Published on January 8, 2008
LCROSS: Overview for Ground-based Observatories: LCROSS: Overview for Ground-based Observatories LCROSS: Lunar CRater Observation and Sensing Satellite Slide2: 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: 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? SP Hydrogen Abundance ( LP data) Each implies different origin and emplacement processes. Slide4: Potential sources and sinks of lunar water ice SOURCES • Comet and asteroid impacts • Reduction of FeO in lunar materials by solar wind hydrogen • Juvenile water released from lunar interior over billions of years SINKS • Meteoritic bombardment • Erosion due to particle sputtering • Photodissociation from interstellar hydrogen Lyman-alpha Slide5: Lunar Ice Summary ICE Nozette, Spudis et al.: • Clementine bistatic radar = ice. • Arecibo = ice. • LP = ice. 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). * H measurements not definitive. Below 1-1.5% H, form of H unknown. Slide6: The LCROSS Mission Slide7: 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 EDUS1 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, (which may include LRO, Chandrayaan-1, HST, etc) 1Launch Vehicle Earth Departure Upper Stage Shepherding Spacecraft Slide8: Mission Hardware EDUS of the LRO EELV: Atlas V rocket ~2000kg (after boil-off) Low risk to LRO due to use of the same adapter (straight load path), interface and separation systems Shepherding Spacecraft & Instruments: ESPA ring spacecraft structure Visual cameras Infrared cameras Near infrared spectrometers Heritage command & data handling avionics Other components common with LRO or already flt qual’d 70 to 80% of software is “reused” Slide9: 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 7 hours before impact, the S-S/C will decelerate to trail the EDUS by 15 minutes and position itself to capture EDUS impact images and impact plume data During the 15 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+15 minutes, but offset by several hundred meters. LCROSS Mission Timeline: Pre-Launch Launch Transfer VIF Ops Pad Ops LV Ascent Park Orbit Coast LRO Inject/Sep Centaur Venting & Re-Target Centaur Handover To LCROSS Checkout TCM-1 TCM-2 TCM-3 Swingby Calibration [TCM-4] Day -1 Day 0 Day 2 Day 1 Day 3 Day 4 Day 5 Day 6 Day 7 Cruise Final Targeting Day 14 Day 35 Day 49 Day 83 Impact and Data Collection Final Targeting Burn EDUS Separation Braking Burn Data Collection EDUS Impact S-S/C Impact Day 100 Day 99 Day 59 [TCM-6] [TCM-7] TCM-9 [TCM-5] TCM-8 Day 90 TCM-11 Timp– 8hrs Timp– 7hrs Timp– 6.5hrs Timp– 2hrs T = 0 Timp+ 4 min TCM-10 LCROSS Mission Timeline SciCal-1 In-flight calibrations S S S S Day 21 Day 42 SciCal-2 SciCal-3 Day 66 Slide11: Ecliptic North Moon’s Orbit BASELINE MISSION: LRO Launch: 10/28,29,30/2008; South Pole impact; Impact date: 2/05/09 To meet the baseline launch window, we will employ a 3.5 month trajectory (red orbit below) Initial injection toward a South Pole LRO insertion Centaur to use DV<50 m/s for partial re-targeting toward a North Pole swing-by. S-S/C to perform remaining DV required achieve desired swing-by conditions Instrument calibration will be over North Pole with South Pole impact 3.5M later The next LRO launch window (11/12,13,14/2006) is actually more desirable, enabling a more robust 3-month mission (green orbit below) Slide12: Mission Operations S-S/C and Instruments: Approx 14 minutes after separation from EDUS, the S-S/C will enter the ejecta cloud created by EDUS impact S-S/C instruments will monitor & measure the ejecta. The S-S/C can be directed to impact within 100m of the EDUS. Expected EDUS impact accuracy of 3km. Anticipated impact velocity > 2km/s at an angle > 70 degrees to the plane of the surface. Slide13: Comparison of LCROSS & Lunar Prospector impacts Impact Model Validation: Impact Model Validation The impact model used to estimated ejecta mass is based on widely used semi-empirical relations (Jay Melosh) Predicted crater size, depth, ejecta mass and velocity were calibrated against highly sophisticated impact models (Eric Asphaug) and experimental data (Peter Schultz) Schultz and Gault (1985) Asphaug (2006) Cratering Efficiency from Experiments Simulations, like