One of two U.S. astronauts who had set out to begin maintenance on the International Space Station Tuesday watched her tool bag float out into orbit after a grease gun she was carrying inside it exploded.

Astronaut Heide Stefanyshyn-Piper rides the International Space Station's Canadarm 2 to space shuttle Endeavour's payload bay on Tuesday. (NASA TV)

Heidemarie Stefanyshyn-Piper was just starting to work on a jammed solar panel joint when the mishap occurred. The astronaut said she was attempting to wipe the oily goo off her gloves and camera when she lost her grip on the backpack-sized bag, which held the rest of her tools.

The tool pack is one of the largest items to be lost by a spacewalker.

"Oh, great," Stefanyshyn-Piper said as she watched her bag drift off into space.

Mission Control did not immediately indicate whether the lost item posed a major risk, but flight controllers were tracking its location in orbit. Mission Control did say the spacewalk will continue as planned.

Stefanyshyn-Piper and Steve Bowen left the station in their spacesuits at 1:09 ET p.m. armed with putty knives and grease guns to scrape away metal grit and lubricate the starboard solar alpha rotary joint.

NASA hoped cleaning and lubricating the joint and replacing its trundle bearings would allow the station's solar panels to once again turn in the direction of the sun and more efficiently draw energy to power the station.

Astronaut Heidemarie Stefanyshyn-Piper, STS-126 mission specialist, attired in her shuttle launch and entry suit, on the middeck of Space Shuttle Endeavour. (NASA)

The two astronauts were also to lubricate the rotary joint on the opposite side of the station in what they hope will prevent similar problems.

The repair work is expected to take up much of all four spacewalks but with one tool bag down, astronauts will now be forced to share supplies, according to Mission Control.

Getting the station's solar panels at maximum efficiency will be essential once the station expands its crew from three to six members in May 2009.

Monday, the shuttle crew and three space station crew began unloading cargo as part of an outer space home-improvement mission that will allow the station crew to expand from three to six members next year.

Among the items unloaded were a second toilet for the station, an exercise machine, kitchenette, two sleeping compartments and a water recycling system that will allow astronauts to purify urine and sweat, turning it into drinkable water.

Tuesday's spacewalk is the first of four scheduled excursions for the crew of the space shuttle Endeavour, which arrived at the station on Sunday and will spend close to two weeks there.

During the spacewalk, which is scheduled for 6½ hours, the two astronauts will also replace a depleted nitrogen tank and a device used to help coolant flow along the station's main truss. They will remove covers from the front of the Japanese Kibo module in preparation for the installation of an exterior addition to the module, scheduled to arrive in 2009.