Commissioning Sequences Pave the Way

Several LROC NAC sequences were acquired looking across the illuminated limb to quantify scattered light. Not only were these excellent engineering test images but they also presented spectacular oblique views across the lunar surface [NASA/GSFC/Arizona State University].

Last week the LRO spacecraft lowered its orbit into the 50-km mapping orbit after three months in an elliptical (30 km by 200 km) commissioning orbit. Many engineering tests were performed with the spacecraft and all the instruments during the busy commissioning phase of the mission. The LROC test images were of deep space, stars, nighttime Moon, and vertical views of the lunar surface. Occasionally some extreme oblique views were shuttered as a result of specific test criteria.

The limb images in this release were part of an image quality test. One measure of a camera's ability to capture fine detail is the amount of light that is scattered in the optics. In a perfect camera, all light rays enter the aperture and end up in focus in the correct pixel. However, in reality some percentage of light is scattered in the optics, off structures in the telescope, or even reflected off the CCD. If there is no scattering, then no light rays strike pixels looking at space - by measuring the signal level in "space pixels" against those seeing the illuminated surface a good measure of light scattering is easily made. As it turns the scattered light in the NACs is very low (a good thing), about one percent.

An added bonus from this engineering test is a spectacular view across the lunar surface! There are no named lunar features in this image, which is centered in the lunar highlands over 450 km northwest of Mare Humboldtianum at approximately 65.5°N, 55.6°E. The large region which appears dark, flat, and level in the distance are not mare basalts as you might expect, but are light plains emplaced as ejecta by large basin-forming impacts. Vast amounts of lunar crust are thrown out in these large basin forming impacts, part of the ejecta travels across the surface in a fluidized manner even though there is no water in the mix. As the flow decelerates it settles to the ground as if it were a fluid, filling low spots and lapping against high standing topography. The exact details of this process are not well understood - another mystery awaiting future lunar explorers!

The original mosaic was shrunk by a factor of eight to fit the format of this webpage.

Published by Mark Robinson on 21 September 2009