the one below for a 2000 kg lunar impact, were used to estimate the impact plume dynamics and characteristics. The figures show the plume 0.01 sec after impact. World Class Impact Science Impact Observation Strategy: Impact Observation Strategy Bright Impact Flash Thermal OH Production Rapid Thermal Evolution Expansion of Plume Thermal Evolution H2O ice sublimation Photo-production of OH 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 Slide16: CBEIM Crater Size The Current Best Estimate Impact Model (CBEIM) 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. Slide17: 2 km 5 km 10 km 15 km 25 km 35 km Ejecta Curtain Characteristics – 1% water content CBEIM and Sensitivity Studies Altitudes Slide18: LCROSS Observational Campaign 0.5 deg = 30’ --> 3476 km 30’ = 1800”: 1800”/3476 km = 0.518”/km Consider different sizes of plume (see previous slide). Plume will also vary in brightness as a function of both time and space. (35 km)(0.518”/pix) = 18.13” (2 km)(0.00863”/km) = 1.036” Rough estimates of plume size Slide19: 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 Slide20: 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. Slide21: Observational Timescales and Platforms Multiple independent measurement methods are used to 1) characterize the impact event 2) provide a definitive understanding of the amount of water contained in the regolith. Slide22: INSTRUMENTS • 2 NIR spectrometers • 1 Visible context imager • 1 Visible total luminance diode • 2 Mid-IR imagers • 2 NIR imagers • 1 Visible spectrometer LCROSS Shepherding Spacecraft Slide23: LCROSS S/S-C Two near-IR spectrometers Monitor spectral bands (every second) associated with water vapor, ice, and hydrated minerals in NIR (1.35-2.25 microns) covering the first overtones of H2O ice (band is free of interference, more brightly illuminated by sunlight than fundamentals near 3 microns). Regions near 1.4 and 1.9 microns (usually obscured by Earth’s atm) also provides sensitive indication of water vapor from ice, shape of band may provide info regarding nature of ice crystals and mineral hydrate. Broad minima at 1.5 and 2.0 microns indicative of water ice Slide24: LCROSS S/S-C Broad minima at 1.5 and 2.0 microns indicative of water ice 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 Reflection spectrum of Quaoar Slide25: 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 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. Ernst and Schultz 2003 Slide26: 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. Slide27: 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). Slide28: 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 Maximum impact time adjustment of 12 hours allowed (due to V limitations) Slide29: Impact Coverage from Hawaii and Chile Analysis Methodology Used nominal LRO launch dates (19 in 2008) Used S. Cooley minimum LCROSS V trajectories to establish nominal impact geometry Adjusted impact time to put Hawaii or Chile in darkness (2 hours from dawn/dusk) Where possible, adjusted impact time to put Moon in Hawaii or Chile view (45 deg elevation) and to achieve dual station coverage Slide31: Backup slides Slide32: 30 45 At impact, want the following proposed viewing conditions: Observatory in darkness (at least 2hrs, or 30 deg from dawn/dusk) Moon elevation angle relative to observatory of at least 45 deg Need to solve for angle, Not to scale! Slide33: rM sin RE sin = = 90 + 45 = 135 deg ==> = 0.65 deg = 180 - - = 44.4 deg = - 30 = 14.4 deg Slide34: 30 45 75.6 deg For impact to be observable by ground telescope with elevation of at least 45 deg and a minimum of 2 hrs before dawn, need to have a Sun-Earth-Moon angle of at least ~76 deg Slide35: 30 45 Additional constraint imposed by Full Moon due to brightness … need to impact at least 30 deg from Full Moon: 76<Lunar Phase Angle<150 210<Lunar Phase Angle<284 Where phase angle is defined as zero for New Moon and 180 for Full Moon 75.6 deg LCROSS Impact Viewing Constraints: LCROSS Impact Viewing Constraints Exclusion Zone Exclusion Zone Slide37: A one day delay in impact time results in a lunar phase angle increase of (13.7 -1) = +12.7 deg A one sidereal month delay in impact time results in a lunar phase angle decrease