The EIT-telescope as well as LASCO have become temporarily blind. An onboard computer remarked a virtual malfunction and ordered the satellite to go into the 'safe' mode in which only vital functions are kept alive.
Newsflash broadcast on the SOHO-webpage on April 21, 2004
"... SOHO entered ESR (safe) mode at 05:37 UT. The ESR was triggered by the FSPAAD (Fine Sun Pointing Attitude Anomaly Detector)."
On April 22, 2004, the following message could be read:
"... Recovery from ESR has been postponed until tomorrow. SOHO entered ESR (safe) mode at 07:27 UT. The ESR was triggered by another spurious FSPAAD (Fine Sun Pointing Attitude Anomaly Detector) anomaly. Recovery is in progress."
The detector FSPAAD, in charge of checking the orientation of the panels of SOHO towards the sun, sent on April 21 and a second time on April 22 a spurious signal triggering the satellite to go into the Emergency Sun Reacquisition (ESR) mode controlled by an on-board computer. ESR is a 'safe' mode in which SOHO stops all observational scientific functions (and transmission of data) and tries to point its solar cells at the Sun again. The last is done by firing small hydrazine thrusters. This action is sometime called "momentum management". By the law of preservation of momentum, the spacecraft can be pushed and rolled into the desired position. The SOHO crew on Earth tries to establish contact with the satellite to put it again in the Mission Mode, the mode from which scientific observations are made.
Above the latest SOHO/EIT movie. Just before satellite entered the first ESR mode, a last picture was taken at 05:36 UT followed by an information gap until 22:56 UT. The end of the movie with the "strange" pictures coincides with the time the satellite went a second time into the safe mode.
