APOLLOAPOLLO Testing Gravity via Laser Ranging to the Moon Testing Gravity via Laser Ranging to the Moon Tom Murphy (UCSD)

APOLLOAPOLLOAPOLLOAPOLLO

Testing GravityTesting Gravity

viavia

Laser Ranging to the MoonLaser Ranging to the Moon

Testing GravityTesting Gravity

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Laser Ranging to the MoonLaser Ranging to the Moon

Tom Murphy (UCSD)

2008.07.07 Q2C3 2

The Full Parameterized Post Newtonian (PPN) Metric

The Full Parameterized Post Newtonian (PPN) Metric

Generalized metric abandoning many fundamental assumptions GR is a special case Allows violations of conservations, Lorentz invariance, etc.

Generalized metric abandoning many fundamental assumptions GR is a special case Allows violations of conservations, Lorentz invariance, etc.

2008.07.07 Q2C3 3

Simplified (Conservative) PPN Equations of Motion

Simplified (Conservative) PPN Equations of Motion

Newtonian piece

2008.07.07 Q2C3 4

Relativistic Observables in the Lunar RangeRelativistic Observables in the Lunar Range

Lunar Laser Ranging provides a comprehensive probe of gravity, currently boasting the best tests of: Equivalence Principle (mainly strong version, but check on weak)

a/a 1013; SEP to 4104

time-rate-of-change of G fractional change < 1012 per year

geodetic precession to 0.5%

1/r2 force law to 1010 times the strength of gravity at 108 m scales

gravitomagnetism (origin of frame-dragging) to 0.1% (from motions of point masses—not systemic rotation)

APOLLO effort will improve by 10; access new physics

Lunar Laser Ranging provides a comprehensive probe of gravity, currently boasting the best tests of: Equivalence Principle (mainly strong version, but check on weak)

a/a 1013; SEP to 4104

time-rate-of-change of G fractional change < 1012 per year

geodetic precession to 0.5%

1/r2 force law to 1010 times the strength of gravity at 108 m scales

gravitomagnetism (origin of frame-dragging) to 0.1% (from motions of point masses—not systemic rotation)

APOLLO effort will improve by 10; access new physics

2008.07.07 Q2C3 6

LLR through the decadesLLR through the decadesPreviously200 meters

APOLLO

2008.07.07 Q2C3 7

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 Operating at 20 pulses/sec Using advanced detector technology Gathering multiple photons/shot

Achieving millimeter range precision Tightly integrating experiment and

analysis Having the best acronym

funded by NASA & NSF

APOLLO offers order-of-magnitude improvements to LLR by: Using a 3.5 meter telescope Operating at 20 pulses/sec Using advanced detector technology Gathering multiple photons/shot

Achieving millimeter range precision Tightly integrating experiment and

analysis Having the best acronym

funded by NASA & NSF

2008.07.07 Q2C3 8

The APOLLO CollaborationThe APOLLO Collaboration

UCSD:Tom Murphy (PI)Eric Michelsen

U Washington:Eric AdelbergerErik Swanson

Harvard:Chris StubbsJames Battat

JPL:Jim WilliamsSlava TuryshevDale Boggs

Lincoln Lab:Brian AullBob Reich

Northwest Analysis:Ken Nordtvedt

Apache Point Obs.Russet McMillan

Humboldt State:C. D. Hoyle

CfA/SAO:Bob ReasenbergIrwin ShapiroJohn Chandler

2008.07.07 Q2C3 9Photo by NASA

2008.07.07 Q2C3 10

Lunar Retroreflector ArraysLunar Retroreflector Arrays

Corner cubes

Apollo 14 retroreflector array



2008.07.07 Q2C3 11

The Reflector PositionsThe Reflector Positions Three Apollo missions left reflectors

Apollo 11: 100-element Apollo 14: 100-element Apollo 15: 300-element

Two French-built, Soviet-landed reflectors were placed on rovers

Luna 17 (lost!) Luna 21 similar in size to A11, A14

Signal loss is huge: 108 of photons launched find

reflector (atmospheric seeing) 108 of returned photons find

telescope (corner cube diffraction) >1017 loss considering other

optical/detection losses

Three Apollo missions left reflectors Apollo 11: 100-element Apollo 14: 100-element Apollo 15: 300-element

