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