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APOLLO: Next-Generation Lunar Laser RangingAPOLLO: Next-Generation Lunar Laser RangingAPOLLO: Next-Generation Lunar Laser RangingAPOLLO: Next-Generation Lunar Laser Ranging
Tom Murphy
UCSDTom Murphy
UCSD
The APOLLO CollaborationThe APOLLO Collaboration
UCSD:Tom Murphy (PI)Eric MichelsenAdam OrinEric WilliamsPhilippe LeBlancEvan Million
U Washington:Eric AdelbergerC. D. HoyleErik Swanson
Harvard:Chris StubbsJames Battat
JPL:Jim WilliamsSlava TuryshevDale BoggsJean Dickey
Lincoln Lab:Brian AullBob Reich
Northwest Analysis:Ken Nordtvedt
Close AssociatesClose Associates
Funding:initially NASA Code Unow split: 60% Code S40% NSF grav. phys.
A Modern, Post-Newtonian ViewA Modern, Post-Newtonian View
The Post-Newtonian Parameterization (PPN) describes deviations from GR
The main parameters are and tells us how much spacetime
curvature is produced per unit mass
tells us how nonlinear gravity is (self-interaction)
and are identically 1.00 in GR Current limits have:
(–1) < 2.510-5 (Cassini) (–1) < 1.110-4 (LLR)
The Post-Newtonian Parameterization (PPN) describes deviations from GR
The main parameters are and tells us how much spacetime
curvature is produced per unit mass
tells us how nonlinear gravity is (self-interaction)
and are identically 1.00 in GR Current limits have:
(–1) < 2.510-5 (Cassini) (–1) < 1.110-4 (LLR)
Relativistic Observables in the Lunar RangeRelativistic Observables in the Lunar Range
Lunar Laser Ranging provides a comprehensive probe of gravity, boasting the best tests of:
Weak Equivalence Principle: a/a 10-13
Strong Equivalence Principle: | | ≤ 410-4
time-rate-of-change of G: ≤ 10-12 per year geodetic precession: 0.35% 1/r2 force law: 10-10 times force of gravity gravitomagnetism (frame-dragging): 0.1%
Equivalence Principle (EP) Violation Happens if gravitational mass and inertial mass are not equal Earth and Moon would fall at different rates toward the sun Would appear as a polarization of the lunar orbit Range signal has form of cosD (D is lunar phase angle)
Lunar Laser Ranging provides a comprehensive probe of gravity, boasting the best tests of:
Weak Equivalence Principle: a/a 10-13
Strong Equivalence Principle: | | ≤ 410-4
time-rate-of-change of G: ≤ 10-12 per year geodetic precession: 0.35% 1/r2 force law: 10-10 times force of gravity gravitomagnetism (frame-dragging): 0.1%
Equivalence Principle (EP) Violation Happens if gravitational mass and inertial mass are not equal Earth and Moon would fall at different rates toward the sun Would appear as a polarization of the lunar orbit Range signal has form of cosD (D is lunar phase angle)
Equivalence Principle SignalEquivalence Principle Signal
If, for example, Earth has greater inertial mass than gravitational mass (while the moon does not):
Earth is sluggish to move Alternatively, pulled weakly by
gravity Takes orbit of larger radius
(than does Moon) Appears that Moon’s orbit is
shifted toward sun: cosD signal
If, for example, Earth has greater inertial mass than gravitational mass (while the moon does not):
Earth is sluggish to move Alternatively, pulled weakly by
gravity Takes orbit of larger radius
(than does Moon) Appears that Moon’s orbit is
shifted toward sun: cosD signal
Sun
Nominal orbit:Moon follows this, on average
Sluggish orbit
LLR through the decadesLLR through the decadesPreviously100 meters
APOLLO
APOLLO: the next big thing in LLRAPOLLO: the next big thing in LLR
APOLLO offers order-of-magnitude improvements to LLR by:
Using a 3.5 meter telescope Gathering multiple photons/shot Operating at 20 pulses/sec Using advanced detector technology Achieving millimeter range precision Tightly integrating experiment and analysis Having the best acronym
APOLLO offers order-of-magnitude improvements to LLR by:
Using a 3.5 meter telescope Gathering multiple photons/shot Operating at 20 pulses/sec Using advanced detector technology Achieving millimeter range precision Tightly integrating experiment and analysis Having the best acronym
Lunar Retroreflector ArraysLunar Retroreflector Arrays
Corner cubes
Apollo 14 retroreflector array
Apollo 11 retroreflector array
Apollo 15 retroreflector array
APOLLO’s Secret Weapon: ApertureAPOLLO’s Secret Weapon: Aperture
The Apache Point Observatory’s 3.5 meter telescope Southern NM (Sunspot) 9,200 ft (2800 m) elevation Great “seeing”: 1 arcsec Flexibly scheduled, high-class
research telescope 7-university consortium (UW, U
Chicago, Princeton, Johns Hopkins, Colorado, NMSU, U Virginia)
The Apache Point Observatory’s 3.5 meter telescope Southern NM (Sunspot) 9,200 ft (2800 m) elevation Great “seeing”: 1 arcsec Flexibly scheduled, high-class
research telescope 7-university consortium (UW, U
Chicago, Princeton, Johns Hopkins, Colorado, NMSU, U Virginia)
APOLLO LaserAPOLLO Laser
Nd:YAG mode-locked, cavity-dumped
Frequency-doubled to 532 nm (green)
90 ps pulse width (FWHM) 115 mJ per pulse 20 Hz repetition rate 2.3 Watt average power GW peak power!!
