Astronauts from the space shuttle Discovery struggled through a difficult day on Sunday thanks to a mysterious pressure leak and worries about some critical flight software.
The major morning job for the Discovery crew, which has been working on the International Space Station for more than a week, was to remove the Italian-made Leonardo cargo module from the station and return it to the shuttle for the return flight to Earth.
But a small vestibule compartment between the space-station and the 21-foot cylindrical module twice began to refill with air after the crew tried to depressurize it.
NASA was about to send an astronaut into the compartment to investigate when one of the space-station crew, James Voss, discovered the problem - a leaky connection on an air hose that could be safely ignored.
Earlier, the Discovery crew spent hours testing the software on two shuttle computers that had been turned on too quickly.
The crew was running more than five hours behind schedule by the time the cargo module began its slow separation from the station, with Earthlight finally peaking through the gap.
Shuttle commander James Wetherbee told ground controllers that he knew the delays were making for a "long day."
"It's just fun working with you," deadpanned Mission Control's Gerhard Thiele, who was working the communications console.
This is not the first time the crew has fallen behind in its work. A 13th day was added to the flight when astronauts could not fill Leonardo with garbage and used station parts in time to make the scheduled departure.
A routine spacewalk stretched into a record nine-hour marathon after an astronaut fumbled a piece of hardware and had to find a replacement.
And the shuttle was even late docking to the station March 10 after one of the station's giant solar arrays failed to lock into a safe position.
Despite the series of delays, the shuttle crew has met all their mission objectives. They delivered about five tons of supplies and hardware to the station along with the second crew to take up residence.
They also installed the first early components of a Canadian-built robot arm that will arrive at the station next month. The arm is a bigger, smarter and more dexterous version of the one that astronaut Andrew Thomas used to pluck Leonardo from the space station's walls and nestle it in the shuttle's payload bay.
When Discovery leaves the station Sunday night, the first three astronauts to live aboard the station will be returning home. The Expedition One team of American William Shepherd and Russians Yuri Gidzenko and Sergei Krikalyov have lived on the station since Nov. 2.
The Expedition Two crew of Russian Yury Usachev and Americans Susan Helms and James Voss will spend four to six months on the orbiting outpost.
The computer software problem could have delayed the shuttle's return by yet another day if the software had been corrupted and had to be reloaded.
Two of the shuttle's General Purpose Computers were turned on too quickly Saturday. Guidelines call for them to be activated 10 seconds apart, but the astronauts, who were not working from a set procedure, took only six seconds, NASA said. Four computers in all might have been effected by corrupted software.
NASA had Wetherbee and pilot James Kelly run the lengthy diagnostic test to reassure ground teams that the computers were operating normally.
"We have full confidence that things will work from now on," Mission Control told the astronauts.
"Thank you very much for all the analysis and help," Wetherbee said.
The problems began when NASA noted some icing along coolant lines that could have harmed the shuttle's electrical systems. Heating the shuttle meant turning on the computers, and that led to the quick start and "an intense day of investigation and analysis," said NASA spokesman Rob Navias.
The $150 million Leonardo module is making its maiden flight.
The space station, still under construction, is a joint project of space agencies in the United States, Russia, Europe, Japan and Canada. NASA expects to spend $95 billion to build and then operate the facility for a decade or more.