Infrared Space Observatory - Launch and Operations

Launch and Operations

After a very successful development and integration phase ISO was finally launched into orbit on November 17, 1995 on board an Ariane-44P launch vehicle. Performance of the launch vehicle was very good with the apogee only 43 km lower than expected. ESA's Space Operations Centre in Darmstadt in Germany had full control over ISO in the first four days of flight. After early commissioning primary control over ISO was handed over to the Spacecraft Control Centre (SCC) at Villafranca in Spain (VILSPA) for the remainder of the mission. In the first three weeks after launch the orbit was fine-tuned and all satellite systems were activated and tested. Cool-down of the cryostat proved to be more efficient than previously calculated, so the anticipated mission length was extended to 24 months. Between November 21 and November 26 all four science instruments were switched on and thoroughly checked out. Between December 9, 1995 and February 3, 1996 the 'Performance Verification Phase' took place, dedicated to commissioning all instruments and fixing problems. Routine observations started from February 4, 1996 and lasted until the last helium coolant depleted on April 8, 1998.

The perigee of ISO's orbit lay well inside the Van Allen radiation belt, forcing the science instruments to be shut down for seven hours during each pass through the radiation belt. Thus, 17 hours in each orbit remained for scientific observation. A typical 24 hour orbit of ISO can be broken down into six phases:

• Acquisition-of-Signal (AOS) by the primary Mission Control Center VILSPA in Spain and activation of the satellite.
• Science operations during the VILSPA window, starting four hours after perigee, and lasting for up to nine hours.
• Handover of operations to the secondary mission control center at Goldstone at apogee. During this 15 minute periode the science instruments could not be operated.
• Science operations during the Goldstone window, lasting up to eight hours.
• De-activation of the instruments upon approach of the Van Allen radiation belt and Loss-of-Signal (LOS) at Goldstone.
• Perigee passage.

Contrary to IRAS, no science data was recorded on-board ISO for later transmission to the ground. All data, both science data and housekeeping data were transmitted to the ground in real-time. The perigee point of ISO's orbit was below the radio horizon of the mission control centers at both VILSPA and Goldstone, thus forcing the science instruments to be switched off at perigee.