The site selection for both robotic and human landings on the Moon may be dependent upon identifying the availability of in- situ lunar resources. Earlier Clementine radar observations and Lunar Prospector neutron spectroscopy suggest that water ice, a potential resource, is found in shadowed craters near the lunar poles. Since these observations are by no means conclusive, we need to determine whether or not ice exists in significant quantities at the lunar poles such that informed decisions can be made regarding 1) its potential utility as a lunar resource, and consequently 2) the potential suitability of the lunar poles for a human outpost from an in-situ resource utilization (ISRU) perspective. To probe the lunar regolith in permanent shadow the mission will utilize two massive impactors to release dust and volatiles from the lunar subsurface into a plume to be observed primarily by the Shepherding Spacecraft (S-S/C), and complimented by observations from ground-based, aerial and orbital telescopes.

Crater Impact Characteristics
The primary goal of LCROSS is to measure the concentration of water ice (ice to dust ratio) in permanently shadowed lunar regolith. Setting constraints on water ice will set a fiducial for the LRO studies of hydrogen neutrons, that are expected to have water ice as a source. Several important processes occur when a body strikes the lunar surface, including the initial impact, ejecta and plume dispersion, and the exposure of fresh subsurface (Figure 1). Continuously monitoring the impact events at a variety of spatial (m to km to exosphere scales) and temporal scales (sec to minutes to days) allows us to understand lunar impact processes and assess the likelihood that water ice, due to impacts occurring within the permanently shadowed target crater, may be distributed non-uniformly.

Figure 1 The life cycle of a lunar impact and associated time and special scales. The LCROSS measurement methods are “layered” in response to the rapidly evolving impact environment.

The lunar impacts of the 2000 kg upper stage and the 700 kg S-S/C will take place at a velocity of 2.5 km/sec and at an angle of ~75°. The resulting impact craters will be ~28m in diameter by ~5m deep (Centaur) and ~18m in diameter by 3.5m deep (Shepherding Spacecraft). The ejected mass from these craters is shown in Figure 2. Analysis to date has shown LCROSS will create a significantly larger crater than Lunar Prospector (LP) that hit the Moon at 1.7 km/s with a 158 kg impactor at a glancing angle of 6°. Temperatures reached will be insufficient to vaporize most (~90%) of the material, though is expected to create a very brief visible flash that will last < 100ms which will be measured by the S-S/C photometer instrument. Most of the ejecta will be thrown upward at a velocity of >250 m/sec, and will contain water in its solid state, as water ice mixed with the dust (refractory minerals).

Figure 2 The predicted ejected mass following the LCROSS Centaur impact is ~230 times larger than that predicted for Lunar Prospector (LP).

Water ice in the ejected dust cloud will sublime (convert from a solid to a gaseous state) under the influence of solar irradiation. The rate of this sublimation depends primarily on the water to ice ratio and particle size, with ~0.1 mm dust rich (1% water ice) particles subliming their water in several minutes. The surface brightness of the cloud increases as the ejecta expands; in 40 seconds, the ejecta cloud will fill a 1 arc second observing aperture as seen from Earth-based telescopes. Subsequently, the water molecules will be dissociated by solar UV radiation and the OH molecule will be observable in emission at 308nm.

A state-of-the-art, multi-phase impact code and NASA Ames Vertical Gun experiments will support payload and mission development as well as analysis of the observations. These tools will be used to refine the current estimates of impact flash, ejecta thermal and trajectory evolution, and assessment of the quantity and physical state of water release.

Technical Information
Overview | Mission Rationale | Spacecraft and System Description | Instrumentation | Water Detection | Targeting
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