Two French-built, Soviet-landed reflectors were placed on rovers

Luna 17 (lost!) Luna 21 similar in size to A11, A14

Signal loss is huge: 108 of photons launched find

reflector (atmospheric seeing) 108 of returned photons find

telescope (corner cube diffraction) >1017 loss considering other

optical/detection losses

2008.07.07 Q2C3 12

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 APOLLO gets 8–10 < 1 hour

sessions per lunar month 7-university consortium (UW

NMSU, U Chicago, Princeton, Johns Hopkins, Colorado, 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 APOLLO gets 8–10 < 1 hour

sessions per lunar month 7-university consortium (UW

NMSU, U Chicago, Princeton, Johns Hopkins, Colorado, Virginia)

2008.07.07 Q2C3 13

APOLLO LaserAPOLLO Laser Nd:YAG; flashlamp-pumped;

mode-locked; cavity-dumped Frequency-doubled to 532 nm

57% conversion efficiency 90 ps pulse width (FWHM) 115 mJ (green) per pulse

after double-pass amplifier 20 Hz pulse 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; flashlamp-pumped; mode-locked; cavity-dumped

Frequency-doubled to 532 nm 57% conversion efficiency

90 ps pulse width (FWHM) 115 mJ (green) per pulse

after double-pass amplifier 20 Hz pulse 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

2008.07.07 Q2C3 14

Catching All the PhotonsCatching All the Photons Several photons per pulse

necessitates multiple “buckets” to time-tag each one

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

Several photons per pulse necessitates multiple “buckets” to time-tag each one

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

2008.07.07 Q2C3 15

Differential Measurement SchemeDifferential Measurement Scheme

Corner Cube at telescope exit returns fiducial pulse

Same optical path, attenuated by 10 O.D.

Same APD detector, electronics, TDC range

Diffused to present identical illumination on detector elements

Result is differential over 2.5 seconds

Must correct for distance between telescope axis intersection and corner cube

Corner Cube at telescope exit returns fiducial pulse

Same optical path, attenuated by 10 O.D.

Same APD detector, electronics, TDC range

Diffused to present identical illumination on detector elements

Result is differential over 2.5 seconds

Must correct for distance between telescope axis intersection and corner cube

2008.07.07 Q2C3 16

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 <60 < 9

Laser Pulse Width 50 7.5

Timing Electronics 30 4.5

GPS-slaved Clock 7 1

Total Random Uncert 143–317 22–48

Ignoring retro array, APOLLO system has 104 ps (16 mm) error per photon

2008.07.07 Q2C3 17

Laser Mounted on TelescopeLaser Mounted on Telescope

2008.07.07 Q2C3 18

Laser Illumination of TelescopeLaser Illumination of Telescope

2008.07.07 Q2C3 19

Gigantic Laser PointerGigantic Laser Pointer

2008.07.07 Q2C3 20

Out the Barn DoorOut the Barn Door

2008.07.07 Q2C3 21

Blasting the MoonBlasting the Moon

2008.07.07 Q2C3 22

Breaking All RecordsBreaking All Records

Reflector

prev. max

photons/run

APOLLO max

photons/run

prev. max

photons/5-min

APOLLO max

photons/5-min

Apollo 11 172 4288 83 2812

Apollo 14 213 5100 131 3060

Apollo 15 603 8937 280 7950

Lunokhod 2 70 310 29 372

APOLLO has seen rates higher than 2 photons per pulse for brief periods max rates for French and Texas stations about 0.1 and 0.02, respectively APOLLO has collected more return photons in 100 seconds than these other stations

typically collect in months or years APOLLO can operate at full moon

other stations can’t (except during eclipse), though EP signal is max at full moon! Often a majority of APOLLO returns are multiple-photon events

record is 11 photons in one shot (out of 12 functioning APD elements) APD array (many buckets) is crucial

APOLLO has seen rates higher than 2 photons per pulse for brief periods max rates for French and Texas stations about 0.1 and 0.02, respectively APOLLO has collected more return photons in 100 seconds than these other stations

typically collect in months or years APOLLO can operate at full moon

other stations can’t (except during eclipse), though EP signal is max at full moon! Often a majority of APOLLO returns are multiple-photon events

record is 11 photons in one shot (out of 12 functioning APD elements) APD array (many buckets) is crucial

2008.07.07 Q2C3 23

Killer ReturnsKiller Returns2007.11.19Apollo 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

which array is physically smaller?red curves are theoretical profiles: get convolved with fiducial to make lunar return

represents systemcapability: laser;detector; timingelectronics; etc.

