* As NASA clears launch, only 8 shuttle missions remain

* Open porch completes Japan's Kibo laboratory

By Irene Klotz

CAPE CANAVERAL, Fla., June 3 (Reuters) - NASA managers cleared the space shuttle Endeavour on Wednesday for a June 13 launch on a mission to outfit the International Space Station with a Japanese-built outdoor porch for science experiments.

Slated to last 16 days, the flight will be among NASA's lengthiest space station servicing missions and, with five spacewalks and simultaneous use of three robotic cranes, one of its more complex.

"I don't think there are any easy flights from now till the end," said Bill Gerstenmaier, NASA associate administrator for space operations. "This one is unique in that it has all the robotics activity."

Endeavour will carry the last of three pieces to complete Japan's Kibo laboratory complex at the space station. The astronauts also plan to replace batteries for the station's solar power system.

The U.S. space agency has eight missions remaining to complete construction of the $100 billion orbital outpost before the shuttle fleet is retired in about 18 months.

Nevertheless, if weather or technical issues keep Endeavour on the ground beyond June 15, NASA plans to reschedule the mission for July so it can launch an unmanned lunar probe.

The Lunar Reconnaissance Orbiter is the debut mission of NASA's new Constellation program, which is intended to push human spaceflight beyond near-Earth orbit. The probe is designed to scout the moon's surface for landing sites.

Endeavour's launch attempt will come less than three weeks after sister ship Atlantis returned from NASA's last mission to refurbish the Hubble Space Telescope.

"We're running on all cylinders right now, in my mind," said shuttle launch director Pete Nickolenko. "We're hitting our stride."

Endeavour has been mostly prepared for flight for about a month as it served as a standby rescue vehicle for the Atlantis astronauts. That crew was too far from the station to seek refuge in case their ship was damaged during flight.

NASA set up emergency rescue options after losing shuttle Columbia and its seven astronauts in 2003 due to damage to the ship's heat shield.

(Editing by Jim Loney and Will Dunham)

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