The unique science results that have been obtained by Lunar Laser Ranging Program (LLRP)
using the Apollo retroreflectors and the science we expect to obtain with the Lunar Laser
Ranging Retroreflector Array for the 21st Century (LLRRA-21) will be discussed. While the
Apollo retroreflector arrays are still in operation and continue to produce new state-of-the-art
science results, the combination of the lunar librations and the design of the Apollo arrays
limit the range accuracy that may be obtained for each single photo-electron return to ~20
mm. A next generation lunar retroreflector (e.g., the Lunar Laser Ranging Retroreflector for
the 21st Century or LLRRA-21) holds promise for significant improvements in the
understanding of the deep interior of the moon, that is, the liquid and solid core, the core
mantel boundary and the inner mantle. The magnitude of these improvements will depend
critically on the method of robotic deployment of the LLRRA-21. The candidate methods of
deployment will be reviewed, especially those that can be supported by the Google Lunar X
Prize flights of the next couple of years. The thermal behavior of the LLRRA-21 is critical to
the ability for a number of ground stations to effectively conduct a ranging program. This
has required the developments to an extensive simulation using a sequence of programs. The
expected magnitude of the return signal from these optical/thermal simulations will be
described in detail. This expected signal return will be similar to signal return that is
currently being obtained from the Apollo 15 array, so we can evaluate the capability of
various ground stations to conduct regular ranging programs. This will address number of
ground stations that can contribute and the frequency of observations what would be
available for the science analysis. Finally, the lifetime issues related to the Apollo arrays and
the projection to the current design of the LLRRA-21 will be discussed. This work has been
supported by the LUNAR team of the NASA/NLSI and the INFN-LNF and ASI.