RMS = 120 ps(18 mm)

2008.07.07 Q2C3 24

Sensing the Array Size & OrientationSensing the Array Size & Orientation2007.10.28 2007.10.29 2007.11.19 2007.11.20

2008.07.07 Q2C3 25

Reaching the Millimeter Goal?Reaching the Millimeter Goal?

1 millimeter quality data is frequently achieved

especially since Sept. 2007 represents combined

performance for single (< 1 hour) observing session

random uncertainty only Virtually all nights deliver

better than 4 mm, and 2 mm is typical

1 millimeter quality data is frequently achieved

especially since Sept. 2007 represents combined

performance for single (< 1 hour) observing session

random uncertainty only Virtually all nights deliver

better than 4 mm, and 2 mm is typical

shaded recent results

median = 1.8 mm1.1 mm recent

2008.07.07 Q2C3 26

Residuals During a Contiguous RunResiduals During a Contiguous Run

Breaking 10,000-shot run into 5 chunks, we can evaluate the stability of our measurement

Comparison is against imperfect prediction, which can leave linear drift

No scatter beyond that expected statistically

Breaking 10,000-shot run into 5 chunks, we can evaluate the stability of our measurement

Comparison is against imperfect prediction, which can leave linear drift

No scatter beyond that expected statistically

15 mm

individual error bars: 1.5 mm

2008.07.07 Q2C3 27

Residuals Run-to-RunResiduals Run-to-RunWe can get 1 mmrange precision insingle “runs” (<10-minutes)

The scatter about a linear fit is small: consistent with estimated random error

0.5 mm effective data point for Apollo 15 reflector on this night

1.45 mm1483 photons; 3k shots




Apollo 15 reflector2008.02.18

2008.07.07 Q2C3 28

Residuals Against JPL ModelResiduals Against JPL Model

APOLLO data pointsprocessed togetherwith 16,000 rangesover 38 years showsconsistency withmodel orbit

Fit is not yet perfect, but this is expected when the model sees high-quality data for the first time, and APOLLO data reduction is still evolving as well

Weighted RMS isabout 8 mm

3 for this fit

plot redacted: no agreement from JPL to make public

2008.07.07 Q2C3 29

APOLLO Impact on ModelAPOLLO Impact on ModelIf APOLLO data isdown-weighted to 15 mm, we see whatthe model would dowithout APOLLO-quality data

Answer: large (40 mm)adjustments to lunarorientation—as seenvia reflector offsets (e.g., arrowed sessions)

May lead to improvedunderstanding of lunarinterior, but alsosharpens the picturefor elucidating grav.physics phenomena

plot redacted: no agreement from JPL to make public

2008.07.07 Q2C3 30

Summary & Next StepsSummary & Next Steps

APOLLO is a millimeter-capable lunar ranging station with unprecedented performance

Given the order-of-magnitude gains in range precision, we expect order-of-magnitude gains in a variety of tests of fundamental gravity

Our steady-state campaign is not quite 2 years old began October 2006, one year after first light

Now grappling with analysis in the face of vastly better data much new stuff to learn, with concomitant refinements to data

reduction and to the analytical model Modest improvements in gravity seen already with APOLLO

data; more to follow in the upcoming months and years Next BIG step: interplanetary laser ranging (e.g., Mars)

see talk by Hamid Hemmati later in this session

APOLLO is a millimeter-capable lunar ranging station with unprecedented performance

Given the order-of-magnitude gains in range precision, we expect order-of-magnitude gains in a variety of tests of fundamental gravity

Our steady-state campaign is not quite 2 years old began October 2006, one year after first light

Now grappling with analysis in the face of vastly better data much new stuff to learn, with concomitant refinements to data

reduction and to the analytical model Modest improvements in gravity seen already with APOLLO

data; more to follow in the upcoming months and years Next BIG step: interplanetary laser ranging (e.g., Mars)

see talk by Hamid Hemmati later in this session

Documents

APOLLOAPOLLO Testing Gravity via Laser Ranging to the Moon Testing Gravity via Laser Ranging to the Moon Tom Murphy (UCSD)