Transcript Shooting the Moon - University of California, Los Angeles

APOLLO
Testing Gravity
via
Laser Ranging to the Moon
Tom Murphy (UCSD)
The Full Parameterized Post Newtonian
(PPN) Metric
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Generalized metric abandoning many fundamental assumptions
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GR is a special case
Allows violations of conservations, Lorentz invariance, etc.
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Simplified (Conservative) PPN Equations
of Motion Newtonian piece
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Relativistic Observables in the Lunar Range
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Lunar Laser Ranging provides a comprehensive probe of
gravity, currently boasting the best tests of:
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Equivalence Principle (mainly strong version, but check on weak)
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time-rate-of-change of G
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to 1010 times the strength of gravity at 108 m scales
gravitomagnetism (origin of frame-dragging)
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to  0.5%
1/r2 force law
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fractional change < 1012 per year
geodetic precession
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a/a  1013; SEP to 4104
to 0.1% (from motions of point masses—not systemic rotation)
APOLLO effort will improve by 10; access new physics
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Previously
200 meters
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LLR through the decades
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APOLLO
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APOLLO: the next big thing in LLR
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APOLLO offers order-of-magnitude
improvements to LLR by:
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Using a 3.5 meter telescope
Operating at 20 pulses/sec
Using advanced detector technology
Gathering multiple photons/shot
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Tightly integrating experiment and
analysis
Having the best acronym
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Achieving millimeter range precision
funded by NASA & NSF
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The APOLLO Collaboration
UCSD:
U Washington:
Apache Point Obs.
Tom Murphy (PI)
Eric Michelsen
Eric Adelberger
Erik Swanson
Russet McMillan
Harvard:
Humboldt State:
Northwest Analysis:
Chris Stubbs
James Battat
C. D. Hoyle
Ken Nordtvedt
JPL:
CfA/SAO:
Lincoln Lab:
Jim Williams
Slava Turyshev
Dale Boggs
Bob Reasenberg
Irwin Shapiro
John Chandler
Brian Aull
Bob Reich
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Photo2008.07.07
by NASA
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Lunar Retroreflector Arrays
Corner cubes
Apollo 11 retroreflector array
Apollo 14 retroreflector array
Apollo 15 retroreflector array
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The Reflector Positions
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Three Apollo missions left reflectors
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Two French-built, Soviet-landed
reflectors were placed on rovers
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Luna 17 (lost!)
Luna 21
similar in size to A11, A14
Signal loss is huge:
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Apollo 11: 100-element
Apollo 14: 100-element
Apollo 15: 300-element
108 of photons launched find
reflector (atmospheric seeing)
108 of returned photons find
telescope (corner cube diffraction)
>1017 loss considering other
optical/detection losses
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APOLLO’s Secret Weapon: Aperture
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The Apache Point
Observatory’s 3.5 meter
telescope
 Southern NM (Sunspot)
 9,200 ft (2800 m) elevation
 Great “seeing”: 1 arcsec
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Flexibly scheduled, high-class
research telescope
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APOLLO gets 8–10 < 1 hour
sessions per lunar month
7-university consortium (UW
NMSU, U Chicago, Princeton,
Johns Hopkins, Colorado,
Virginia)
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APOLLO Laser
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Nd:YAG; flashlamp-pumped;
mode-locked; cavity-dumped
Frequency-doubled to 532 nm
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90 ps pulse width (FWHM)
115 mJ (green) per pulse
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Beam is expanded to 3.5
meter aperture
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after double-pass amplifier
20 Hz pulse repetition rate
2.3 Watt average power
GW peak power!!
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57% conversion efficiency
Less of an eye hazard
Less damaging to optics
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Catching All the Photons
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Several photons per pulse
necessitates multiple “buckets” to
time-tag each one
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Lincoln Lab prototype APD arrays
are perfect for APOLLO
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44 array of 30 m elements on
100 m centers
Lenslet array in front recovers full
fill factor
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Avalanche Photodiodes (APDs)
respond only to first photon
Resultant field is 1.4 arcsec on a side
Focused image is formed at lenslet
2-D tracking capability facilitates
optimal efficiency
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Differential Measurement Scheme
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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
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APOLLO Random Error Budget
Error Source
Time Uncert. (ps)
(round trip)
Range error (mm)
(one way)
100–300
15–45
APD Illumination
60
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APD Intrinsic
<60
<9
Laser Pulse Width
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7.5
Timing Electronics
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4.5
GPS-slaved Clock
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1
143–317
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Retro Array Orient.
Total Random Uncert
Ignoring retro array, APOLLO system has 104 ps (16 mm) error per photon
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Laser Mounted on Telescope
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Laser Illumination of Telescope
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Gigantic Laser Pointer
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Out the Barn Door
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Blasting the Moon
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Breaking All Records
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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
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372
APOLLO has seen rates higher than 2 photons per pulse for brief periods
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APOLLO can operate at full moon
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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
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
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record is 11 photons in one shot (out of 12 functioning APD elements)
APD array (many buckets) is crucial
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Killer Returns
Apollo 15
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Apollo 11
red curves are theoretical
profiles:
convolvedsmaller?
with fiducial to make lunar return
which
arrayget
is physically
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
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• 2344 photons in 5000 shots
• 369,817,674,951.1  0.7 mm
• 1 detection with 8 photons
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Sensing the Array Size & Orientation
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Reaching the Millimeter Goal?
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median = 1.8 mm
1.1 mm recent
1 millimeter quality data is
frequently achieved
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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
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Residuals During a Contiguous Run
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15 mm
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individual error bars:   1.5 mm
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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
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Residuals Run-to-Run
1.16 mm
2269 photons; 3k shots
Apollo 15 reflector
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1.73 mm
901 photons; 2k shots
0.66 mm
8457 photons; 10k shots
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 mm
1483 photons; 3k shots
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We can get 1 mm
range precision in
single “runs” (<10minutes)
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Residuals Against JPL Model
APOLLO data points
processed together
with 16,000 ranges
over 38 years shows
consistency with
model orbit
plot redacted: no agreement from JPL to make public
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 is
about 8 mm
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  3 for this fit
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APOLLO Impact on Model
If APOLLO data is
down-weighted to
15 mm, we see what
the model would do
without APOLLOquality data
plot redacted: no agreement from JPL to make public
Answer: large (40 mm)
adjustments to lunar
orientation—as seen
via reflector offsets
(e.g., arrowed
sessions)
May lead to improved
understanding of lunar
interior, but also
sharpens the picture
for elucidating grav.
physics phenomena
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Summary & Next Steps
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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
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Now grappling with analysis in the face of vastly better data
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began October 2006, one year after first light
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)
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see talk by Hamid Hemmati later in this session
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