HOUSTON —  A spacewalking astronaut accidentally let go of her tool bag Tuesday after a grease gun inside it exploded, and helplessly watched as the tote and everything inside floated away.

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 on a solar panel.

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 wiping off herself, the white, backpack-size bag slipped out of her grip, and she lost all her other tools.

"Oh, great," she mumbled.

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Stefanyshyn-Piper was carrying out the spacewalk with Stephen Bowen. He had his own tool bag with another grease gun, putty knife and oven-like terry cloth mitts to wipe away metal grit from a clogged joint at the space station.

Mission Control agreed the spacewalk would continue as planned, and that the two astronauts would share tools. Flight controllers were assessing the impact the lost bag would have on the next three planned spacewalks.

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.

Mission Control said the screw was not considered a serious hazard, but did not immediately elaborate on the missing tool bag. Flight controllers were tracking its location in orbit.

The lost bag marred what had been a near-flawless mission by Endeavour and its seven-member crew.

Putting her disappointment aside, Stefanyshyn-Piper — the first woman to be assigned as lead spacewalker for a shuttle flight — carried out her work on the joint with Bowen.

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 the gear inside 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 shuttle will remain docked through until at least Thanksgiving. The next spacewalk is set for Thursday.