Roscosmos flight director Vladimir Solovyov said an autopilot signal went off just a few minutes before the scheduled docking showing that one of the engines had failed.
He said engineers then ordered cosmonaut Gennady Padalka to manually dock because of the possibility that an emergency computer program would have fired the Soyuz engines and sent it moving away from the orbiting station.
The docking by Padalka appeared otherwise smooth and slightly ahead of schedule, coming roughly two days after blasting off from the Baikonur cosmodrome in Kazakhstan's barren steppe. Russian cosmonauts typically receive extensive training in the event Soyuz's autopilot fails or some other problem pops up.
"We have to figure out what happened," he told a news conference at Russia's mission control in Korolyov, on Moscow's outskirts.
Applause broke out among space officials and crew relatives gathered at mission control after the hookup was announced.
The crews of the capsule and the station were to spend around three hours checking seals and instruments before opening the air locks and meeting up face-to-face.
Padalka and U.S. astronaut Michael Barratt will join the station's current crew, while Simonyi, who is making his second trip as a paying customer to the space station, returns to Earth on April 7 along with cosmonaut Yuri Lonchakov and NASA astronaut Michael Fincke.
Simonyi, a Hungarian-born software designer who helped build software for Microsoft Corp., is expected to be the last paying customer to travel aboard a Russian spacecraft to the station for the foreseeable future since the station's permanent crew is expanding from three to six.
During his time on the station, Simonyi plans to conduct medical and radiation experiments and chat with schoolchildren and family via communication links.
The arrival of the three Soyuz crew comes just three days after the departure of the U.S. space shuttle Discovery after wrapping up a 13-day mission highlighted by the successful installation and unfurling of the station's last pair of solar wings.
The touchdown of the Discovery and its crew was scheduled for late Saturday at NASA's spaceport.
On the Net:
NASA Web site: http://spaceflight.nasa.gov
Roscosmos Web site: http://www.roscosmos.ru/
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