Chandra's observing schedule was interrupted at 8:28 EDT on July 6 due to a spacecraft safemode event. The transition to safemode was detected in the subsequent ground contact with the spacecraft about 9 hours later.
Analysis of real time and playback data indicate that the safemode transition was nominal and all systems functioned as expected. Pointing control of the Observatory switched from the On-board Computer to the Control Processing Electronics (CPE) which is designed to keep Chandra stably oriented so as to assure power from its solar arrays during such events.
The cause for the safemode transition is under investigation. The On-board Computer (OBC) reported cause was that the measured change in spacecraft angular momentum relative to that predicted was greater than allowed. However, telemetry from the gyroscopes and reaction wheels did not show any anomalous behavior in the time preceding the safemode event. Analysis of telemetry leading to the safemode event and the contents of the OBC memory (halted at the time of the safemode) are on-going. At this time the leading candidate for the cause of the safemode is an upset or corruption of data within the OBC affecting its processing.
Besides analysis and eventual understanding of the safemode event, a major current focus for the team is on managing momentum build-up as the Observatory passes through its relatively low perigee with substantial torques and corresponding reaction wheel responses every 2.5 days. Several viable options are available to manage the momentum and the team is sorting through them to pick the safest for the near-term. The next perigee pass occurs on the night of July 8 and morning of July 9.
It is not presently known when Chandra will resume science operations - a week more or less is a reasonable first estimate as the team deciphers the cause of the safemode and then determines the appropriate (safest) configuration and steps for returning Chandra to normal operations.
As an aside we note that this is the first time ever for Chandra to be in safe-mode due to an actual spacecraft anomaly and the first time in safe mode since Feb 2000 - over 11 years ago.