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LADEE Science Objectives

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  1. LADEEScience Objectives LADEE Mission Concept Review (MCR)December 10, 2008 Greg Delory – Deputy Project Scientist Rick Elphic – Project Scientist Tom Morgan – Program Scientist

  2. Summary Overview • The top eleven science goals identified in the National Research Council’s report, “Scientific Context for the Exploration of the Moon” include: • Determine the global density, composition, and time variability of the fragile lunar atmosphere before it is perturbed by further human activity • Determine the size, charge, and spatial distribution of electrostatically transported dust grains and assess their likely effects on lunar exploration and lunar-based astronomy. • The LADEE mission was designed to begin to address these objectives.

  3. Lunar Exosphere/Dust Environment

  4. Lunar Exosphere (1) Post Apollo Ground-based ALSEP-LACE Detections on Apollo 17 Remote observations of the Na exosphere (Potter & Morgan, 1998) Global distribution of Ar-40 at 30 km and 50 km (D. Hodges). • The Exosphere is a collisionless atmosphere. Atoms due not collide with each other to thermalize the gas ̶ terms like temperature must be treated with care. • Some of the exosphere is externally derived ̶ Solar Wind and Interplanetary dust ̶ but most is originated from the regolith and reflects composition. • There are various source and sink mechanisms, dependent on species, location on the Moon, position of the Moon in its orbit, internal activity and external events (meteor showers, solar conditions)

  5. Lunar Exosphere (2) Lunar Atmospheric and Composition Experiment (LACE) 40Ar Measurements • Identified Constituents are: 40Ar, 36Ar, 222Rb, He, H, Na, K; the upper limit on density of the exosphere is a few times 107 atoms /cm3 • Upper limits established for most other species – but solid detections of important components such as C, N, O, CO2, CH4 + metals remain elusive

  6. Lunar Dust • All size fractions are present, but the very smallest are difficult to collect or to adequately count. Based on mechanical techniques the accepted wisdom is that: Regolith is fine-grained Roughly: - 10% smaller than 10 mm - 20% smaller than 20 mm 3. Predictable physical properties (porosity, thermal conductivity, shear and bearing strength, angle of repose, tribology) 4. Notice the shape/surface area Plenty of dust, particularly in the size range below 10 - 20 um for which electrostatic forces dominate over gravity. 100 mm

  7. Evidence of Dynamic Dust …and on the lunar surface From lunar orbit… Gene Cernan sketches from Apollo Command Module Lunar Ejecta and Meteorites experiment (LEAM) Terminators Berg et al., 1976 McCoy and Criswell, 1974

  8. SDT Findings - Exosphere • Species that make up exosphere: • prevalent at 50 km altitude • maximize at sunrise terminator • peak densities at equator • Species can be categorized by their sources, including solar wind, regolith and radiogenic • Ar, He, H/H2, OH, CH4, CO, CO2, Na, K, Si, Al, Fe all of interest • No one instrument/technique can obtain all species of interest • A NMS will likely detect Ar, He, H2 but will have great difficulty with trace species requiring a supporting instrument • Mission lifetime of a year is ideal, but new, interesting science can be done in 3 mo. Model results @50 km from R. Hodges for the LADEE SDT

  9. SDT Findings - Dust Stubbs et al., 2006 • Two dust components: • Dust of Lunar Origin (DoLO) • Interplanetary Dust (IDP) • DoLO: Electrostatically lofted or secondary ejecta • DoLO may peak near terminator with densities at ~10-4/cc at 50 km • IDPs/secondary ejecta detected at all longitudes • Distinct Targets-of-Opportunity for improved dust observations: • Known meteor shower/comet tails • Magnetotail/plasma sheet crossings • Solar storms • DoLO single impacts very difficult to detect since grains are submicron and slow • Remote sensing UV/VIS instrument will provide critical complementary data to in situ observations

  10. Baseline Science In order to accomplish the science objectives, the LADEE mission shall meet the following baseline science requirements: • Measure spatial and temporal variations of Ar, He, Na, and K over time scales from several (3) lunar orbits to one lunation. • Detect or obtain new lower limits for other species for which observations have been made. These include the following elements or compounds and the current limit* (part/cm3); CH4(1104), S(150), O(1103), Si(48), Kr(2104), Xe(3103), Fe(3.8x102), Al(55), Ti (1), Mg(6103), OH(1106), and H2O(100). • Search for other species (beyond those listed in the previous two bullets) or positive ambient ions of these species and other atoms or compounds in the 2-150 Da mass range. • Detect or set upper limits as small as 10-4 dust particles /cm3 from 1.5 to 50 km altitude for particles as small as 100 nm via occultation measurements. • Detect or set upper limits on the dust population at 50 km. *Limits measured against Table 1.1, S. A. Stern, Reviews of Geophysics, 37, 453, 1999. (Stern states no limit for H20)

  11. Science Flowdown - Payload UV Spectrometer (UVS) Neutral Mass Spectrometer (NMS) MSL/SAM Heritage LCROSS heritage In situ measurement of exospheric species Dust and exosphere measurements 300 Dalton range/unit mass resolution Lunar Laser Com Demo (LLCD) Dust Detector (DD) – TBD Detect sub-micron dust (size, charge) Technology demonstration To be selected 3 kg, 5W envelope Several well-understood candidates available Science synergy: high data rates 51-622 Mbps

  12. Science Flowdown – s/c & Trajectory • Ideal orbit: Circular, retrograde, low inclination • Most active science location: Periseline at < 50 km over sunrise terminator • Retrograde orbit keeps instruments in ram but out of sunshine • Equatorial orbit preferred over polar orbit: Densest portion of exosphere, don’t expect emitted polar water from cold traps. LADEE can contribute to search for water by “following the OH” and examining the terminator desorption processes • Spacecraft is “dirty” and will have the potential to contaminate instruments via outgassing, thruster firings, and EMI

  13. Science Flowdown - Operations • Guiding Considerations • Spatial and temporal variations crucial to LADEE science objectives • Science operations need to cover variations ranging from ~3 orbits to ~1 month or more. • Spatial variations across noon-terminators-midnight important • UVS: • Sufficient time to achieve SNR ~5 for most species • Occultation (dust) and limb (dust, exosphere) modes • NMS: • Sufficient time to resolve major species • Ram, nadir, and rotisserie modes • DD: • Similar to NMS Operations Profiles Duty cycled profiles to optimize observations vs. bus resources NMS/DD: 40% duty cycle UVS: 1500-3000 s integration times w/cycled occultations

  14. Example Orbits UVS Orbit NMS/DD Orbit Sub-pointing Ram-pointing configuration Limb-pointing LLCD Orbit

  15. Follow-On Analysis • Pointing w.r.t. lunar limb • Uncertainty in position of the Moon (lunar ephemeris) • Active trade underway examining slewing/coverage/SNR • Altitude knowledge (ties into previous bullet) • 3 km is current goal; 1 km desired • Part of larger trajectory/navigation trade analysis • Continue orbit/science observation trade • Trajectory/fuel trade may lead to more variable orbits • OK with science with modified observation profiles • Potential coverage at low (20 km) altitudes very compelling from PSWG perspective

  16. Summary • Top level spacecraft and mission capabilities meet the baseline science objectives • Current activities focused on optimization • Implementation of baseline science remains flexible and can be responsive to changes in the mission design