Published: Apr 12, 2010 11:52 AM EDT

(AP) Spacewalking astronauts had to pull out a hammer and pry bar while attaching a big, new tank full of ammonia coolant to the International Space Station on Sunday.  They successfully drove in a stiff bolt, after two frustrating hours.

The 215-mile-high action unfolded on the 40th anniversary of the launch of Apollo 13.

The spacewalk stretched on for a marathon 7 1/2 hours, and the astronauts inside kept urging the spacewalkers to take a break and rest their hands.  However, they insisted they weren't too tired.
    
Several chores were put off until Tuesday's spacewalk, the third of the mission.  An empty tank will be placed into the docked shuttle for return to earth during that walk. NASA plans to refill the ammonia tank and fly it back to the space station this summer as a spare.  That will be the next-to-last shuttle flight.
    
Discovery will remain at the space station until saturday, and landing is scheduled for April 19.