Beam is expanded to 3.5 meter aperture
Less of an eye hazard Less damaging to optics
Nd:YAG mode-locked, cavity-dumped
Frequency-doubled to 532 nm (green)
90 ps pulse width (FWHM) 115 mJ per pulse 20 Hz repetition rate 2.3 Watt average power GW peak power!!
Beam is expanded to 3.5 meter aperture
Less of an eye hazard Less damaging to optics
Catching All the PhotonsCatching All the Photons Several photons per pulse
necessitates multiple “buckets” to time-tag each
Avalanche Photodiodes (APDs) respond only to first photon
Lincoln Lab prototype APD arrays are perfect for APOLLO
44 array of 30 m elements on 100 m centers
Lenslet array in front recovers full fill factor
Several photons per pulse necessitates multiple “buckets” to time-tag each
Avalanche Photodiodes (APDs) respond only to first photon
Lincoln Lab prototype APD arrays are perfect for APOLLO
44 array of 30 m elements on 100 m centers
Lenslet array in front recovers full fill factor
• Resultant field is 1.4 arcsec on a side• Focused image is formed at lenslet• 2-D tracking capability facilitates optimal efficiency
Laser Mounted on TelescopeLaser Mounted on Telescope
First Light: July 24, 2005First Light: July 24, 2005
First Light: July 24, 2005First Light: July 24, 2005
Blasting the MoonBlasting the Moon
APOLLO Random Error BudgetAPOLLO Random Error Budget
Error Source Time Uncert. (ps)(round trip)
Range error (mm)(one way)
Retro Array Orient. 100–300 15–45
APD Illumination 60 9
APD Intrinsic <50 < 7
Laser Pulse Width 45 6.5
Timing Electronics 20 3
GPS-slaved Clock 7 1
Total Random Uncert 136–314 20–47
Example Data From Recent RunExample Data From Recent Run
Randomly-timed background photons (bright moon)
Return photonsfrom reflector
width is < 1 foot
2150 photons in14,000 shots
APOLLO SuperlativesAPOLLO Superlatives
More lunar return photons in 10 minutes than the McDonald station gets in three years
best single run: >2500 photons in 10,000 shots (8 minutes) Peak rates of >0.6 photons per shot (12 per second)
compare to typical 1/500 for McDonald, 1/100 for France Range with ease at full moon
APOLLO’s very first returns were at full moon other stations can’t fight the high background
As many as 8 photons detected in a single pulse! APD array is essential
Centimeter precision straight away Millimeter-capable beginning April 2006
More lunar return photons in 10 minutes than the McDonald station gets in three years
best single run: >2500 photons in 10,000 shots (8 minutes) Peak rates of >0.6 photons per shot (12 per second)
compare to typical 1/500 for McDonald, 1/100 for France Range with ease at full moon
APOLLO’s very first returns were at full moon other stations can’t fight the high background
As many as 8 photons detected in a single pulse! APD array is essential
Centimeter precision straight away Millimeter-capable beginning April 2006
Future DirectionsFuture Directions
LLR tests gravity on our doorstep Although additional “doorstep” opportunities via lunar landing missions
sparse arrays, transponders There’s also a back yard: the solar system Interplanetary laser ranging offers another order-of-magnitude
Measure via Shapiro delay Measure strong equivalence principle as Sun falls toward Jupiter Multi-task laser altimeters as asynchronous transponders
incredible demonstration to MESSENGER: 24 million km 2-way link Piggyback on optical communications/navigation
Other methods for probing local spacetime Weak equivalence principle tests Solar-induced curvature via interferometric angular measurements Clocks in space to test Lorentz invariance/SME
LLR tests gravity on our doorstep Although additional “doorstep” opportunities via lunar landing missions
sparse arrays, transponders There’s also a back yard: the solar system Interplanetary laser ranging offers another order-of-magnitude
Measure via Shapiro delay Measure strong equivalence principle as Sun falls toward Jupiter Multi-task laser altimeters as asynchronous transponders
incredible demonstration to MESSENGER: 24 million km 2-way link Piggyback on optical communications/navigation
Other methods for probing local spacetime Weak equivalence principle tests Solar-induced curvature via interferometric angular measurements Clocks in space to test Lorentz invariance/SME