Postmortems

SOHO observatory five hour loss of contact (1998)

Twenty-odd years ago the two tonne SOHO observatory was launched into orbit a million miles away. It’s still working. But three years after launch it had an upset and stopped communicating. The recovery story is quite heroic. But first, the three errors which almost lost it: “the Investigation Board has determined that the first two errors were contained in preprogrammed command sequences executed on ground system computers, while the last error was a decision to send a command to the spacecraft in response to unexpected telemetry readings. … two anomalous command sequences, in combination with a decision to send a command to SOHO to turn off a gyro in response to unexpected telemetry values, caused the spacecraft to enter a series of ESRs, and ultimately led to the loss of control.”

“At any time during the over five hour emergency situation, the verification of the spinning status of Gyro A would have precluded the mishap.”

“The SOHO Mission was interrupted on June 24, 1998, while the SOHO Team was conducting a series of spacecraft gyroscope calibrations and manoeuvres. 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 Goldstone Solar System Radar combined 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 revolutions every 53 seconds. 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”

  • wikipedia

Much more detail elsewhere:


SOHOspin

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“At any time during the over five hour emergency situation, the verification of the spinning status of Gyro A would have precluded the mishap.”

“June 25th. The loss of communication occurred because of a faulty
recovery attempt from the second of the two ESRs, which had a disastrous result. In brief, SOHO’s roll rate began increasing dangerously, and then attitude control failed completely as SOHO rolled into a tumble while still thrusting. In short order, there was a loss of high gain antenna (HGA) coverage of the Earth and, worse, loss of solar power as well. In the early aftermath, repeated attempts to communicate with SOHO failed, and the situation appeared truly grim.”