Launch and Orbit
Launch
The IUE satellite was launched by a Delta rocket on 26 January 1978 at 17:36 UT from the Kennedy Space Centre, Cape Canaveral, United States.
The first two stages of the Delta rocket put the third stage and IUE spacecraft assembly in a parking orbit. Prior to the Delta third stage burn for injection into the transfer orbit, the spacecraft and third stage assembly were spun-up to 60 rpm by the Delta spin table.
After injection from the parking orbit into the transfer orbit and separation from the Delta's third stage, an automatic nutation control (ANC) system on the spacecraft was initiated because the moment of inertia ratio with respect to the spin vector was unfavourable. This spin mode lasted about 21 hours.
Along the transfer orbit, range and range rate measurements were made to accurately predict the orbit and time of apogee motor firing. Additionally precession of the spin vector was determined to align the apogee motor in the proper direction for boost into the geo-synchronous orbit.
At apogee, the motor was commanded to ignite by ground command. When the desired station was obtained, the spacecraft was despun in two phases to gain three-axis gyro rate control. In the first phase, the IUE was spun up to 2 to 5 degrees per second, and the solar arrays were deployed. In the second phase, the IUE was rate damped to 0.25 degrees per second. The early orbit phase was concluded after the spacecraft was aligned with the sunline normal to the primary plane of the solar arrays.
Orbit
IUE was put in a geo-synchronous orbit positioned over the Atlantic around 300° East longitude. The orbital parameters achieved after reaching the geo-synchronous orbit were very close to the desired nominal values. On 27 January 1978 the measured values were:
| Orbital parameter | Predicted | Achieved |
| Semi-Major axis (km) |
42 164 |
42 156 |
| Eccentricity |
0.250 |
0.239 |
| Inclination (°) |
28.7 |
28.63 |
| Argument of perigee (°) |
257 |
257.04 |
| Period (h) |
23.93 |
23.927 |
| Perigee height (km) |
25 230 |
25 669 |
| Apogee height (km) |
46 340 |
45 887 |
During the nearly 19-year lifetime of the mission, the orbit evolved by external forces that all Earth-orbiting satellites experience. The main contributions to the orbit perturbation were:
- Gravitational influence of the Earth (due to the fact that the Earth is not an exact sphere, but slightly flattened)
- Gravitational influence of the Sun and Moon
- Solar radiation pressure
The combined effect of these forces were to increase the inclination (from 28.6° to 35.9°) and decrease the eccentricity (from 0.239 to 0.137) of the IUE orbit, as well as pushing IUE's position west-ward from the initial 300° East longitude. This drift was periodically corrected for by firings of on-board thrusters.