Solar and Heliospheric Observatory - Near Loss of SOHO

Near Loss of SOHO

The SOHO Mission Interruption sequence of events began on June 24, 1998, while the SOHO Team was conducting a series of spacecraft gyroscope calibrations and maneuvers. Operations proceeded until 23:16 UTC when SOHO lost lock on the Sun, and entered an emergency attitude control mode called Emergency Sun Reacquisition (ESR). The SOHO Team attempted to recover the observatory, but SOHO entered the emergency mode again on June 25 02:35 UTC. Recovery efforts continued, but SOHO entered the emergency mode for the last time at 04:38 UTC. All contact with SOHO was lost, and the mission interruption had begun. SOHO was spinning, losing electrical power, and no longer pointing at the Sun.

Expert ESA personnel were immediately dispatched from Europe to the United States to direct operations. Days passed without contact from SOHO. On July 23, the Arecibo Observatory and DSN antennas were used to locate SOHO with radar, and to determine its location and attitude. SOHO was close to its predicted position, oriented with its side versus the usual front Optical Surface Reflector panel pointing toward the Sun, and was rotating at one RPM. Once SOHO was located, plans for contacting SOHO were formed. On August 3 a carrier was detected from SOHO, the first signal since June 25. After days of charging the battery, a successful attempt was made to modulate the carrier and downlink telemetry on August 8. After instrument temperatures were downlinked on August 9, data analysis was performed, and planning for the SOHO recovery began in earnest.

The SOHO Recovery Team began by allocating the limited electrical power. After this, SOHO's anomalous orientation in space was determined. Thawing the frozen hydrazine fuel tank using SOHO's thermal control heaters began on August 12. Thawing pipes and the thrusters was next, and SOHO was re-oriented towards the Sun on September 16. After nearly a week of spacecraft bus recovery activities and an orbital correction maneuver, the SOHO spacecraft (bus) returned to normal mode on September 25 at 19:52 UTC. Recovery of the instruments began on October 5 with SUMER, and ended on October 24, 1998 with CELIAS.

Only one gyro remained operational after this recovery, and on December 21 that gyro failed. Attitude control was accomplished with manual thruster firings that consumed 7 kg of fuel weekly, while ESA developed a new gyroless operations mode that was successfully implemented on February 1, 1999.