On August 24, 2022, after experiencing a change in the frictional torque in one of the grating wheels of the Medium Resolution Spectrometer (MRS) on MIRI, the JWST team paused science observations using this mode. Since then, a team of experts has carried out an in-depth investigation of the mechanism design, as well as historical and post-launch data. After weeks of intensive work, the team concluded that the most likely root cause of the observed issue is increased contact forces between the wheel central bearing assembly’s sub-components under certain conditions. Based on this, a set of recommendations on how to use the affected mechanism during science operations is being defined.

An engineering test demonstrating new operational parameters for the grating wheel mechanism was successfully executed on November 2, 2022. MIRI is resuming MRS science observations, including taking advantage of a unique opportunity to observe Saturn’s polar regions. The JWST team will schedule additional MRS science observations, initially at a highly-orchestrated cadence with additional trending measurements to monitor the new operational regime of the mechanism to prepare MIRI’s MRS mode for a return to full science scheduling.

STScI will be in contact with individual PIs regarding the re-scheduling of their observations.