WASHINGTON – Two American astronauts began on Monday the first of two spacewalks to install a permanent spare closet on the orbiting International Space Station and also to bottle some outer space for Japan.

The unusual project is part of a team effort with the Japan Aerospace Exploration Agency, JAXA, to open up a metal cylinder that has been signed by other astronauts, and bring it back for public display.

The so-called "Message in a Bottle" experiment, in which they will "expose a metal canister to capture the vacuum of space," is planned for the end of the six-hour spacewalk, NASA said.

In the meantime, astronauts Steve Bowen and Alvin Drew set about taking care of some more technical matters, by attaching a new extension power cable for backup purposes between the Unity node and the Tranquility module.

Next they would install "a pump module vent tool, a camera wedge and extensions to the mobile transporter rail," as preparation for the setup of the Italian-built Leonardo module as a permanent fixture at the lab, NASA said.

The walk began at 1546 GMT, about a half hour earlier than scheduled, and is set to end at 2048 GMT. A second spacewalk is planned for Wednesday.

Bowen and Drew are part of a six-member US crew that arrived at the ISS on Saturday aboard the space shuttle Discovery, which launched Thursday on its final mission.

After it wraps up this journey, Discovery will become the first of three spacecraft -- the other two are Endeavour and Atlantis -- to retire this year as the American shuttle program ends.

Discovery crew is at the ISS to deliver the Permanent Multipurpose Module, with extra storage space and an area for experiments, and the Express Logistic Carrier, an external platform for large equipment.

It is also carrying the first humanoid robot to the ISS.

The spacewalk is the sixth for Bowen, who replaced astronaut Tim Kopra after a bike accident in January, and the first for Drew, an African-American astronaut who has now become the 200th person to walk in space, NASA said.