It was one of the largest items ever to be lost by a spacewalker, and occurred during an unprecedented attempt to clean and lube a gummed-up joint at the international space station.
Heidemarie Stefanyshyn-Piper was just starting to work on the joint when the mishap occurred.
She said her grease gun exploded, getting the dark gray stuff all over a camera and her gloves. While she was wiping everything off, the white, backpack-size bag slipped out of her grip, and she lost all her tools.
"Oh, great," she mumbled.
She and her fellow spacewalker, Stephen Bowen, then finished their tasks in almost seven hours by sharing tools. Bowen had his own tool bag with another set of grease guns, putty knives and oven-like terry cloth mitts to wipe away metal grit from the joint.
"Despite my little hiccup, or major hiccup, I think we did a good job out there," Stefanyshyn-Piper said after returning to the space station.
Flight controllers were assessing the impact the lost bag would have on the next three planned spacewalks. The astronauts may be asked to keep sharing tools or use caulking-style guns intended for repairs to the shuttle's thermal shielding.
Earlier, the spacewalkers spotted a screw floating by, but were too far away to catch it. "I have no idea where it came from," Stefanyshyn-Piper told Mission Control.
Flight director Ginger Kerrick said neither the bag nor the screw posed hazards to the spacecraft. By late Tuesday, the bag was already 2½ miles in front of the shuttle-station complex. "It's well on its way away from us," she assured reporters.
NASA was not sure how the bag got loose; it should have been tethered to a larger equipment bag. Another unknown: why the grease gun discharged.
"It is a human endeavor. Mistakes can happen. Equipment can fail," said John Ray, the lead spacewalk officer in Mission Control. He noted that Stefanyshyn-Piper showed "real character and great discipline" by continuing on and doing a fine job for the rest of the spacewalk. She was the first woman to be assigned as lead spacewalker for a shuttle flight.
The lost bag marred what had been a near-flawless mission by Endeavour and its seven-member crew.
For more than a year, the jammed joint has been unable to automatically point the right-side solar wings toward the sun for maximum energy production. The repair work - expected from the outset to be greasy and hand-intensive - is supposed to take up much of all four spacewalks.
The joint is located near the extreme reaches of the 220-mile-high outpost. The spacewalkers had 85-foot safety tethers to keep them connected to the mother ship at all times.
NASA suspects a lack of lubrication caused the massive joint to break down; grinding parts left metal shavings everywhere and prompted flight controllers to use the joint sparingly. Besides scraping and wiping away the grit and applying grease, the spacewalkers will replace the bearings.
As a precaution, extra grease will be applied on a later spacewalk to the joint on the opposite side of the space station that has allowed those solar wings to produce ample electricity.
As the action unfolded outside, the astronauts inside the shuttle-station complex started unloading gear from a huge trunk that was brought up by Endeavour.
The big-ticket item - and one of the first things to be hooked up - is a recycling system that will convert astronauts' urine and sweat into drinking water. It is essential if NASA is to double the size of the space station crew to six next June.
Endeavour also delivered an extra bathroom, kitchenette, two bedrooms, an exercise machine and refrigerator that will allow space station residents to enjoy cold drinks for the first time.
The additions - coming exactly 10 years after the first space station piece was launched - will transform the place into a two-bath, two-kitchen, five-bedroom home.
Endeavour arrived at the space station Sunday. The next spacewalk is set for Thursday.
On Tuesday, mission managers cleared Endeavour to return to Earth at the end of the month. A thorough inspection of images of the shuttle showed no evidence of any damage to its heat shield like the kind that doomed Columbia in 2003.
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