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Lunar Crater Observation and Sensing Satellite (LCROSS) Mission: Opportunities for Observations of the Impact Plumes from Ground-based and Space-based Telescopes American Astronomical Society Honolulu, HI 30 May 2007 Dr. Jennifer L. Heldmann NASA Ames Research Center T. Colaprete1, D. Wooden1, E. Asphaug2, P. Schultz3, C.S. Plesko2, L. Ong2, D. Korycansky2, K. Galal1, G. Briggs1 1 NASA Ames Research Center, 2 UC Santa Cruz, 3 Brown University
Basic Science Questions Addressed by LCROSS • Nature and origin of polar hydrogen • Distribution • Concentration • Origin • Impact cratering dynamics - Plume evolution / ejecta curtain dynamics - Crater formation processes - Thermal evolution (plume, crater, remnant ejecta) 3) Target material properties - Geotechnical properties - Dust (particle size distribution, thermal properties, etc.)
Outstanding science questions Lunar Prospector detected enhanced levels of hydrogen near the lunar poles. What is the Quantity, Form, and Distribution of the polar hydrogen?? The answers are currently unknown. Possible forms of the H: 1) Water ice? 2) Hydrated minerals? 3) Organics? 4) Solar wind hydrogen? Each implies different origin and emplacement processes. SP Hydrogen Abundance ( LP data)
Lunar Ice Summary Not ICE • Clementine bistatic radar = irreproducible results for ice, same signals seen in sunlit areas. • Arecibo = not ice, same signals seen in sunlit regions, not anomalous in Shackleton. • LP = why more Hydrogen detected in the north when more permanent shadow in the south? • Theory = H2O evolution in lunar cold trap reaches equilibrium over time (diffuse deposits, 0.41% by mass). ICE • Clementine bistatic radar = ice. • Arecibo = ice. • LP = ice. * H measurements not definitive. Below 1-1.5% H, form of H unknown.
Mission Description • Lunar CRater Observing and Sensing Satellite (LCROSS) • The LCROSS Mission is a Lunar Kinetic Impactor employed to reveal the presence & nature of water on the Moon • LCROSS Shepherding S/C (S-S/C) accurately directs the 2000 kg EDUS into a permanently shadowed region at a lunar pole, creating a substantial cloud of ejecta (~60 km high, >200x the energy of Lunar Prospector) • The S-S/C decelerates, observes the EDUS plume, and then enters the plume using several instruments to look for water • The S-S/C itself then becomes a 700 kg secondary impactor • Lunar-orbital and Earth-based assets will also be able to study both plumes Shepherding Spacecraft 1Launch Vehicle Earth Departure Upper Stage
Mission Timeline • Lunar Gravity Assist, Lunar Return Orbit (LGALRO): Following the release of LRO, the S-S/C & EDUS will enter a ~86 day orbit (5 day lunar swing-by, 81 day earth orbit): • Allows for LRO to become operational • Allows for EDUS propellant boil-off • Allows for impact targeting • Upon separation from EDUS, about 8 hours before impact, the S-S/C will decelerate to trail the EDUS by 4 minutes and position itself to capture EDUS impact images and impact plume data • During the 4 minutes after EDUS impact, the S-S/C will be collecting and transmitting data, then slightly divert its trajectory to impact the same general area at T+4 minutes, but offset by several hundred meters. Nominal launch date: Oct 2008 Nominal impact date: mid-Feb 2009
Impact Observation Strategy • Expansion of Plume • Thermal Evolution • H2O ice sublimation • Photo-production of OH • Bright Impact Flash • Thermal OH Production • Rapid Thermal Evolution • Residual Thermal Blanket • Expanding OH Exosphere The combination of ground-based, orbital and in-situ platforms span the necessary temporal and spatial scales: from sec/meters to hours/km
LCROSS Shepherding Spacecraft INSTRUMENTS • 2 NIR spectrometers • 1 Visible context imager • 1 Visible total luminance diode • 2 Mid-IR imagers • 2 NIR imagers • 1 Visible spectrometer
LCROSS S/C Measurements Ice: Near-IR spectroscopy of the scattered sunlight absorption (fundamental and overtone) features of water ice in situ Vapor: Near-IR spectra of H2O vapor (sublimed ice) emission bands (overtone vibration bands at 1.4 and 1.9 microns) in situ, and of fundamental bands near 3 microns from ground-based 10 m class telescopes * Note no sharp water bands at 1.4 and 1.9 microns (overtone). Small feature at 2.9 microns (fundamental) is due to terrestrial water contamination. Pieters et al., LPSC 2006
LCROSS S/C Measurements Ice: Near-IR spectroscopy of the scattered sunlight absorption (fundamental and overtone) features of water ice in situ Vapor: Near-IR spectra of H2O vapor (sublimed ice) emission bands (overtone vibration bands at 1.4 and 1.9 microns) in situ, and of fundamental bands near 3 microns from ground-based 10 m class telescopes Measurement of an extended OH- atmosphere via spectroscopy at the 308 nm OH- band at UV-visible wavelengths along with nearby scattering continuum Spectroscopy covering the 619 nm H2O+ band and adjacent scattering continuum Narrow band imaging at mid-IR wavelengths to follow thermal evolution of plume and newly deposited regolith, which will be affected by water vapor in the ejecta.
