Corner-Cube Retroreflector Instrument for Advanced Lunar Laser Ranging
- Tuesday, 03 January 2012
A paper describes how, based on a structural-thermal-optical-performance analysis, it has been determined that a single, large, hollow corner cube (170-mm outer diameter) with custom dihedral angles offers a return signal comparable to the Apollo 11 and 14 solid-corner-cube arrays (each consisting of 100 small, solid corner cubes), with negligible pulse spread and much lower mass. The design of the corner cube, and its surrounding mounting and casing, is driven by the thermal environment on the lunar surface, which is subject to significant temperature variations (in the range between 70 and 390 K). Therefore, the corner cube is enclosed in an insulated container open at one end; a narrow-bandpass solar filter is used to reduce the solar energy that enters the open end during the lunar day, achieving a nearly uniform temperature inside the container. Also, the materials and adhesive techniques that will be used for this corner-cube reflector must have appropriate thermal and mechanical characteristics (e.g., silica or beryllium for the cube and aluminum for the casing) to further reduce the impact of the thermal environment on the instrument’s performance.
The instrument would consist of a single,
open corner cube protected by a
separate solar filter, and mounted in a
cylindrical or spherical case. A major
goal in the design of a new lunar ranging
system is a measurement accuracy
improvement to better than 1 mm by
reducing the pulse spread due to orientation.
While achieving this goal, it was
desired to keep the intensity of the
return beam at least as bright as the
Apollo 100-corner-cube arrays. These
goals are met in this design by increasing
the optical aperture of a single corner
cube to approximately 170 mm outer
diameter. This use of an “open” corner
cube allows the selection of corner cube
materials to be based primarily on thermal
considerations, with no requirements
on optical transparency. Such a
corner cube also allows for easier pointing
requirements, because there is no
dependence on total internal reflection,
which can fail off-axis.
This work was done by Slava G. Turyshev, William M. Folkner, Gary M. Gutt, James G. Williams, Ruwan P. Somawardhana, and Richard T. Baran of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47489
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