Transcript Lunar Laser Ranging (LLR) within LUNAR

Lunar Laser Ranging (LLR) within LUNAR
Tom Murphy1
Doug Currie2
Stephen Merkowitz3
D. Carrier, Jan McGarry3, K. Nordtvedt, Tom Zagwodski3
with help from:
E. Aaron, N. Ashby, B. Behr, S. Dell’Agnello, G. Della Monache,
R. Reasenberg, I. Shapiro
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UCSD;
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U Md;
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GSFC
LLR Science Motivations
• Fundamental incompatibility of QM and GR
– Improve our tests of GR
• Dark Energy may be misunderstanding of large-scale gravity
– Dvali idea replaces with leaky gravity  lunar precession
• Inflation may have left residual scalar fields (inflaton)
– generic result is violation of EP and changing constants
• Dark Matter inspires alternative gravity models (MOND)
– test of inverse square law could reveal
• Lunar Science
– probe properties of liquid core
– measure dissipation and core-mantle boundary interaction
– get interior structure through Love numbers and gravity field
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How Does LLR Work?
Short laser pulses and time-of-flight
measurement to high precision
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Previously
200 meters
LLR through the decades
big telescope,
fat laser pulse
small telescope, narrow laser pulse
APOLLO
big telescope, narrow laser pulse
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Dominant Uncertainty
near corner
tilted
reflector
array
Laser Pulse
far corner
fat laser pulse:
return uncertainty
dominated by pulse
medium laser pulse:
return uncertainty
dominated by array
short laser pulse:
return uncertainty
dominated by pulse
array irrelevant/resolved
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APOLLO Example Data
Apollo 15
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Apollo 11
red curves are theoretical profiles: get convolved with fiducial to make lunar return
represents system
capability: laser;
detector; timing
electronics; etc.
RMS = 120 ps
(18 mm)
• 6624 photons in 5000 shots
• 369,840,578,287.4  0.8 mm
• 4 detections with 10 photons
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• 2344 photons in 5000 shots
• 369,817,674,951.1  0.7 mm
• 1 detection with 8 photons
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Sensing Array Size and Orientation
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Sparse Array Solves Problem
• A sparse (even random) array of corner cubes will temporally
separate individual returns
– now dominated by ground station characteristics
– moderate advances in ground technology pay off
• Can either build deliberately sparse array, or scatter at random
– will figure out each reflector’s position after the fact
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Extracting Science
• Ground station records photon times: launch and return
• Build a sophisticated parameterized model to try to mimic time
series, including:
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model for gravity (equations of motion)
solar system dynamics
body-body interactions
dissipative physics (tidal friction)
crustal loading phenomena (atmosphere, ocean)
relativistic time transformation (clocks)
relativistic light propagation
atmospheric propagation delay
• Minimize residuals between obs. and model in least-squares fit
– result is a bunch of initial conditions, physical scales, gravity model
• Analysis is currently behind observation (recent development)
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Our Mission
• LLR has been a foundational technique in studying gravity
• Today’s precision is limited by the arrays
– designed for 1970 laser
• Now that we have millimeter range precision, the model is the
limiting factor in extracting science
• We should design a new system that will outlive 2010 lasers and
timing systems
– passive reflectors are long-lived
– 10 m emplacement is an appropriate goal
• We should develop the science case and expand our ability to
model LLR for a new regime of high precision
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Our Team
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Doug Currie (UMd) part of original Apollo reflector/LLR team
Stephen Merkowitz (GSFC) LISA, transponders, gravity
Tom Murphy (UCSD) is PI for APOLLO; millimeter LLR
Ken Nordtvedt: master gravitational phenomenologist/theorist
David Carrier: Apollo drilling expert
Jan McGarry (GSFC): Satellite Laser Ranging & transponders
Tom Zagwodski (GSFC): Satellite Laser Ranging & transponders
Ed Aaron (ITE): Corner cube fabrication
Neil Ashby (U Colorado): tests of relativity
Brad Behr (Maryland): thermal modeling
Simone Dell’Agnello & Giovanni Della Monache (LNF, Italy):
Corner cube testing and LLR modeling
• Bob Reasenberg & Irwin Shapiro (Harvard/CfA): LLR modeling
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Our Plan, In Overview
• Development of theoretical tools
– hone science case for sub-millimeter LLR
– develop a next-generation LLR model and use for science simulation
• Next-generation corner cube and array design
– optimize designs, initially following parallel tracks of solid cube (Currie)
and hollow cube (Merkowitz)
