File image of a Progress spacecraft just prior to docking with the space station Alpha last year. Photo: NASA

An unmanned cargo freighter was unable to firmly latch onto the International Space Station on Wednesday following its successful automated rendezvous with the orbiting outpost.

The shuttle Endeavour, scheduled for blastoff Thursday, cannot dock with the station if the Progress is not firmly attached. If the issue is not resolved by Thursday morning, NASA managers could opt to delay Endeavour's fueling and, in a worst-case scenario, the launch itself.

Several hours after the 2:43 p.m. EST (1943 GMT) docking, Russian flight controllers were still unable to determine if hooks had driven closed to complete the two-step docking sequence.

The initial "contact and capture" of the Progress to the aft docking port on the station's Zvezda service module appeared normal as the two spacecraft flew 250 miles above Central Asia.

"Thanks guys. Good job," Russian mission control radioed the station crew moments after the docking.

But in the subsequent minutes when the hooks were supposed to engage to hold the Progress and station tightly together, engineers at the Russian mission control center outside Moscow didn't see the expected indication of a "hard dock".

As a result, the station remained in its "free drift" mode with its with attitude control turned off, which began at docking. This mode prevents the station from maneuvering itself during the crucial link up by the Progress vehicle.

Without seeing the indications that the Progress was firmly mated to the station, controllers kept the station's orientation control system off three hours longer than planned while troubleshooting was underway.

At about 6 p.m. EST, Russian flight controllers said the Progress was attached securely enough to allow the station's steering system to return to normal operation.

The extended length of the "free drift" period forced a reduction in power generation for the station, prompting some non-critical items onboard to be temporarily turned off to conserve electricity.

NASA said the items included heaters that control condensation on the shell of the Destiny laboratory module, Unity node and U.S. docking ports were turned off for a few hours along with a backup cabin air assembly and a contaminant monitor in the U.S. Quest airlock. Once the station returned to its normal orientation, all equipment temporarily powered off was powered back on with no impact to station operations.

Engineers will spend the night analyzing the situation with the Progress and some troubleshooting might be performed aboard the station on Thursday morning when the complex next moves within range of Russian ground tracking sites.

NASA sources say flight rules require the Progress to be hard docked to the station before a space shuttle would be cleared to visit the outpost. Mission managers have an already-scheduled meeting at 10 a.m. EST Thursday to review the status of the station and shuttle countdown before giving final approval to fuel Endeavour for launch.

If the Progress situation remains unresolved, it is possible managers might decide to postpone Endeavour's launch. A shuttle docking imparts forces on the station that could cause a wobbly Progress to damage the complex.

At this time, the shuttle is still slated for blastoff at 7:41 p.m. EST bound for Alpha carrying the Expedition Four crew.

The Progress, the sixth launched to the station, is carrying two-and-a-half tons of supplies, food, equipment and fuel for Alpha. Its docking capped a two-day trek to the station that began with launch from Baikonur Cosmodrome in Kazakhstan on Monday.

The supplies will be used by the Expedition Four crew of commander Yuri Onufrienko and flight engineers Carl Walz and Dan Bursch when they begin their five-month mission aboard Alpha following Endeavour's docking, which is scheduled for this weekend.

The craft's load of propellant will be transferred into the Russian segment of the station for use by the outpost's jet thrusters.

Progress 6P is planned to remain attached to the station for 80 days before undocking on February 16 to make way for the next cargo ship. The craft will be loaded with trash before departing the station to burn up in the atmosphere.