The two five-second engine firings, performed 45 minutes apart on December 21, slightly raised the station's orbit. The firings, which used both of Zarya's large thrusters simultaneously, checked the performance of the station software that will be used for docking with the Russian-provided Service Module, an early living quarters for the station. At this time, no further engine firings are planned prior to the start of a rendezvous with the Service Module next year.
The Service Module 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.
Following the engine tests, flight controllers in the Zarya Flight Control Room at Mission Control, Korolev - near Moscow, Russia - maneuvered the station back into a naturally stable spinning orientation to conserve propellant and moderate temperatures on the spacecraft. Called an X-nadir spin, the 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 is the standard orientation for the station until the arrival of Discovery in May. About once each week, however, controllers 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.
Other activities performed on the station this week included deep-cycling, individually fully discharging and then recharging, each of Zarya's six batteries. The battery deep-cycling procedure is a standard housekeeping activity that will be performed a couple of times each month to optimize the batteries' performance. Flight controllers also successfully tested the Telerobotically Operated Rendezvous (TORU) system on Zarya, a manually-operated backup rendezvous system for which two stuck antennas were freed by astronauts Jerry Ross and Jim Newman during a spacewalk on Space Shuttle mission STS-88.
Flight controllers in Houston and Moscow continue to monitor the station around the clock. No major activities or checkouts are planned for next week, and all station systems are operating normally. The International Space Station is in an orbit with a high point of 256 statute miles and a low point of 248 statute miles, circling Earth once every 92 minutes.