LADEE
Science Objectives
LADEE Mission Concept Review
(MCR)
December 10, 2008
Greg Delory – Deputy Project Scientist
Rick Elphic – Project Scientist
Tom Morgan – Program Scientist
Summary Overview
• The top eleven science goals identified in
the National Research Council’s report,
“Scientific Context for the Exploration of
the Moon” include:
a. Determine the global density,
composition, and time variability of the
fragile lunar atmosphere before it is
perturbed by further human activity
b. 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.
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Lunar Exosphere/Dust Environment
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Lunar Exosphere (1)
ALSEP-LACE Detections on Apollo 17
Post Apollo Ground-based
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)
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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
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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:
1. Regolith is fine-grained
2. 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.
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100 mm
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Evidence of Dynamic Dust
From lunar orbit…
…and on the lunar surface
Gene Cernan sketches from Apollo
Command Module
Lunar Ejecta and Meteorites experiment (LEAM)
Terminators
McCoy and Criswell, 1974
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Berg et al., 1976
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SDT Findings - Exosphere
• Species that make up exosphere:
1) prevalent at 50 km altitude
2) maximize at sunrise terminator
3) peak densities at equator
• Species can be categorized by their
sources, including solar wind, regolith
and radiogenic
Model results @50 km
from R. Hodges for the
LADEE SDT
• 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.
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SDT Findings - Dust
Stubbs et al., 2006
• Two dust components:
1) Dust of Lunar Origin (DoLO)
2) 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
• 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
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• Distinct Targets-of-Opportunity for
improved dust observations:
˗ Known meteor shower/comet
tails
˗ Magnetotail/plasma sheet
crossings
˗ Solar storms
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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)
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Science Flowdown - Payload
Neutral Mass Spectrometer (NMS)
UV Spectrometer (UVS)
MSL/SAM Heritage
LCROSS heritage
In situ
measurement
of exospheric
species
Dust and exosphere
measurements
300 Dalton range/unit mass resolution
Dust Detector (DD) – TBD
Detect sub-micron dust (size, charge)
Lunar Laser Com Demo (LLCD)
Technology demonstration
100 mm Optical
Module
To be selected
3 kg, 5W envelope
Science synergy:
high data rates
s
dem
Mo
Several well-understood
candidates available
ol
ntr
Co onics
ctr
Ele
60 c
m
51-622 Mbps
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
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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 noonterminators-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
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Operations Profiles
1
UVS
2
NMS/DD
3
UVS
4
Comm/PWR
5
NMS/DD
6
UVS
7
NMS/DD
Duty cycled profiles
to optimize
observations vs.
bus resources
1
UVS
2
Comm/PWR
3
UVS
4
NMS/DD
5
Comm/PWR
6
NMS/DD
NMS/DD: 40% duty cycle
UVS: 1500-3000 s integration
times w/cycled occultations
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Example Orbits
UVS Orbit
NMS/DD Orbit
Sub-pointing
Ram-pointing
configuration
Limb-pointing
LLCD Orbit
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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
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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
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