updated 9/23/2010 11:15:30 PM ET 2010-09-24T03:15:30

This story was updated at 10:50 p.m. ET.

A docking port malfunction is preventing a Russian Soyuz spacecraft from leaving the International Space Station as planned tonight (Sept. 23), forcing its three-person crew to remain in space for at least two extra orbits as engineers on Earth study the glitch.

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. But a set of hooks locking it to its docking port have refused to unlatch, NASA officials said.

"It is not clear as to why the hooks won't drive open, but we have passed the time for a nominal undocking," NASA commentator Rob Navias said in a NASA TV broadcast.  Engineers at Russia's Mission Control center near Moscow are studying the issue and working on backup plans, he added.

Russian flight controllers are now aiming for the second of two backup landing opportunities, which would call for an undocking time of about 12:35 a.m. EDT (0435 GMT) on Friday, and a landing time of just before 4:06 a.m. EDT (0806 GMT) on the Central Asian steppes of Kazakhstan. [ Graphic Inside and Out: The International Space Station ]

The landing will be some 248 miles (400 km) north of the original landing target zone, NASA officials said.

The Soyuz spacecraft is slated to return American astronaut Tracy Caldwell Dyson to Earth alongside her Russian crewmates Alexander Skvortsov and Mikhail Kornienko. They are wrapping up a nearly six-month mission to the International Space Station.

But in order to return to Earth, the hooks securing their Soyuz to the space station must be released.

The stuck hooks are on the space station's Poisk docking module, a chamber mounted to the top of the space station Russian segment. These hooks, along with others on the Soyuz side, ensure the spacecraft and station are securely attached while docked.

Russian flight controllers assured the Soyuz crew that they are still slated to return home tonight.

"There is no cancellation in the cards for the moment," a flight controller said.

Skvortsov said he and his Soyuz crewmates are doing well. Mission Control told them to take off their landing spacesuit gloves and get comfortable while they wait.

"We're feeling fine and we're ready to proceed," he said.

NASA will broadcast the upcoming Soyuz spacecraft undocking and landing  live on NASA TV. Undocking and landing coverage begins at 9:15 p.m. ET (0115 Sept. 24 GMT).  Click here  live space station mission updates and a link to NASA TV.

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