gradually bring Discovery to a point about eight nautical miles behind the station, the starting point for a final approach to the station.


Terminal Phase

About three hours before the scheduled docking time on Flight Day Three, Discovery will reach the point about eight nautical miles behind the ISS. At that time, Discovery's jets will be fired in a Terminal Phase Initiation (TI) burn to begin the final phase of the rendezvous. Discovery will close the final eight nautical miles to the station during the next orbit. As Discovery closes in, the shuttle’s rendezvous radar system will begin tracking the station and providing range and closing rate information to the crew.



As Discovery begins to close in on the station, flight controllers will command the station to the docking orientation, perpendicular to the Earth's surface with the Unity module pointed toward space and Zarya toward Earth. For Discovery's final approach, the solar panels on the station will be "feathered" to have their edge facing the shuttle. Also, when within about 1,000 feet of the station, Commander Kent Rominger will use Discovery's steering jets in a mode called "Low Z," a mode that fires jets offset rather than pointed directly at the station to control the shuttle's approach. This jet mode together with the solar panel's feathering prevent exhaust from shuttle jets from flexing and overstressing the station's solar panels.


Proximity Operations

As Discovery reaches close proximity to the station, the Trajectory Control Sensor, a laser ranging device mounted in the shuttle payload bay, will supplement the shuttle’s onboard navigation information by supplying additional data on the range and closing rate. As Discovery closes in, the shuttle will have the opportunity for several small successive engine firings to fine-tune its approach using its onboard navigation information. As Discovery moves to within about a half-mile of the station, Rominger will take over manual control of the shuttle's approach, flying the spacecraft from the aft flight deck and monitoring the approach through the rear and overhead windows.

Rominger will slow Discovery's approach and fly to a point about 500 feet directly below the station, from which he will begin a half-circle of the station. During the rendezvous, Rominger will be assisted by Pilot Rick Husband in controlling Discovery's approach. Mission Specialists Tammy Jernigan and Ellen Ochoa also will play key roles in the rendezvous, with Jernigan operating the shuttle's docking mechanism and Ochoa assisting with the rendezvous navigation. The crew will use a handheld laser pointed through the shuttle windows to provide supplemental information on the station's range and closing rate.

Rominger will fly Discovery to cross about 350 feet in front of the station and then continue to reach a point about 250 feet directly above the station. He will stop Discovery and then slowly descend toward the station to a point about 170 feet away from it. At approximately 170 feet distant, Rominger will hold Discovery's position for about a half-hour to allow the station to move within range of Russian communications stations. Also at this time, Jernigan will power up the docking mechanism and prepare it for contact and capture with the station. A day ahead of the rendezvous, the docking mechanism is scheduled to be checked out and its docking ring extended.


Final Approach

Once the station is in communications with Russia and flight controllers have confirmed it is in the proper configuration for docking, Rominger will begin the final approach.

Using the view from a centerline camera fixed in the center of Discovery's docking mechanism, Rominger will precisely align the shuttle mechanism with the docking mechanism on Unity's conical mating adapter as he approaches. At a distance of about 30 feet, Rominger will stationkeep momentarily to adjust the alignment, if necessary.


Docking

For Discovery's docking, Rominger will maintain the shuttle's speed relative to the station at about one-tenth of a foot per second, and keep the docking mechanisms aligned to within three inches of one another. As Discovery moves to within 25 feet, Rominger will switch the shuttle steering jets back to normal mode from "Low Z" to provide better control of Discovery for the final docking.

When Discovery makes contact with the station, latches will automatically attach the two spacecraft together. Immediately after Discovery docks, the shuttle's steering jets will be deactivated and ground controllers will command off the station's steering jets to reduce the forces acting at the docking interface. Shock absorber-type springs in the docking mechanism will dampen any relative motion between the shuttle and the station.

Once relative motion between the spacecraft has been stopped, Jernigan will command the docking ring on Discovery's mechanism to retract and close latches in the mechanism to firmly secure the shuttle to the station. Once the two vehicles are firmly secured, the shuttle’s steering jets will be reactivated to control both spacecraft for the duration of the docked operations.