For a split second, astronaut Heide Stefanyshyn Piper thought about going after the tool bag that had floated away.
"I thought maybe I can jump for it and grab it," she said. "But then we would have two floating objects, and one of them would be me -- so the best thing was just let it go, and it was very disheartening to watch it float away."
The price tag for the lost bag was $100,000. It wasn't even a designer bag. But NASA can't buy parts off the rack, and everything that flies into space must be certified.
Piper was several hours into a grueling seven-hour spacewalk outside the International Space Station when a grease gun exploded in her tool bag.
"Oh great," she muttered, clearly frustrated with the unexpected mess and the expected loss.
She was trying to clean the mess off her gloves and a camera when the tool bag slipped away and drifted off into space.
The tool bag is one of the largest items to be lost on a spacewalk. It contained two grease guns, a putty knife and some cleaning wipes.
The loss of the tool bag, which had apparently slipped from its tether, complicates the remaining spacewalks, which are needed to finish work on a broken joint on the solar array. The Space Shuttle Endeavour didn't carry up extra grease guns or much spare grease.
In lieu of the grease guns, flight controllers are considering using caulk guns that were designed to fix a hole on the space shuttle. NASA has to trade the risk of using one of those guns to fix the solar array, with the possibility of needing the gun to fix a space shuttle.
"What it boils down to, all it takes is one small mistake for a tether not to be hooked up correctly," spacewalk Officer John Ray said. He complimented Piper, though. "Heide did a great job on the rest of the EVA [extravehicular activity]. She showed great character and recovered well."
Spacewalks look deceptively easy when people are watching them on TV -- with astronauts floating around above Earth with great views in zero gravity. But the truth is, an EVA is hard work.
Astronaut Piers Sellers has spent 41 hours on spacewalks. He told ABC News that before STS 121, his last mission, what makes it such a challenge.
"It's a little bit like standing in a little rowboat with an ocean swell, trying to paint the side of a ship that you're up against," he said. "You have to compensate for the waves and the sways. But you can do that, people do it every day with rowboats. And we'll see if we can do it in space."
Sellers lost a spatula on one of his spacewalks. He called it his favorite spatula. Once something slips out of your hands on orbit, it is gone, destined to become just another piece of space junk -- what is known in NASA-speak as "orbital debris."
Veteran spacewalker Jim Reilly said it is quite tiring to work in a spacesuit. It may look like a big, fluffy marshmallow -- but it is really a small spaceship.
"The thing that probably affects you more and makes you more tired is operating in a low vacuum, or low pressure," he said. "We are working inside of a suit that is pressurized at 4.3 PSI, so it is fairly rigid, so you have to learn how to do things differently. You just don't reach up and grab it like this, otherwise you will wear yourself out."
NASA is tracking the lost tool bag to make sure it doesn't crash into the International Space Station or Endeavour. Flight controllers said it is orbiting somewhere near the space station and expect it to deorbit soon and burn up on re-entry through Earth's atmosphere.