Expedition_Zero_part

June 1999: Long, Lonely Orbits

Following the departure of Discovery at the end of the STS-96 mission on 3 June, a delay in further orbital operations was a possibility. But no-one could have guessed that there would be almost a year of inactivity before the next Shuttle arrived. In a period that should have seen the flight of STS-101, the docking of the Zvezda Service Module, two robotic Progress re-supply craft, STS-92 and the first resident expedition, there was not a single launch associated with ISS.

Ongoing problems with the preparation of Zvezda, the Proton launch vehicle and the Shuttle fleet added delay after delay to the already much-postponed launch schedule. As one NASA official explained: "Building ISS is like stacking up a row of dominoes; if one tumbles, it has a knock-on effect down the line." The delay of just one element or the postponement of a single launch would have serious consequences for the first dozen or so missions.

After Discovery pulled away from the ISS, the re-stocked station was left in its routine, crewless orientation. This consisted of Unity pointed towards Earth and Zarya out towards space, with the station put into its slow spin to spread the heat distribution across its surfaces. All onboard systems were funtioning normally, with about 865 watts of power to provide the appropriate thermal conditions.

The crew of STS-96 had restored the Early Communications System (ECS) to full working order by changing out a switch inside the power distribution box, although the exact cause of this failure remained unsolved and under investigation. The astronauts had also been able to complete in-flight maintenance on the six batteries in Zarya, meaning that the flight controllers only needed to charge each battery once every six months instead of every week.

About ten days after STS-96 undocked from ISS, the USAF Space Command informed ground controllers of a close-proximity pass by a spent Russian rocket stage. Though not a frequent event, Shuttle missions had sometimes been required to shift orbital position occasionally to avoid larger pieces of orbital debris. In this case, a closest approach to within 1 km (0.6 miles) of ISS had been estimated for 13 June.

Following the uplink of the command to initiate the manoeuvring burn, one of Zvezda's engines was programmed to burn for longer than was permitted by the onboard computer. The system recognised the error and correctly cancelled the burn. Consequently, no manoeuvre took place. The debris eventually passed the station at 7 km (4.3 miles), so the burn was fortunatley not needed on this occasion. A review of debris avoidance manoeuvres was initiated by both the American and Russian controllers.

July - October 1999

The first full charge and discharge programme for the six batteries on Zarya was begun in early July. While Battery 6 completed the procedure, the discharge of Battery 1 did not reach the expected value. Ironically, when controllers cycled Battery 2 later in the month, they noted that Battery 1 was also discharging and decided to allow the previously sluggish battery to complete is cycle. The series was continued with Battery 3 and then Battery 5 in July and Battery 4 in early August. This 'maintenance training' was compared to routines that occur in any modern home on Earth. According to NASA, this was akin to turning off the lights, fans or equipment not being used and re-charging a cordless tool or cellphone. The agency stated: "The current power requirements of the station can be met on as little as three batteries by careful management of power usage."

The only other problem on ISS was an old one: a faulty ECS antenna, though this time on the port side. The system was working correctly through the starboard one. Despite small repairs conducted by the STS-96 astronauts, it was expected that the antenna might need replacing on a future EVA if the problem re-occurred, even though it had no impact on ISS operations. A replacement battery was also added to the STS-101 payload manifest, as Battery 1 was continuing to show slightly different voltage output to the other five, although it was operating satisfactorily.

During August, a successful test of the automatic docking system marked a further milestone in preparations to receive the Zvezda Service Module, which was being prepared for a launch in mid-November. The ISS docking systems were prepared by control teams in Houston and Moscow. The Russian control centre then commanded the station to move to its docking orientation before performing a test of the Zarya module's automatic docking system. Following the test, the station was returned to its normal spin-stabilised orientation.

Battery 1 continued to show a degraded performance, which resulted in the decision to remove the troublesome unit from operational use early in August. To gain a better understanding of the problem, a test was devised that saw the battery reconnected to the station's power system for one 92-minute orbit. This would allow ground controllers to document its performance - and that of its associated electronics - to try to identify the cause of the fault.

Results from the test indicated that the battery could be used in the future if required until it could be replaced. Data from the test indicated normal operation, but the battery's performance remained slightly below what was expected. Engineers evaluated that this could possibly be due to one faulty cell within the battery rather than the whole unit. The problem that led to its disconnection was thought to be an isolated one and was not expected to return. The battery remained disconnected for the time being while the problem was analysed further. Engineers planned a second test of 4.5 hours (three orbits) to gather more information. This second test (and another smaller one a few hours later) still failed to provide full power and a decision was made to leave Battery 1 offline and to replace it during STS-101.

