Space station getting a new doghouse

The International Space Station has a new doghouse.

It's not for orbiting canines or astronauts who have misbehaved.

The "doghouse" is simply the nickname for a large oxygen tank, so named for its shape.

Atlantis spacewalkers Randy Bresnik and Bobby Satcher are installing the doghouse -- a spare that will be the fifth on station -- outside the Quest airlock.

The spacewalkers unbolted the 1,240-pound boxy tank structure, which measures 5 feet by 6.2 feet by 4.5 feet, from one of the two platforms of spare parts hauled up by Atlantis.

The three-foot diameter, high-pressure tank includes 220 pounds of gaseous oxygen at 2,450 pounds per square inch. It will be used to replenish air that escapes from the airlock when hatches are opened for spacewalks.

It's an example of the large spares NASA wanted to deliver to the station while it still has the shuttle's large cargo volume, so they'll be available if parts begin to fail in the coming years. Five more shuttle flights are planned.

After unbolting the doghouse from the station's starboard truss, Satcher and Brenik held it gently while mission specialist Leland Melvin captured it with the station's 58-foot robotic arm.

He has swung the doghouse over to the airlock, where the spacewalkers will bolt it into place.

Meanwhile, on the same platform vacated by the doghouse, Bresnik is installing a two suitcase-like containers of materials science experiments, called the Materials International Space Station Experiment-7, or MISSE-7.

More than 700 material samples include some NASA is testing for possible use with future spacesuits and spacecraft. The experiments are expected to return to Earth on the second-to-last shuttle flight.

The mission's third and final planned spacewalk began at 8:24 a.m. EST and is expected to last under six hours.