Cosmonauts Pavel Vinogradov and Roman Romanenko ventured outside the Russian segment of the International Space Station Friday to retrieve two experiment packages, to install a space weather monitoring instrument and to replace a navigation aid needed by an unmanned European cargo ship set to arrive in June.

Wearing Russian Orlan spacesuits, the cosmonauts opened the hatch of the Pirs airlock module at 10:03 a.m. EDT (GMT-4) to officially kick off a planned six-hour spacewalk, the first of up to eight excursions planned for 2013.

A few minutes later, Romanenko and Vinogradov floated outside as the space station sailed into an orbital dawn 250 miles above the south Pacific Ocean.

This is the 167th spacewalk devoted to station assembly and maintenance since construction began in 1998, the first EVA of 2013, the seventh for Vinogradov and the first for Romanenko, a second-generation cosmonaut whose father logged four spacewalks.

Vinogradov and Romanenko have four primary objectives:

• Install an instrument package known as "Obstinofka," or "environment," on the forward part of the Zvezda module to study the electrically charged plasma wave environment and its interaction with the ionosphere.
  
  
• Replace a laser retro-reflector on the back end of Zvezda that is part of a system approaching European cargo ships use to home in on the command module's aft docking port. The work is needed before the arrival of the European Space Agency's Einstein Automated Transfer Vehicle in June.
  
  
• Retrieve a "Biorisk" experiment package mounted on the hull in 2011 to measure the effects of microbial activity on spacecraft structures and whether solar activity has any impact on microbial growth.
  
  
• Retrieve one set of materials science space exposure samples, part of the "Vinoslivost," or "endurance," experiment, that was mounted on the station's hull in 2012.

Cosmonaut Alexander Misurkin and Chris Cassidy planned to spend the day in the Poisk module, attached to Zvezda's upper docking port, giving them access to their Soyuz TMA-08M spacecraft.

Expedition 35 commander Chris Hadfield and Thomas Marshburn were isolated in the forward part of the station, with access to the Soyuz TMA-07M spacecraft docked to the Russian Rassvet module.

Going into today's outing, 109 astronauts and cosmonauts had logged 1,049 hours -- 43.7 days -- of station EVA time building and maintaining the station. Up to six Russian spacewalks are planned for 2013, with two U.S. excursions on tap this summer.