SPACE CENTER, Houston: Two NASA astronauts have stepped out of space shuttle Atlantis and into history. Linda Godwin and Michael "Rich" Clifford are the first American astronauts to perform space walk outside of a Russian space station. This morning's walk outside of Mir was the first NASA spacewalk outside a space station since 1974. Godwin and Clifford attached cosmic dust catchers, paint samples and other materials to Mir. This is a stepping stone to building a mammoth international space station starting at the end of next year. The project is expected to be completed early next century. The six- hour spacewalk had an element of danger as Atlantis was docked to Mir and could not immediately dash after Godwin or Clifford in the unlikely event that their tether broke. NASA took extra precautions, giving the two astronauts jet packs in case their lifelines broke. Also, the hatches between shuttle Atlantis and Mir were closed prior to the spacewalk to allow for a quick shuttle getaway in the unlikely event that a tether broke and the jet pack failed. Atlantis will depart Mir on Thursday, leaving astronaut Shannon Lucid for her five- month stay on Mir.