of 27.3 deg Slide38: Brief Tutorial: Using Neutron Data to Detect Hydrogen -The moon itself emits neutrons (galactic cosmic rays from space hit the Moon and knock neutrons out of regolith). -These neutrons move fast at first, then lose energy as they collide with nearby atoms until they finally reach the same temperature as the surrounding material. Midway between this change (fast to slow), the neutrons are “warm” or “epithermal”. -If you observe a lot of epithermal neutrons --> the initial fast neutrons must be taking awhile to lose energy and become thermal neutrons. -If you observe few epithermal neutrons --> change from fast to thermal energy levels happens fast. -The role of hydrogen: An atom of hydrogen has similar mass as a neutron, so when a neutron collides with a hydrogen atom, the neutron loses most of its kinetic energy instantly. -Therefore by measuring the fluxes of neutrons at several energies we can estimate the amount of hydrogen in the regolith. “Ice at the Lunar Poles”, Vondrak and Crider, New Scientist 91 2003. Slide39: “Integration of lunar polar remote-sensing data sets: Evidence for ice at the lunar south pole” Nozette, Spudis, et al., JGR 106, 2001. - LP: hydrogen detected within permanent shadow at south pole, especially at Shackleton crater. Clementine: Same areas correlate with Clementine bistatic radar data indicating ice. Arecibo: Same areas correlate with “anomalous” high values observed by Arecibo on the lower, sun-shadowed wall of Shackleton crater. Estimates from Arecibo and Clementine suggest ~10 km2 of ice may be present on the Earth-facing wall of Shackleton crater. None of the data is definitive but taken together it is plausible that ice occurs in the cold traps on the Moon (notably in Shackleton crater). LP neutron data, Arecibo, and Clementine data --> Ice in Shackleton. Slide40: “Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles” Feldman et al., Science 281, 1998. H detected at both poles. Observations are consistent with water ice covered by as much as 40 cm of desiccated regolith within permanently shadowed craters near both poles. However, this model is not unique. Could get similar results from Lower water ice abundances in buried deposit Different surface area and surface distribution of the deposit Multilayered geometry (alternating layers of ice and dry regolith) Discrepancy: The neutron data suggests more H in the north yet Clementine data suggests there is more area of permanent shadow in the south. All excess H is not in the form of water ice? Clementine data is incomplete? (south pole was observed by Clementine in winter, so some regions may get sunlight in summer) LP neutron data --> Could be ice at poles. Slide41: “Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles” Feldman et al., Science 281, 1998. Hydrogen abundances NORTH -North facing rim of Peary Crater -Linear trend parallel to 130º meridian extending to 77ºN -Rims of Hermite, Rozhdestvenskiy, and Plaskett craters SOUTH -Rim of the South Pole-Aitken basin -Patches along rim of Shrodinger crater Slide42: “Arecibo Radar Mapping of the Lunar Poles” Stacy, Campbell, Ford, Science 276, 1997. Used the Arecibo 12.6-cm radar system with resolution of 125 m. No areas greater than 1 km2 found with properties suggestive of the presence of ice. Several areas smaller than 1 km2 were found with these properties, but some of these areas are in sunlight (Clementine, Lunar Orbiter data). Features with similar properties were also observed at 47°N (Sinus Iridum). Highest backscatter comes from steep crater walls, not crater floor in several cases. These observations suggest these are regions of rough surfaces and/or blocky areas rather than icy deposits. Clementine radar data is consistent with but not unique to ice deposits. Rock surfaces rough on the scale of the radar wavelength and observed at high incidence angles can result in similar signals. Arecibo data --> Not necessarily ice. Slide43: “Radar Imaging of the lunar poles” Campbell, Nature 426, 2003. Used Arecibo telescope at 70cm for 300 m resolution (can penetrate several meters of lunar dust) Areas of crater floors near poles in permanent shadow do not yield strong radar echoes (like