See http://cxc.harvard.edu/mta/SOT_reports/sot-shift.html 

Shift L5815. 
    Start at 174:13:00 GMT

Prior to the 173/2200-2300z pass the radiation environment was elevated. The
ACE P3 fluence on ACIS had already reached 1e9 and if rates didn't drop would
exceed 2e9 by the next comm.

At AOS observing was still proceeding with the DDT observation (ObsID 17696)
but ACIS telemetry was saturated.

SCS-107 was commanded from the ground via CAP 593A at ~2015:173:22:40z (6:40pm
EDT) and executed nominally. As a clean-up action SCS-29 must be disabled
(pre-approved CAP).

The initial plan to get back to observing is to pick-up with the nominal
schedule using the 174/2015-2115z pass for load uplink, subject to the
radiation environment. SOT-MP will check whether time lost on the DDT should
be rescheduled.