HOUSTON —  The astronaut who lost her tool bag on a spacewalk admitted Wednesday that she made a mistake by not checking to see if the sack was tied down, and said she's still smarting over the whole thing.

Heidemarie Stefanyshyn-Piper said in an interview with The Associated Press that it was "very disheartening" to lose her bag full of tools.

She was trying to clean up grease that had oozed out of a grease gun in the backpack-size bag, when the tote and everything in it floated away Tuesday.

The bag was one of the largest items ever lost by a spacewalking astronaut.

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For a split second, she thought she might be able to grab it and she tried to judge how far away it was.

Just as quickly, "I thought, no, that would probably just make things worse and the best thing to do would be to just let it go."

"There's still the psychological thing of knowing that we made a mistake and having to live through that," she said. "During the spacewalk ... it was easy to put it aside because I knew that we still had five hours of spacewalk work to do and the work needed to get done and you can't dwell on a mistake. It was hardest coming back in and having to face everybody else."

She noted there were three more spacewalks and promised not to let the mistake happen again.

"You're not going to see us lose another bag. We're going to double- and triple-check everything from here on out," she said.

The next spacewalk is Thursday; Stefanyshyn-Piper will venture back out of the international Space Station for more work on a jammed joint at the space station that controls some of the solar wings.

Her spacewalking partner Tuesday, Stephen Bowen, also took the blame for the mishap.

"I didn't go back and triple-check everything. So I'm just as guilty at this as Heide is," Bowen told AP.

In the packing and repacking of all the tools and sacks, it's possible that bag became untethered, Stefanyshyn-Piper said.

Flight director Ginger Kerrick was withholding judgment.

"We don't know that this incident occurred because they forgot to tether something. We don't know if perhaps the hook just came loose inside the bag," Kerrick stressed at a news conference. "You've got to remember, we are working with humans here and we are prone to human error. We do the best we can, and we learn from our mistakes."

Kerrick said precautions will be taken to help prevent any more leaking grease guns or loose bags.

The grease guns will be attached to the outside of the bag rather than packed inside, to prevent the plungers from being activated accidentally.

Stefanyshyn-Piper said some air might have gotten in with the grease and forced the grease out. She said it seeped out in half-inch and one-inch bits and was all over the inside of the bag.

Two grease guns were in the bag that got away. That leaves just two guns aboard the space shuttle Endeavour and space station, each with a different nozzle for reaching different parts of the clogged joint.

Meanwhile, the bag that got away was still in the neighborhood of the shuttle-station complex but was expected to fall out of orbit fairly soon.

During Thursday's spacewalk, Stefanyshyn-Piper and Shane Kimbrough will share the grease guns and alter their work in order to cut down on the guns' use.

There is a caulking-style gun on Endeavour that is intended for shuttle repairs, but space station flight controllers won't borrow it unless absolutely necessary, Kerrick said.

The gummed-up joint is full of metal shavings, the result of grinding parts. It's been used sparingly for more than a year and, as a result, has hindered energy production at the space station.

Over the course of four spacewalks, astronauts will clean and lubricate the joint and replace the bearings inside, and do a little work on another joint. The first three spacewalks concentrate on the bad joint; the fourth will focus on the good joint.

The joint is supposed to keep the solar wings on the right side of the space station pointed toward the sun. A twin on the left side is operating fine, but the astronauts will squirt on some extra grease as a precaution.

In other space station business Wednesday, the astronauts hooked up some of the gear that was delivered by Endeavour, including a new recycling system for turning urine into drinking water.