Mercury) Therefore any lunar ice (if present) must be in the form of distributed grains or thin layers (centimeters or less in thickness). This scenario could satisfy the LP results without strong radar backscatter enhancement. Slide44: “Radar Imaging of the lunar poles” Campbell, Nature 426, 2003. NORTH Areas of permanent shadow near 85ºN, 63ºE, floor of Hermite crater, several small craters within large polar crater Peary --> radar backscatter is no different than typical lunar highland terrain SOUTH Floors of Shoemaker and Faustini craters (permanent shadow) have no strong radar echoes. Interior wall of Shackleton crater has a brighter radar signal - could be ice but is also consistent with radar returns from crater walls not in permanent shadow (therefore attributed to rougher terrain). Floor of Shackleton is not visible to the radar. Arecibo data --> Not necessarily ice. Slide45: “The Clementine Bistatic Radar Experiment” Nozette et al., Science 274, 1996. Observed enhancement is localized to the permanently shadowed regions of the south. No enhancement is seen in permanently shadowed regions of the north pole or in sunlight areas. These observations can be explained by the presence of ice in the permanently shadowed regions of the south pole. Clementine data --> Ice in Shackleton. Slide46: “Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry” Campbell and Campbell, in press, 2005. w Used Arecibo and Greenbank telescopes at 70 cm, 450 m resolution for latitudes 60º-90ºS, can probe up to 10s of meters below the surface. Radar variations attributed to variations in surface and subsurface rock populations. Small areas of high enhancement are on shadowed and sunlit terrain, associated with small craters. Arecibo & Greenbank data --> Not ice in Shackleton. Slide47: “Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry” Campbell and Campbell, in press, 2005. w CPR values: larger, old craters w/terraces = moderate CPR; young craters = higher CPR due to more near-surface rugged blocks; smaller craters with sharp rims (e.g. Shackleton) = high CPR. Since CPR values can be high for both shadowed and sunlight regions, likely is not due to ice but rather surface morphology. High CPR values are observed in patchy clusters on the floors of both shadowed and sunit craters. Based on Lunar Orbiter photos, high resolution radar data, and the radar scattering properties of terrestrial rugged terrain, the lunar patterns are likely due to proximal ejecta blankets of abundant small craters. Arecibo and Greenbank data --> Not ice in Shackleton. Slide48: “Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry” Campbell and Campbell, in press, 2005. w Shackleton crater: -Lower portion of the interior wall is not significantly different in 70 cm scattering properties than sunlit areas of craters with similar morphology. Arecibo and Greenbank data --> Not ice in Shackleton. Slide49: “Reanalysis of Clementine bistatic radar data from the lunar south pole” Simpson and Tyler, JGR 104, 3845-3862, 1999. Reanalysis of Clementine bistatic radar data reported by Nozette et al. (1996). Unable to reproduce the results of Nozette et al. (1996) Any observed backscatter enhancements are not unique to the south pole. Observations “easily attributable” to local terrain variations, topography, surface roughness, etc. Clementine data --> Not ice in Shackleton. Slide50: “Space weathering effects on lunar cold trap deposits” Crider and Vondrak, JGR 108, 3845-3862, 2003. - A detailed study by Crider & Vondrak simulates the evolution of a H2O column in a lunar cold trap over time as a function of depth with H2O arriving from both the solar wind and from comets. - They conclude that the regolith would reach an equilibrium concentration of H2O at 4100 ppm (0.41% per unit mass). This equilibrium value would be reached from solar sources alone and comets essentially are superfluous. Time merely increases the thickness of the layer in which ice will be harbored. In 1 billion years the layer would be 1.6 m thick. The ice would be diffuse. - Their results are consistent with Arecibo observations and within a factor-of-2 LP neutron spectrometer values. Theory --> Not much ice (if present) in Shackleton.