LCROSS S/S-C Reflection spectrum of Quaoar Red line is reference spectrum for water ice. A sharper minimum at 1.65 microns shows that the ice is crystalline in structure, rather than amorphous. Jewitt and Luu, Nature 2004 Broad minima at 1.5 and 2.0 microns indicative of water ice
LCROSS S/S-C Camera VIS: context camera to 1) observe EDUS impact, 2) observe ejecta cloud morphology and evolution. Luminance Diode VIS: observe impact flash • light flash due to thermal heating & vaporization • shape of the flash’s light curve can be used to determine certain initial conditions of the impact • flash peak intensity depends on impact velocity angle, target & projectile types Ernst and Schultz 2003 Light curve as recorded from a photodiode of a typical Pyrex impact into pumice dust at the NASA Ames Vertical Gun Range. Two components can be seen: as intensity peak lasting 50-100 s that depends on projectile parameters, and a long-lasting decaying blackbody signal dependant on target parameters.
LCROSS S/S-C Two cameras MID-IR(2 wavelengths): look down on permanently shadowed lunar surface to map pre-impact terrain (warmer vs cooler = rocks vs regolith), thermal evolution of plume (dependent upon H2O vapor concentration in plume), ejecta blanket, and freshly exposed regolith.
LCROSS S/S-C Two cameras NIR:2 wavelengths– obtain spatial distribution data regarding the H2O (vapor and ice) content. One spectrometer VIS: look for H2O+ (619 nm) and OH– (308 nm) radicals from sunlight-ionized and sunlight-dissociated H2O vapor molecules; look for evidence of organics (e.g. CN = 380 nm).
Ground-Based Observatory Viewing • Observatory Viewing Constraints • Observatory needs to be 2 hours from dawn/dusk at impact • Impact must occur when Moon is more than 30 deg away from New Moon or Full Moon • Elevation angle of Moon at impact relative to observatory must be greater than 45 deg • Impact optimized for observing from Hawaii • Maximum impact time adjustment of 12 hours allowed (due to V limitations)
Ground-Based Observatory Viewing Observatory Viewing Constraints Phases of the Moon Effective Ground Obs Exclusion Zones
Site selection activities have resulted in five potential impact sites: * Shoemaker * Shackleton * Cabeaus * Faustini * Peary
LCROSS Modeling to Support Observations • The LCROSS Project is actively conducting both laboratory and numerical modeling simulations to support the planning of ground-based and space-based observations. • This information will be made available to all astronomers interested in observing the LCROSS impacts.
Solar Backscatter Projected column annulus at time, t 0.1 km Measurement Studies - Sensitivity • To simulate the solar illumination of the ejecta curtain a well known (and tested) multi-stream scattering code is used (DISORT). • Estimates of the ejecta dust and water ice optical depths are made based on the expected total ejected mass and assumptions of the mean particle size and optical properties. • Linear mixing is used to combine dust and water ice cloud optical properties. Calculated Backscatter Flux t, w,g(dust,ice)
The Current Best Estimate Impact Model (CBEIM) CBEIM Crater Size The CBIEM summarizes the results of numerous impact models / assessments. Used as the base to drive mission design and instrument selection. Efforts continue to refine the model.
CBEIM and Sensitivity Studies Ejecta Curtain Characteristics – 1% water content Altitudes 2 km 5 km 10 km 15 km 25 km 35 km
Plume Brightness Irradiance at time of impact plus 10 (purple), 20 (blue), 30 (green), and 60 (red) seconds.
0.1 km Ejecta Mass The ejecta cloud will more-or-less look like an expanding conical section (an upside-down lampshade). The figure below (images from a hypervelocity shot at the NASA AVG) demonstrates this geometry. Ejecta cloud optical depth modeled with a truncated conical section, the “upside-down lampshade” model. t+2 Solar Scatter t+1 t Projected column annulus at time, t
Funding • Funding is required to support astronomer time, travel, data analysis, etc.
Funding • Funding is required to support astronomer time, travel, data analysis, etc. • NASA will provide astronomer funding through ROSES R&A: new LASER program (Lunar Advanced Science and Exploration Research).
Funding • Funding is required to support astronomer time, travel, data analysis, etc. • NASA will provide astronomer funding through ROSES R&A: new LASER program (Lunar Advanced Science and Exploration Research). • One proposal will be submitted to ROSES (coordinated by J. Heldmann at NASA Ames). Astronomers are responsible for securing observing time at telescopes; astronomers with telescope time are then eligible to be included in the LCROSS LASER proposal.
Proposals • The LCROSS Project will make available all modeling results pertaining to the impacts for use in observation planning. • The LCROSS Project is considering holding a Workshop for all interested observations to discuss issues pertaining to planning of observations, instruments available for observing, scheduling, science background, etc. • The goal of the Workshop is to help facilitate the preparation of proposals to secure telescope observing time. COMMENTS??
For more information… • Website: lcross.arc.nasa.gov • Contact me: Dr. Jennifer L. Heldmann LCROSS Observation Campaign Coordinator NASA Ames Research Center Mail Stop 245-3 Moffett Field, CA 94035 650-604-5540 firstname.lastname@example.org