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 LCROSS - Lunar CRater Observation and Sensing Spacecraft
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Lunar CRater Observation and Sensing Satellite (LCROSS)

Mission Rationale

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 needed 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 ISRU perspective. To probe the lunar regolith in permanent shadow we proposed 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.

The lunar impacts of the 2000 kg upper stage and the 700 kg S-S/C were to take place at a velocity of 2.5 km/sec and at an angle of 75°. The resulting impact craters would 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 had shown LCROSS would 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 would be insufficient to vaporize most (~90%) of the material, though was expected to create a very brief visible flash that would last < 100ms which would be measured by the S-S/C photometer instrument. Most of the ejecta would be thrown upward at a velocity of >250 m/sec, and would contain water in its solid state, as water ice mixed with the dust (refractory minerals).

Graph of ejected mass as a function of impact angle
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 would sublime (convert from a solid to a gaseous state) under the influence of solar irradiation. The rate of this sublimation would depend 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 would increase as the ejecta expaned; in 40 seconds, the ejecta cloud would fill a 1 arc second observing aperture as seen from Earth-based telescopes. Subsequently, the water molecules would be dissociated by solar UV radiation and the OH molecule would be observable in emission at 308nm.

A state-of-the-art, multi-phase impact code and NASA Ames Vertical Gun experiments supported payload and mission development as well as analysis of the observations. These tools were 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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Editor: Brian Day
NASA Official: Daniel Andrews
Last Updated: December 2009