Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p51d..07a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P51D-07
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6250 Moon (1221)
Scientific paper
The primary objective of the Lunar Crater Observation and Sensing Satellite (LCROSS) is to confirm the presence or absence of water ice that might have trapped out over time from the lunar exosphere into permanently shadowed inter-crater regolith near the lunar poles. It will provide a critical ground-truth for Lunar Prospector and LRO neutron and radar maps, making it possible to assess the total lunar water inventory and to provide significant insight into the processes that delivered hydrogen to the polar regions. Non-detection of water could lead to significant changes in the architecture of lunar operations and settlement. Ong and Asphaug (LPSC 2008) study the fraction of volatile material that remains bound to the Moon and Mars during comet and asteroid impacts and calculate a mass of water retained over the past 2 Ga of order 1E10-1E11 tons, a few times the water ice inferred by Feldman et al. (Science 1998) on the basis of Lunar Prospector neutron detection. This flux includes small contemporary events but is dominated by major discrete contributions in the past. Whatever the mechanism for the delivery and possible retention of lunar water, interest in the possible presence of water ice has both scientific and operational foundations. If water is present in the upper meters to the few percent level, LCROSS will find it by using a 2000 kg kinetic impactor -- the empty Atlas V Centaur upper stage -- to excavate more than 250 metric tons of regolith. The thermal and spectral signature of the impact flash and the crater ejecta that gets launched into sunlight will be studied in detail, and the results transmitted to Earth before the 700 kg shepherding spacecraft also impacts the Moon. These two impact experiments and their aftermaths will also be observed from a number of Lunar-orbital and Earth-based assets. For the purpose of mission planning, asset security, and scientific prediction, we have conducted a variety of calculations based upon several models covering different aspects of the event. Crater scaling laws are used to obtain fundamental estimates of crater diameter and ejecta mass. We also apply the RAGE adaptive mesh hydrocode to model the short-timescale (0.1 s) thermal plume that is expected to occur immediately after the impact. We also conduct a number of large scale (millions particle) smooth-particle hydrodynamics (SPH) calculations, which take into account impactor geometry and realistic terrain (e.g. hills or troughs). We also apply the ZEUSMP hydrocode to model crater formation and ejecta mass-velocity distribution. We have also developed two semi-analytic models, the first being a Monte Carlo model of the distribution of expected ejecta, based on scaling models using a plausible range of crater and ejecta parameters, and the second being a simple model of observational predictions for the shepherding spacecraft that will impact the lunar surface 4 minutes later. Results of these calculations will be presented. For the initial thermal plume we predict an initial expansion velocity of 7 km/s and a maximum temperature of 1200 K. Scaling laws for crater formation and the SPH calculation predict a crater with a diameter of 15 m, a total ejecta mass of 1000 tons, with 10 tons reaching an altitude of 2 km above the target. There is no risk of LCROSS ejecta impacting Moon-orbiting assets. Both the SPH and ZEUSMP calculations predict a maximum ejecta velocity of 1 km/s. The semi-analytic Monte Carlo calculations produce more conservative estimates (by a factor of 5) for ejecta reaching 2 km.
Asphaug Erik
Colaprete Anthony
Jutzi Martin
Korycansky Don
Plesko Catherine
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