This story was updated at 2:00 p.m. EDT.
Three astronauts on the International Space Station are set to return to Earth tonight (Sept. 24) ? a full day late - but not because of problems with their Soyuz landing capsule. It was a set of stuck hooks on the station that thwarted their first homecoming attempt.
But how did the unprecedented undocking glitch happen in the first place? Russian engineers narrowed the list of electronic suspects to a faulty microswitch that may have caused an open circuit in the docking port's wiring.
At the core of the malfunction is a set of hooks and latches on the space station's Russian Poisk docking port, which refused to release the astronauts' Soyuz capsule last night.
The glitch was fixed overnight and the astronauts will try to leave the space station again tonight at 10:02 p.m. EDT (0200 GMT) and land early Saturday on the steppes of Kazakhstan in Central Asia.
The microswitch issue popped up early in Thursday night's undocking preparations when astronauts had trouble confirming the hatch between the Soyuz capsule and space station had a tight seal. After extended leak checks, however, the astronauts pressed ahead with their undocking plans.
What happened next was unexpected.
Russian flight controllers were unable to command the hooks and latches on the Poisk module to release the Soyuz spacecraft. The Soyuz has its own system of hooks and latches as well, but the ones on the Poisk module must be released first. [Graphic ? Inside and Out: The International Space Station]
"Basically, two sets of hooks close around each other in a docking interface," NASA spokesman Rob Navias told SPACE.com. "Last night, the microswitch indicator was not providing any feedback that it was receiving electrical continuity through the circuitry."
This was the same component that had trouble confirming the seal between the Soyuz capsule and Poisk module, Navias said. At the moment, engineers are unsure if the two hiccups are related.
"It's possible," Navias said. "It's not conclusive at this point but the data certainly point to a similar issue that would lead back to that microswitch indicator."
The glitch caused hours of delays and, eventually, forced Mission Control and the astronauts to scrap undocking and landing plans for a day. The Soyuz crew ? NASA astronaut Tracy Caldwell Dyson and Russian cosmonauts Alexander Skvortsov and Mikhail Kornienko ? returned inside the space station, and 12 Russian recovery helicopters awaiting their landing in Kazakhstan went back to their home airfields.
Overnight, the space station crew rigged up cables to fix the docking port to allow tonight's Soyuz departure.
"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," NASA officials said in a status update. "Once the jumpers were in place, the Poisk module hooks and latches were successfully opened."
The hook-and-latch system on the Soyuz remains closed, securing the spacecraft to the station, they added.
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.
This will be the first undocking conducted at the Poisk module, so Russian engineers will be closely analyzing the problem, Navias said. The Poisk module will also be the same port that the next trio of space station crewmembers arrive at on Oct. 10.
"The current plan is to keep these jumper cables in place, so that there is a commanding path that is assured to reclose the hooks after the new Soyuz arrives at the station on October 10," Navias explained. "They'll bring up some spare parts and do some troubleshooting so that this problem is alleviated in the future."
The module is just over 13 feet (4 meters) long, about 8 feet (2.5 meters) wide and weighs about 8,000 pounds (17,636 kg). It has about 380 cubic feet (10.7 cubic meters) of living space inside.
While trying to identify the cause of the docking problem, Russian cosmonaut Fyodor Yurchikhin found a small star-shaped gear with two broken teeth, though it was 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 later this year.
- 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.