Predictions for the LCROSS mission

Abstract

We describe the results of a variety of model calculations for predictions of observable results of the LCROSS mission to be launched in 2009. Several models covering different aspects of the event are described along with their results. Our aim is to bracket the range of expected results and produce a useful guide for mission planning. In this paper, we focus on several different questions, which are modeled by different methods. The questions include the size of impact crater, the mass, velocity, and visibility of impact ejecta, and the mass and temperature of the initial vapor plume. The mass and velocity profiles of the ejecta are of primary interest, as the ejecta will be the main target of the S-S/C observations. In particular, we focus on such quantities as the amount of mass that reaches various heights. A height of 2 km above the target is of special interest, as we expect that the EDUS impact will take place on the floor of a moderate-sized crater ~30 km in diameter, with a rim height of 12 km. The impact ejecta must rise above the crater rim at the target site in order to scatter sunlight and become visible to the detectors aboard the S-S/C. We start with a brief discussion of crater scaling relationships as applied to the impact of the EDUS second stage and resulting estimated crater diameter and ejecta mass. Next we describe results from the RAGE hydrocode as applied to modeling the short time scale (t ≤ 0.1 s) thermal plume that is expected to occur immediately after the impact. We present results from several large-scale smooth-particle hydrodynamics (SPH) calculations, along with results from a ZEUS-MP hydrocode model of the crater formation and ejecta mass-velocity distribution. We finish with 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 (S-S/C) that will follow the impact for several minutes until its own impact into the lunar surface. For the initial thermal plume, we predict an initial expansion velocity of ~7 km s^(-1), and a maximum temperature of ~1200 K. Scaling relations for crater formation and the SPH calculation predict a crater with a diameter of ~15 m, a total ejecta mass of ~106 kg, with ~10^4 kg reaching an altitude of 2 km above the target. Both the SPH and ZEUS-MP calculations predict a maximum ejecta velocity of ~1 km s^(-1). The semi-analytic Monte Carlo calculations produce more conservative estimates (by a factor of ~5) for ejecta at 2 km, but with a large dispersion in possible results.