During the week of 13 September, controllers completed a pump test designed for transferring propellant among the station's tanks, to prepare it for the docking of Zvezda. Nitrogen was transferred through portions of the propellant feed lines as part of a combined test of the pump, electrical support systems and the flow lines. A few days later, similar tests were performed with the fuel and oxygen tanks and associated systems.

Towards the end of the month, controllers recorded a slight decrease in power output when one of the remaining five online batteries was undergoing routine cycling. At the same time, the rotational rate of ISS was being decreased slightly as part of the regular housekeeping chores. To help alleviate these power reductions, the solar arrays on Zarya were orientated into a more favourable angle to the sun for power generation. The power reduction was determined to be due to one battery undergoing cycling and a higher level of power being supplied to non-essential equipment. A review resulted in the removal of power from some of this equipment until normal power levels were restored.

After two weeks of Technical Integration Meetings between NASA and the Russian Aviation and Space Agency during the month, there was a Joint Program Review on 30 September. It addressed the current status of the American Shuttle fleet, including the problems with the electrical wiring and the main engine malfunction discovered during the launch of STS-93 in July. The status of the Zarya Service Module and the U.S. elements were also discussed, as was the operation of ISS on orbit.

It was decided that the launch window for Zvezda would be moved to between 26 December 1999 and 16 January 2000, with a more precise launch date to be manifested following the General Design Review planned for later in 1999. NASA stated that the delay in launching Zvezda in no way impacted on the overall launch and assembly schedule. Launch dates for subsequent flights were expected to be adjusted due to the ongoing wiring inspections on all orbiters, as well as recent work delays caused by Hurricane Floyd which had circulated around KSC earlier in the month. No hardware had been damaged, but work schedules had been disrupted by the severe storm.

The decision also pushed the next Shuttle service mission (STS-101) well into January 2000, following the flights of STS-103 to service the Hubble Space Telescope and STS-99 carrying the Shuttle Radar Topography hardware.

During early October, controllers continued monitoring onboard systems, including the ECS on Unity and the status of all batteries on Zarya. With the station orbiting Earth every 92 minutes in a 399 x 370 km (248 x 230 mile) orbit, the 5,000th revolution was completed on 7 October.

By the middle of the month, NASA had demonstrated the command capability from Houston to Zarya's computers, via the ECS aboard Unity. The use of the TDRSS network for ISS operations was demonstrated during a test of the motion control system to measure the station's spin rate accurately and make any necessary adjustments. TDRSS was only a part-time element of ISS, to be used only two or three tmes a week over the coming months. Used in other programmes, its connection to ISS was turned off to avoid conflicting with other requirements, such as the ten-minute data transmissions from the Galileo space probe as it passed close to Jupiter's moon Io.

October also saw further tests of the docking systems aboard the station, the passive avionics and acceleration measurement components and the motion control system, as well as the continued cycling of the five remaining batteries. At 09:36 CDT on 26 October, the station's orbit was raised slightly as a precaution to avoid a further piece of space debris.

NORAD (North American Aerospace Defense Command) had indicated on 22 October that a spent Pegasus rocket body would pass within 14 km (8.5 miles) of the station at about 03:00 CDT on 27 October. The station was manoeuvred to the burn attitude using Zarya's motion control system at 08:30 CDT the previous day. The five-second burn, initiated 33 minutes later, changed the station's overall velocity by 1 m/sec (3.2 fps) and its altitude by 1.6 km (1 mile). It was then reorientated into its normal slow spin position. The Pegasus stage was expected to pass by the station at about 25 km (15 miles) separation.

Tracking data indicated that the rocket stage passed by the ISS at 140 km (87 miles). Had the burn not occurred, the miss distance would have been no more than 800 m (0.5 miles). Raising the orbit had also been a useful exercise in preparing for a rendezvous test planned for early 2000.

During 28 October, routine tests were conducted on the command path to Zarya from Mission Control in Korolyov outside Moscow, using Unity's ECS. A similar test, to Unity from Mission Control in Houston, used the Komparus system inside Zarya. The same day, a Russian Proton rocket carrying a communications satellite was lost during launch. Proton was the launch vehicle designated to carry the much-delayed and crucial Zvezda Service Module to ISS a few weeks later. The full investigation by carried out by Russian officials was expected to be closely monitored by NASA.