Webb Mid-Infrared Instrument Mode Returns to Functionality

The James Webb Space Telescope’s Mid-Infrared Instrument (MIRI) has four observing modes. On Aug. 24, after measuring increased friction in one of the grating wheels used in MIRI’s medium resolution spectrometry (MRS) mode, the Webb team paused science observations using this specific mode. Since then, a team of experts has carried out an in-depth investigation that has reviewed instrument design as well as historical and postlaunch data.

The team concluded the issue is likely caused by increased contact forces between sub-components of the wheel central bearing assembly under certain conditions. Based on this, the team developed and vetted a plan for how to use the affected mechanism during science operations.

An engineering test was executed Wednesday, Nov. 2, that successfully demonstrated predictions for wheel friction. Webb will resume MIRI MRS science observations by Saturday, Nov. 12, starting with a unique opportunity to observe Saturn’s polar regions, just before they become unobservable by Webb for the next 20 years. The team will schedule additional MRS science observations, initially at a limited cadence, following a plan to keep the affected wheel in balance, monitor wheel health, and prepare MIRI MRS for a return to full science operations.