* Astronauts begin last scheduled spacewalk of mission

* Two former teachers paired for spacewalk

* Fixing cargo platform attachment may take brute force

By Irene Klotz

HOUSTON, March 23 (Reuters) - Two astronauts toted pry bars to apply brute force to a balky platform as they left the International Space Station on Monday for a six-hour spacewalk to prepare the outpost for its last phase of construction.

Soaring 220 miles (355 km) over northern China, visiting space shuttle Discovery astronauts Richard Arnold and Joseph Acaba, both first-time fliers and former high school science teachers, floated outside the station's airlock just before noon EDT (1600 GMT) for a day's work outside the complex.

"Thanks again for going outside today," station commander Mike Fincke radioed to the spacewalkers. "This is probably the last EVA (extravehicular activity or spacewalk) for this Discovery mission. We just want to say take your time, enjoy it and do good work."

It was the second spacewalk for both men, having each been paired with Discovery's senior spacewalker Steven Swanson during previous outings to upgrade the station's power system, loosen battery connections and complete other tasks.

During Monday's spacewalk, the last of three planned during Discovery's eight-day stay at the station, Arnold and Acaba will wrestle with a cargo platform attachment that defied spacewalkers' efforts on Saturday to slip it into position.

NASA believes the rotating mechanism needs more force -- with a pry bar if necessary -- to move into its proper latching position.

The astronauts also plan to move one of the station's two rail carts from one end of the station's truss to the other where it will be used by the next visiting shuttle crew to help install an outdoor porch for experiments on a Japanese laboratory.

Other chores include lubricating a part of the space station's robot arm and rewiring a circuit breaker to separate the wiring for the station's gyroscopes so that a problem with one of the positioning devices will not make two inoperable.

The shuttle is scheduled to depart the station on Wednesday and return to the Kennedy Space Center in Florida on Saturday.

NASA has up to nine more missions to the space station, as well as a final servicing call to the Hubble Space Telescope, planned before it retires the shuttle fleet in 2010. (Editing by Jim Loney and Mohammad Zargham)

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