Stefanyshyn-Piper's slip occurred as she was heading to grease a rotary joint on the station's giant starboard solar array system. The joint has been unable to automatically point the solar wings toward the Sun for maximum energy production for over a year.
The tool bag became untethered from a larger kit case and floated away along with a pair of grease guns, wipes and a putty knife.
She reached out, but to no avail. The lost bag is now moving ahead of the station and increasing its distance to the platform, giving flight controllers confidence it poses no danger to the ISS.
"Despite my little hiccup, or major hiccup, I think we did a good job out there," Stefanyshyn-Piper said when she returned to the space station.
With her co-walker Steve Bowen, she spent some seven-and-a-half hours working outside the station. As well as lubricating the joint, the pair replaced a nitrogen tank assembly.
The flight is the fourth and final shuttle mission of the year.
Endeavour and its crew are to due to land back at Kennedy on 30 November.
Nasa had hoped to fly a servicing call to the Hubble Space Telescope last month but delayed the mission to May 2009 to prepare for some additional repair work on the observatory.
The space station marks its 10th anniversary on Thursday. The first component of the platform - the Russian Zarya module - was launched on 20th November 1998. The construction of the station is nearly complete and the international partners expect to be using it well into the next decade.
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