A pair of astronauts had to pull out a hammer and crowbar while attaching a big new tank full of ammonia coolant to the International Space Station.
Making their second spacewalk in three days, Rick Mastracchio and Clayton Anderson banged and pulled and shoved, with no success, on a stuck bolt.
They undid the good bolts and jostled the 771 kg refrigerator-size tank in case it was misaligned. Finally, the troublesome bolt slid into its slot after two frustrating hours.
The astronauts inside kept urging the spacewalkers to take a break and rest their hands. But they insisted they were not too tired.
The 215-mile-high action unfolded on the 40th anniversary of the launch of Apollo 13.
Mission Control put off the fluid line hookups for the tank and a few other chores, saying they could be completed in the third and final spacewalk on Tuesday.
The empty tank will then be placed into the docked shuttle for return to Earth. Nasa plans to refill the tank and fly it back to the space station this summer as a spare, on what will be, the next-to-last shuttle flight.
Discovery will remain at the space station until Saturday. Landing is scheduled for April 19.