Lunar Laser Ranging (LLR) within LUNAR

1
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. DellAgnello, G.
  Della Monache,
• R. Reasenberg, I. Shapiro

1 UCSD 2 U Md 3 GSFC
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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What has LLR done for us lately?

• LLR provides a comprehensive suite of
  gravitational tests
• The earth-moon system is a pristine laboratory
  for investigating gravity (with bonus of lunar
  interior studies)
• moon is massive enough to be stubborn against
  drag/pressure
• moon is far enough to be in a solar orbit (weakly
  bound)
• LLR currently provides our best tests of
• The weak equivalence principle (WEP) ?a/a lt
  1.3?10-13
• The strong equivalence principle (SEP)
  ??????????? lt 4?10?4
• Time-rate-of-change of G to lt 7?10?13 per year
• Inverse square law to 3?10?11 at 108 m scales
• Geodetic precession to 0.6
• Gravitomagnetism to 0.1
• Lunar Science
• Gravity harmonics (J2 to lt 0.5), Love numbers
  (1), liquid core (3?), core-mantle boundary,
  free physical librations (gt 10 m ampl.), tidal
  dissipation (Q30)

similar precision to lab experiments, though
not optimal mass pair in test
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How Does LLR Work?
Short laser pulses and time-of-flight measurement
to high precision
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LLR through the decades
Previously 200 meters
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 11
Apollo 15
2007.11.19
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

• 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
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 reflectors 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
• Todays 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 DellAgnello 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

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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 moon
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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Whats 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 4?K 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 suns binding energy U? ? 4?10?6
• effectively probing the coupling between the
  suns and the earths 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 moons 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

• 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