It was the second day in a row that a bolt was lost, astronauts on the Atlantis mission’s first spacewalk Tuesday also lost one. NASA says their loss poses no danger to either the ISS or the shuttle Atlantis.
During their time outside the ISS, which lasted seven hours and 11 minutes, astronauts Dan Burbank and Canadian Steven MacLean worked on the installation of a 16-tonne truss segment with two power-producing solar panels on the orbiting ISS. The two men were wearing spacesuits equipped for eight hours of survival.
Apart from the bolt loss and the tool which broke, "the spacewalk has gone very smoothly in terms of the activities and tasks," a NASA spokesman said.
It was the first spacewalk for both astronauts and after entering the station's decompression chamber, Mr MacLean said, "It was amazing over Canada."
Solar mission
Their mission was to prepare for the activation of a 360-degree rotation system for solar arrays so that it can rotate to track the sun.
They will eventually double the orbiting laboratory’s power capabilities. Once unfurled Thursday, the arrays will measure 73 metres (240 feet) long.
Mr Burbank and Mr MacLean, each armed with a large screwdriver, undid four restraints installed on the rotation mechanism to prevent damage when it was removed from the Atlantis cargo bay.
The restraints are located under covers held in place by four bolts that the astronauts were to remove and later replace. Two mishaps marred the first four hours of their mission.
A pistol-grip tool being used by MacLean broke and had to be replaced, NASA said.
Mr MacLean noticed that one of the bolts was missing on a cover. "There is a missing bolt on the blanket, I did not see it go," he told Mission Control at the Johnson Space Centre in Houston, Texas.
Space debris can be dangerous if it punctures space station walls or spacesuits and can jam crucial mechanisms. However, spacewalkers have a long history of losing things in space. In July, Discovery spacewalkers lost a 35 cm spatula that floated away.
This time NASA says there is little danger, "There is no indication the bolt is inside the rotary mechanism or in the ISS," a spokesman for the National Aeronautics and Space Administration said on NASA television. "We assume it has floated away in space."
Not easy
Removing the bolts was not easy for the astronauts who were wearing bulky spacesuit gloves. One stuck bolt on a launch restraint slowed down the process as the two astronauts struggled together to finally get it loose. Astronaut Joe Tanner from inside the space station helped out.
"We sure appreciate you answering that age-old question from Mission Control - how many astronauts does it take to unscrew a bolt," said astronaut Pam Melroy in Mission Control in Houston.
"Apparently, it takes three - two outside and one inside."
Tuesday’s spacewalk by Joe Tanner and Heidemarie Stefanyshyn-Piper also lost a bolt, when Mr Tanner saw a bolt glide away but was unable to retrieve it. Mr Tanner even asked his Mr MacLean to be on the lookout for it during his time out in space.
"I've been looking for that bolt all along but I haven't seen it," Mr MacLean said.
Mr MacLean also got the opportunity to informally inspect his shuttle’s underside, when he posed for a spacewalking photo. "That's about as photogenic as I get," he said.
Mr Tanner and Ms Stefanyshyn- Piper, the only woman in the six-member crew, will perform the mission’s last spacewalk Friday.
NASA plans 15 more shuttle trips to complete the ISS by 2010, when the three-shuttle fleet is due to be retired. Completing the space station is central to US ambitions to fly humans back to the Moon and eventually to Mars.