Due to evidence that the inertial measurement unit (IMU) aboard the Lunar Reconnaissance Orbiter (LRO) spacecraft is nearing its end of life, the LRO team has powered down the unit to reserve the remaining IMU life for highly critical events, such as lunar eclipses and safe mode entries.
The IMU contains a set of three ring laser gyroscopes which are used to measure the rotation of the spacecraft in each of three axes. LRO, now in its ninth year of lunar operations, was designed for a one-year mission. Consequently, it is largely single-string, meaning that the spacecraft was built with very little redundancy. There is only one IMU on-board LRO, so the LRO team must now manage spacecraft operations without a direct measurement of spacecraft rotation rates.
In place of the IMU, the project is using successive estimates of spacecraft orientation from two onboard star trackers to derive an estimated rotation rate. While this new method of operation is undergoing testing and software updates are being made to improve performance, the spacecraft will remain in nadir-only (looking straight down at the Moon) mode for the near future. LRO was designed to map the Moon using nadir pointing and science measurements will continue to be gathered.
Off nadir (reorienting the spacecraft to point other than down) slews for special science observations, including LRO Camera stereo and oblique imaging (http://lroc.sese.asu.edu/posts/1000), are expected to resume in the Fall.
Last Updated: May 7, 2018
Editor: Karl Hille