– extensive thermal modeling and testing (partly at the Space Climatic
Facility in Frascati, Italy)
• Transponder design
– develop plans for an architecture suitable for LLR via active
transponders
• Environment/Emplacement
– develop strategies for dust mitigation
– work out emplacement scheme, aiming for 10 m stability
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Progress Toward LUNAR Goals
Lunar Environment
LRO 2-way Ranging
Theoretical Tools
Model Development
Degradation of Apollo CCRs
• We see strong evidence for degraded performance of the Apollo
arrays after 40 years on the moon
• Signal response down by factor of ten at all phases
• Signal suffers additional factor of ten loss near full moon
– yet eclipse measurements are fine  thermal problem
• Can see this effect begin as early as 1979
• Lunokhod reflector has degraded far faster than Apollo reflectors
related to environment mitigation part of work plan
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full moon
APOLLO rates on Apollo 15 reflector
background level
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More on the deficit
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APOLLO system sensitivity is not
to blame for full-moon deficit
– background is not impacted
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Early LLR data trucked right
through full-moon with no
problem
The deficit began to appear
around 1979
No full-moon ranges from 1985
until 2006, except during eclipse
Lunokhod 2 was once 25%
stronger than Apollo 15; now 10
weaker than Apollo 15
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What’s causing the degradation?
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The full-moon deficit, together with
normal eclipse behavior, gives us
the best clues:
– thermal nature
– absorbing solar flux
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Modification of the front surface by
dust deposition or abrasion would
change the thermal properties
– so would bulk absorption in the CCR
– a 4K gradient is all it takes to
reduce response by 10
– would also account for overall deficit
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Lunokhod worse off, because more
exposed (not recessed)
– also silvered back, not TIR
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Preparations for LRO 2-way ranging
• The Lunar Reconnaissance Orbiter (LRO)
included a CCR array on board
– 12 31.7 mm unspoiled TIR corner cubes
• Only APOLLO is capable of ranging to it
• APOLLO is being retooled to the task
– wider gate (800 ns vs. 100 ns) to deal with
range uncertainty
– developing tracking capability
• Aside from the gains cm-level precision will
offer to LRO, APOLLO can verify link strength
to pristine, well-characterized CCRs
• Modifications will also assist in finding the lost
Lunokhod 2 reflector
– LRO imaging may beat us to it!
not explicitly part of work plan, but highly relevant
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Exploring New Science Paradigms
• Nordtvedt has examined a second-order effect that modifies PPN
 and  by an amount proportional to the sun’s binding energy: U
 4106
– effectively probing the coupling between the sun’s and the earth’s
gravitational binding energies
– any experiment reaching 4106 in  or  will become sensitive to this
second-order PPN effect (equiv. to EP test to 21015)
–  is now determined to 2.5105 by Cassini
–  is now determined to 104 by LLR at the centimeter level
• Nordtvedt is also looking at how solar tidal energy in the lunar
orbit effects the way the moon falls toward the sun
– the solar tidal energy is sourced from the sun, and will not contribute
to the moon’s orbital inertia like the other energies involved
– the effect is at the level of 71014, not far from the 1.31013 EP limits
to date
part of theoretical tools work plan
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Development of Analysis Tools
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New physics ideas must be coded into an analysis model
Currently, we lack an openly available and modern platform for LLR
analysis
– JPL has best code, but the code is unavailable
– PEP is semi-functional, open to us, but needs modernization
• PEP is currently the most attractive option
– Jürgen Müller in Germany has modern code, unavailable
– GEODYN is used for SLR in earth-center frame, may be adaptable to LLR
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The models currently lack:
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ocean and atmospheric loading
geocenter motion (1 cm)
latest atmospheric propagation delay (and gradient) models
tie to local gravimeter/GPS to inform site motion
and plenty more (many sub-centimeter effects previously ignored)
But mm-quality data is a recent development: the model effort lags
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Model Tasks
• We are exploring which model/code is worth putting our efforts
into (Y1 task)
• Once settled, we will begin to perform simulations of submillimeter LLR datasets to learn what the science potential
might be (Y2 task)
• Finally, we will code-in new physics so that we may simulate
sensitivities (Y3+ task)
part of theoretical tools work plan
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