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

1 UCSD; 2 U Md; 3 GSFC

2009.09.21 2

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-flightmeasurement to high precision

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LLR through the decadesPreviously200 meters

APOLLO

big telescope,fat laser pulse

small telescope, narrow laser pulse

big telescope, narrow laser pulse

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Dominant Uncertaintytilted

reflectorarray

fat laser pulse:return uncertainty

dominated by pulse

medium laser pulse:return uncertainty

dominated by array

short laser pulse:return uncertainty

dominated by pulsearray irrelevant/resolved

far corner

near corner

Laser Pulse

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APOLLO Example Data2007.11.19Apollo 15 Apollo 11

• 6624 photons in 5000 shots• 369,840,578,287.4 0.8 mm• 4 detections with 10 photons

• 2344 photons in 5000 shots• 369,817,674,951.1 0.7 mm• 1 detection with 8 photons

red curves are theoretical profiles: get convolved with fiducial to make lunar return

represents systemcapability: laser;detector; timingelectronics; etc.

RMS = 120 ps(18 mm)

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Sensing Array Size and Orientation 2007.10.28 2007.10.29 2007.11.19 2007.11.20

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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:– 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

• 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

Progress Toward LUNAR Goals

Lunar Environment

LRO 2-way Ranging

Theoretical Tools

Model Development

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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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APOLLO rates on Apollo 15 reflector

full

mo

on

background level

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More on the deficit

• APOLLO system sensitivity is not to blame for full-moon deficit

– background is not impacted

• 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?

• The full-moon deficit, together with normal eclipse behavior, gives us the best clues:

– thermal nature– absorbing solar flux

• Modification of the front surface by dust deposition or abrasion would change the thermal properties

– so would bulk absorption in the CCR– a 4K gradient is all it takes to reduce

response by 10– would also account for overall deficit

• 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

4106

– effectively probing the coupling between the sun’s and the earth’s gravitational binding energies

– any experiment reaching 4106 in or will become sensitive to this second-order PPN effect (equiv. to EP test to 21015)

is now determined to 2.5105 by Cassini is now determined to 104 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 71014, not far from the 1.31013 EP limits to date

part of theoretical tools work plan

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Development of Analysis Tools

• 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

• The models currently lack:– 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 sub-millimeter 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