Controllers also performed a successful check of Zarya's Kurs rendezvous system and the module's docking system, an automated Russian system that eventually will steer the station to dock with the third station module, the Service Module. The Service Module, an early crew quarters and station core that is the primary Russian contribution to the station, is targeted for a July 1999 launch aboard a Russian Proton rocket. Prior to the Service Module's launch, the Space Shuttle Discovery is scheduled to visit the station in May 1999, carrying supplies to be stored in the interior and a Russian-built spacewalkers' cargo crane to be installed on the exterior.
A few hours after Endeavour's undocking from the station on Sunday, flight controllers at Mission Control, Korolev, maneuvered the station into a naturally stable spinning orientation to conserve propellant and moderate temperatures on the spacecraft. Called an X-nadir spin, that orientation has the Unity module pointed toward Earth and Zarya pointed toward deep space with the station slowly spinning a few tenths of a degree per second. It will be the standard orientation for the station until the arrival of Discovery in May. About once each week, however, controllers will turn on the station's steering jets and maneuver it into position to update the guidance system and perform other checkouts or activities as needed.
On Wednesday, Zarya's steering jets were activated and the station's guidance updated prior to the successful test of Zarya's large thrusters. The test firing on Wednesday checked the operation of both thruster engines simultaneously for the first time, firing them for about 9 seconds, as will be needed to periodically reboost the station's orbit. Previously, all station engine firings had been performed with only one of Zarya's large engines. The Wednesday test firing raised the station from an orbit that had a high point of about 252 statute miles and a low point of 244 statute miles to its current orbit of 256 by 247 statute miles.
Two more engine test firings are planned for Monday that will again check the operations required to reboost the station as well as the operations required for a rendezvous next year with the Service Module.
All systems aboard the International Space Station are performing well, and the spacecraft is completing one orbit of Earth every 92 minutes.