By Conor Humphries

KOROLYOV, Russia (Reuters) - Two Russian cosmonauts and a U.S. astronaut aborted a return to Earth on Friday when their space capsule failed to separate from the International Space Station.

"This situation has never occurred before," a spokeswoman at Russian Mission Control near Moscow said, as technicians and space officials tried to discover the cause. The descent was rescheduled for Saturday, roughly 24 hours after the initial attempt.

Latches holding a Soyuz capsule to a docking port failed to open, the spokeswoman said, preventing Russians Alexander Skvortsov and Mikhail Korniyenko, and NASA's Tracy Caldwell Dyson from returning to Earth after nearly six months in orbit.

NASA later said the latches had successfully opened after crew members rigged up a solution to the problem.

"Space station crew members installed a series of jumpers, bypassing a failed component that had prevented commands from being received by the Russian Poisk module's docking mechanism," the U.S. space agency said on its website nasa.gov.

Russia's space agency chief Anatoly Perminov said the docking mechanism did not function because the station's computer sent a false signal indicating the hatch between station and capsule was not fully sealed.

Technicians had found no problem with the seals, he said, and suggested they were still puzzling over exactly went wrong.

A second undocking attempt "could have gone ahead today, but we need additional time to make sure we have reliable information about the problem," Perminov told a news conference at Mission Control.

"There is no point in rushing," he said. Perminov refused to take questions, saying he did not want to fuel rumors.

NASA said the crew had sent down video and photographs of a small star-shaped gear with two broken teeth. Experts were trying to determine whether it was related to the problem.

MISHAPS

Dyson, Skvortsov and Korniyenko clambered out of the Soyuz and back into the station after the aborted departure. NASA said they would get some sleep before starting preparations for the fresh attempt.

The other three crew members, Russian Fyodor Yurchikhin and NASA astronauts Doug Wheelock and Shannon Walker, would remain aboard the station as planned after Saturday's departure.

"I see no technical problem on the station or anywhere that would threaten the crew," Perminov said.

Another Russian space agency official, Alexei Krasnov, said the landing should now take place on Saturday at about 9:20 a.m. Moscow time (0520 GMT).

The U.S.-Russian trio boarded the space station on April 4 after a flight up together in the Soyuz TMA-18 craft, which will also be used for their return to Earth.

At least two mishaps have already marred their stay.

Failure of one of two cooling systems in August set off equipment shutdowns. In July, a faulty radio link forced an unmanned cargo ship to delay docking for two days.

U.S. space shuttles have delivered some astronauts to the station, but single-use Russian Soyuz craft will ferry all crews after the NASA retires its shuttle fleet next year.

Earlier this year, Russia announced a halt to trips by millionaire space tourists to free capacity on Soyuz flights as the station has expanded to accommodate a permanent crew of six.

(Writing by Conor Humphries and Steve Gutterman; Editing by David Stamp)