An Unusual View: MISR sees the Moon

The job of the Multiangle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra satellite is to view Earth. For more than 17 years, its nine cameras have stared downward 24 hours a day, faithfully collecting images used to study Earth's surface and atmosphere. On August 5, however, MISR captured some very unusual data as the Terra satellite performed a backflip in space. This maneuver was performed to allow MISR and the other instruments on Terra to catch a glimpse of the Moon, something that has been done only once before, in 2003. Why task an elderly satellite with such a radical maneuver? Since we can be confident that the Moon's brightness has remained very constant over the mission, MISR's images of the Moon can be used as a check of the instrument's calibration, allowing an independent verification of the procedures used to correct the images for any changes the cameras have experienced over their many years in space. If changes in the cameras' responses to light aren't properly accounted for, the images captured by MISR would make it appear as if Earth were growing darker or lighter, which would throw off scientists' efforts to characterize air pollution, cloud cover and Earth's climate. Because of this, the MISR team uses several methods to calibrate the data, all of which involve imaging something with a known (or independently measured) brightness and correcting the images to match that brightness. Every month, MISR views two panels of a special material called Spectralon, which reflects sunlight in a very particular way, onboard the instrument. Periodically, this calibration is checked by a field team who measures the brightness of a flat, uniformly colored surface on Earth, usually a dry desert lakebed, as MISR flies overhead. The lunar maneuver offers a third opportunity to check the brightness calibration of MISR's images. While viewing Earth, MISR's cameras are fixed at nine different angles, with one (called An) pointed straight down, four canted forwards (Af, Bf, Cf, and Df) and four angled backwards (Aa, Ba, Ca, and Da). The A, B, C, and D cameras have different focal lengths, with the most oblique (D) cameras having the longest focal lengths in order to preserve spatial resolution on the ground. During the lunar maneuver, however, the spacecraft rotated so that each camera saw the almost-full Moon straight on. This means that the different focal lengths produce images with different resolutions. The D cameras produce the sharpest images. These grayscale images were made with raw data from the red spectral band of each camera. Because the spacecraft is constantly rotating while these images were taken, the images are "smeared" in the vertical direction, producing an oval-shaped Moon. These have been corrected to restore the Moon to its true circular shape. https://photojournal.jpl.nasa.gov/catalog/PIA21876