LRO Helps LCROSS With Crater Selection; Will Observe Impact
image of lunar south polar region with potential LCROSS targets

This image of the moon's south polar region shows a number of potential LCROSS targets. The crater labeled "SP_C" is Cabeus Proper, the final selection. Credit: NASA
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LRO has made detailed observations to support the LCROSS selection of a lunar south pole crater for its October 9 impact. The LRO team used laser altimetry data from both the LRO LOLA and JAXA Kaguya laser altimeters to determine regions of permanent shadows that would be the most likely regions to harbor frozen volatiles (such as water ice), if any are preserved in significant concentrations. The topography measurements also pointed out regions within the polar impact craters where the local slope is sufficiently small to maximize the transfer of kinetic energy from LCROSS impact into the lunar regolith target.

LRO’s LEND neutron spectrometer provided maps of enhanced hydrogen concentrations that could indicate water ice embedded in the upper meter of the lunar regolith. LRO Diviner temperature measurements of the south polar region reveal the extremely low temperatures of cold traps that can potentially preserve volatiles in ice form. Mini-RF dual-frequency radar polarization imaging provided information that indicates the blockiness of the impact site and which can be used to further test for the presence of significant water ice. These combinations of LRO measurements as well as other factors lead the LCROSS team to choose the south polar crater Cabeus for its Oct. 9, 2009, impact. Before the impact occurs the Cabeus target area will be exhaustively observed by the LRO LROC, LOLA, and Mini-RF instruments in detail to characterize the pre-impact geology.

The LRO spacecraft orbit will be adjusted to pass at closest approach to the Cabeus target site 90 seconds after the LCROSS Centaur impact. At and just after the impact, the LRO LAMP far UV spectrometer will search for evidence of significant volatiles and how they spread in the moon's tenuous, almost nonexistent atmosphere. LRO’s Diviner radiometer will peer into the impact site to measure the heating effects of the impact and how it changes over time. LRO plans to have a campaign of extensive observations of the post-impact surface geological effects by all instruments over the course of the days and weeks following the LCROSS impact event. Such LRO-based observations will provide important measurements in support of the LCROSS team's analysis of the physics of the impact and how volatile materials may have been mobilized.
Nancy N. Jones
NASA's Goddard Space Flight Center