November 29, 1999

On Sunday, 1999 November 28, while in normal mode collecting routine science data, the computer for the spacecraft’s Attitude Control Unit A (ACU) reset for an undetermined reason.
This automatically triggered a switch to the Emergency Sun Reacquisition (ESR) mode. ESR is an automatic reconfiguration of the Attitude and Orbit Control Subsystem (AOCS). During
ESR, the spacecraft broadcasts low-rate engineering data only (no science data). Three-axis attitude control is accomplished via thrusters and the ground system is required to provide roll axis
control. A spacecraft emergency was declared in order to insure adequate Deep Space Network (DSN) coverage.

ESR recovery procedures were initiated to retrieve data for analysis and allow open-loop roll control. Roll rate adjustment commands were successfully uplinked and the gyroless ACU flight
software patch was successfully reloaded to ACU A in an inactive mode. ACU A nominal status was confirmed.

Today we plan to execute the transition from ESR to Coarse Roll Pointing (CRP) mode. CPR allows medium rate telemetry through the high-gain antenna and most instruments should be
able to operate. The SOHO spacecraft emergency will be lifted as soon as SOHO safely transitions to CRP mode. This is the first ESR encountered since the upload of the ‘gyroless’ flight
software in 1999 October. An ACU-A reset for undetermined reasons also occurred in 1997 November.

If CRP recovery occurs on schedule, a spacecraft momentum management will be performed on Tuesday, 1999 November 30, and the instruments returned to normal scientific operations
shortly thereafter. We expect a complete recovery well before the 4th anniversary of SOHO’s launch on December 2.