Endeavour’s astronauts boosted the fledgling International Space Station to a higher altitude Tuesday and had a chance to relax for a few hours as the first station assembly flight neared the halfway mark.

Commander Bob Cabana and Pilot Rick Sturckow fired Endeavour’s steering jets in a staccato fashion for about 22 minutes Tuesday to gradually raise the highest point of the shuttle and attached station’s orbit about 5½ statute miles to 248 miles. Throughout its lifetime, the station’s altitude will be periodically raised to maintain a proper orbit.

With all Endeavour and station systems in excellent shape, astronauts Jerry Ross and Jim Newman prepared for a second space walk later today to set up a communications system in the Unity module that will allow U.S. flight controllers to monitor Unity’s systems. The so-called S-band early communications system includes two boxy antennas that will be installed on the outside of Unity as one of today’s first tasks, and avionics gear that will be set up inside Unity on Thursday after astronauts enter the module for the first time. A test of the system’s videoconferencing capability is planned after Thursday’s installation.

Ross and Newman also will remove launch restraints over four hatchways on the Unity connecting node to which future station modules and an airlock will be attached. The hatchways, or Common Berthing Mechanisms, serve as docking ports for new hardware that will be delivered to the station over the next 18 months.

Near the end of today’s space walk, Newman will use a grappling hook in an attempt to unfurl one of two balky antennas on Zarya’s backup rendezvous navigation. The antennas would be used to transmit range and closure rate information to approaching spacecraft heading for dockings with the Russian control module. It is believed that stiff cabling or interference from thermal blankets on Zarya may be preventing the antennas from fully extending, even though pyrotechnic pins have fired to enable the antennas to roll free from their spools. Pending final approval from mission managers, Newman will attempt to free the antenna on the nadir port of Zarya today. If successful, the same procedure may be used to free the antenna on the zenith port Saturday.

Before the astronauts began an eight-hour sleep period, International Space Station flight controllers in Houston reported that Unity’s lower Pressurized Mating Adapter had warmed enough to permit astronauts to enter the module Thursday. Heaters were activated for the first time late Monday after power from Zarya was connected to the U.S. component.

The astronauts are scheduled to be awakened at 10:36 a.m. Central time to begin space walk preparations. The space walk by Ross and Newman is scheduled to get under way about 3 p.m. Central time, but could begin as soon as the astronauts are ready to depressurize Endeavour’s airlock.

The next STS-88 mission status report is expected shortly after crew wakeup.

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