Astronauts Tammy Jernigan and Daniel Barry will conduct a six-hour space walk on flight day four to install crane systems that will be used on future missions to assist in assembly and transfer operations. The two astronauts will be assisted by Flight Engineer Ellen Ochoa, who will operate the Shuttle’s robotic arm from inside Discovery.


Getting Ready

Preparations for the space walk begin early in the mission, on flight day two, with the checkout of the space suits, also called extravehicular mobility units, or EMUs. Later that day, the air pressure in the crew cabin will be reduced from the standard operating pressure at 14.7 psi to 10.2 psi in order to gradually condition the astronauts to an atmospheric environment more closely resembling that of the space suits, themselves, which operate at 4.3 psi. This technique reduces the time required for the astronauts to "prebreathe" pure oxygen so that nitrogen may be cleansed from their bloodstream to prevent a condition commonly called the "bends." The astronauts will prepare their EVA tools and review procedures for their installation operation on the third day of the mission.


The Space Walk

On flight day four, Jernigan and Barry will don their space suits, pre-breathe pure oxygen to cleanse the remaining nitrogen from their blood streams, and exit the Orbiter into the payload bay for six hours of preparation, installation and cleanup. Jernigan, designated "EV1" will exit the airlock first and, after configuring tethers for the start of the space walk, will be joined by Barry, designated "EV2." Jernigan will be visibly distinguished by a red stripe around the legs on her space suit.

Preparations will include the placement of a work station and portable foot restraints into position for the installation of an American-built crane, called the ORU Transfer Device, or OTD, and the Russian-built crane called Strela (Russian, for "arrow"). Installation of the OTD and Strela are estimated to take about 1 hour each.

Ready to begin the installation work, Jernigan will fix her boots to a foot restraint on the end of the Shuttle's robotic arm, and Ochoa will maneuver her from a foot restraint at Pressurized Mating Adapter (PMA-1) to the Integrated Cargo Carrier (ICC). PMA-1 is a passageway between Unity and Zarya, and the ICC is a flatbed pallet attached in Discovery's payload bay.

Jernigan will meet Barry on the Integrated Cargo Carrier, where they will retrieve the small, 209-pound U.S. OTD. While holding the crane, Jernigan will be transported back to PMA-1. Barry will join her to attach the crane to an EVA worksite fixture on the PMA.

The two will return to the Integrated Cargo Carrier and retrieve the 165-pound operator post and 33-pound grapple fixture adapter plate for the Russian crane, Strela. They will assemble those pieces on one of the two PMA-2 grapple fixtures. PMA-2 connects the Shuttle external airlock to Unity.

Strela is an updated version of the crane used on Mir. It will primarily be used for Science Power Platform operations. Assembly of the three-metric-ton capacity crane will be completed during this year's second ISS logistics mission. A 45-ft. telescopic boom will then be added to the operator post, and the completed assembly will be moved to a grapple fixture on Zarya.

The astronauts will then return to the ICC to retrieve two portable foot restraints and transfer them into PMA-1. They will return to the Integrated Cargo Carrier, one last time, to remove three tool and hardware bags and three additional pieces of EVA support equipment from the SPACEHAB Oceaneering Space System box. The bags will contain tools for handling ISS replacement parts and to assist in the assembly of the Space Station, as well as on-orbit installable handrails. Jernigan and Barry will stow them on Unity.

If time permits, the two space walkers will also install a thermal cover on a Unity trunion pin, inspect Zarya's paint, checkout the ODT and survey the Early Communications System antenna on the starboard side of Unity.

Click here for background information on EVA suits, tools and techniques.