International Ultraviolet Explorer - Mission Termination

Mission Termination

The IUE was designed to have a minimum lifetime of three years, and carried consumable sufficient for a five year mission. However, it lasted far longer than its design called for. Occasional hardware failures caused difficulties, but innovative techniques were devised to overcome them. For example, the spacecraft was equipped with six gyros to stabilise the spacecraft. Successive failures of these in 1979, 1982, 1983, 1985 and 1996 ultimately left the spacecraft with a single functional gyro. Telescope control was maintained with two gyros by using the telescope's Sun sensor to determine the spacecraft's attitude, and stabilisation in three axes proved possible even after the fifth failure, by using the Sun sensor, the Fine Error Sensors and the single remaining gyro. Most other parts of the telescope systems remained fully functional throughout the mission.

In 1995, budget concerns at NASA almost led to the termination of the mission, but instead the operations responsibilities were redivided, with ESA taking control for 16 hours a day, and GSFC for the remaining 8 only. The ESA 16 hours was used for science operations, while the GSFC 8 hours was used only for maintenance. In February 1996, further budget cuts led ESA to decide that it would no longer maintain the satellite. Operations ceased in September of that year, and on 30 September all the remaining hydrazine was discharged, the batteries were drained and switched off, and at 1844 UT, the radio transmitter was shut down and all contact with the spacecraft was lost.

It continues to orbit the Earth in its geosynchronous orbit, and will continue to do so more or less indefinitely as it is far above the upper reaches of the Earth's atmosphere. Anomalies in the Earth's gravity due to its non-spherical shape meant that the telescope tended to drift west from its original location at approximately 70°W longitude towards approximately 110°W. During the mission, this drift was corrected by occasional rocket firings, but since the end of the mission the satellite has drifted uncontrolled to the west of its former location.