This story was updated at 7:08 a.m. ET.
An undocking system malfunction on the International Space Station has delayed the return to Earth for three astronauts in a Soyuz capsule by at least one day as Russian engineers try to understand what went wrong.
The problems began late Thursday (Sept. 23) when a set of hooks and latches on the space station's Russian Poisk docking port refused to release their grip on the astronauts' Soyuz spacecraft linked to the port.
The Soyuz crew ? two Russian spaceflyers and one American ? had hoped to undock and land on the steppes of Kazakhstan in Central Asia early Friday morning local time. They are now slated to land no earlier than early Saturday.
But flight controllers were unable to command the hooks into the undocking position, causing hours of delays and ultimately forcing the astronauts to abandon their undocking and landing plans for the day. Russian engineers suspect open electrical circuit may be to blame, NASA officials said. [Graphic ? Inside and Out: The International Space Station]
"Well, we're not going to have another attempt today for the undocking," a Russian flight controller radioed the station crew. "We're going to give the guys the go for the opening of the hatch and coming back into the ISS."
A fleet of 12 Russian recovery helicopters and other aircraft were gearing up for the astronauts' return to Earth aat the time.
Russian engineers at the station's Mission Control near Moscow are now aiming to try again late Friday night to undock the spacecraft.
The Soyuz TMA-18 spacecraft was slated to undock from the space station at 9:34 p.m. EDT (0134 Sept. 24 GMT) in order to return two cosmonauts and one American astronaut back to Earth after six months in orbit.
Aboard the craft were American astronaut Tracy Caldwell Dyson and her Russian crewmates Alexander Skvortsov and Mikhail Kornienko.
While trying to identify the cause of the glitch, Russian cosmonaut Fyodor Yurchikhin found a gear with two broken teeth, though it is unclear what role the broken part may have played, if any, in the evening's troubles.
Yurchikhin and two NASA astronauts ? Douglas Wheelock and Shannon Walker ? are remaining behind on the International Space Station to complete their own space mission.
The space station's Poisk docking port is on the top of the orbiting laboratory's Russian segment. It was delivered in 2009 and entered use for visiting Russian spaceships earlier this year.
The stuck hooks are on Poisk side of the docking port. These hooks, along with others on the Soyuz side, ensure the spacecraft and station are securely attached while docked.
During the undocking attempts, Russia's Mission Control center radioed words of encouragement to the Soyuz crew, telling the astronauts they could take off spacesuit gloves and get comfortable while they wait.
Skvortsov, who is commanding the Soyuz trip to Earth, said he and his Soyuz crewmates were doing well.
The scrapped undocking attempt capped a night of delays for the space station's full six-person crew. Earlier in the night, they had trouble sealing the hatch cover on the Poisk module docking port.
- Graphic ? Inside and Out: The International Space Station
- Video: Astronaut Describes Riding Home on a Rocket
- Gallery - Soyuz Spaceship's Snowy Landing
NASA will broadcast the upcoming Soyuz spacecraft undocking and landing live on NASA TV. Click here space station mission updates and a link to NASA TV.