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APOLLOAPOLLOAPOLLOAPOLLO
Testing GravityTesting Gravity
viavia
Laser Ranging to the MoonLaser Ranging to the Moon
Testing GravityTesting Gravity
viavia
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 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 10
Lunar Retroreflector ArraysLunar Retroreflector Arrays
Corner cubes
Apollo 14 retroreflector array
Apollo 11 retroreflector array
Apollo 15 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 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
0.66 mm8457 photons; 10k shots
1.73 mm901 photons; 2k shots
1.16 mm2269 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