Astronauts Mike Massimino and Michael Good began the second of five STS-125 spacewalks at 8:49 a.m. EDT.
   
  Massimino is wearing a spacesuit marked with broken horizontal stripes.  He has two previous spacewalks in support of Hubble during STS-109.
 
  This is Good's first spacewalk. He is wearing a spacesuit with barber pole red stripes.
 
  Massimino and Good will spend the bulk of their spacewalk time  replacing three rate sensor units. Each unit is part of a rate gyro  assembly, which sense vehicle motion and provide rate data for the  telescope.
 
  The replacement units are stored inside a protective  enclosure inside the shuttle's cargo bay. Massimino will open the lid  of the enclosure to allow Good, who will be riding the space shuttle's  robotic arm, to retrieve the first unit and carry it to the telescope.  Massimino also will retrieve a gripper tool that Good will use to  maneuver the units into place.

 source: nosa.gov