APOLLO: Next-Generation Lunar Laser Ranging Tom Murphy UCSD The APOLLO Collaboration UCSD: U Washington: Harvard: Tom Murphy (PI) Eric Michelsen Adam Orin Eric Williams Philippe LeBlanc Evan Million Eric Adelberger C.
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Transcript APOLLO: Next-Generation Lunar Laser Ranging Tom Murphy UCSD The APOLLO Collaboration UCSD: U Washington: Harvard: Tom Murphy (PI) Eric Michelsen Adam Orin Eric Williams Philippe LeBlanc Evan Million Eric Adelberger C.
APOLLO: Next-Generation Lunar Laser Ranging
Tom Murphy
UCSD
The APOLLO Collaboration
UCSD:
U Washington:
Harvard:
Tom Murphy (PI)
Eric Michelsen
Adam Orin
Eric Williams
Philippe LeBlanc
Evan Million
Eric Adelberger
C. D. Hoyle
Erik Swanson
Chris Stubbs
James Battat
Northwest Analysis:
initially NASA Code U
now split: 60% Code S
40% NSF grav. phys.
Funding:
Ken Nordtvedt
Close Associates
JPL:
Lincoln Lab:
Jim Williams
Slava Turyshev
Dale Boggs
Jean Dickey
Brian Aull
Bob Reich
A 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)
Relativistic 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)
Equivalence Principle Signal
Sluggish orbit
If, for example, Earth has
greater inertial mass than
gravitational mass (while the
moon does not):
Nominal orbit:
Moon follows this, on average
Sun
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
Previously
100 meters
LLR through the decades
APOLLO
APOLLO: 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
Lunar Retroreflector Arrays
Corner cubes
Apollo 11 retroreflector array
Apollo 14 retroreflector array
Apollo 15 retroreflector array
APOLLO’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)
APOLLO Laser
Nd:YAG mode-locked, cavitydumped
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 Photons
Several photons per pulse
necessitates multiple “buckets” to
time-tag each
Lincoln Lab prototype APD arrays
are perfect for APOLLO
Avalanche Photodiodes (APDs)
respond only to first photon
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 Telescope
First Light: July 24, 2005
First Light: July 24, 2005
Blasting the Moon
APOLLO Random Error Budget
Error Source
Time Uncert. (ps)
(round trip)
Range error (mm)
(one way)
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
136–314
20–47
Retro Array Orient.
Total Random Uncert
Example Data From Recent Run
Return photons
from reflector
width is < 1 foot
2150 photons in
14,000 shots
Randomly-timed background photons (bright moon)
APOLLO Superlatives
More lunar return photons in 10 minutes than the McDonald station
gets in three years
Peak rates of >0.6 photons per shot (12 per second)
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!
compare to typical 1/500 for McDonald, 1/100 for France
Range with ease at full moon
best single run: >2500 photons in 10,000 shots (8 minutes)
APD array is essential
Centimeter precision straight away
Millimeter-capable beginning April 2006
Future Directions
LLR tests gravity on our doorstep
There’s also a back yard: the solar system
Interplanetary laser ranging offers another order-of-magnitude
Although additional “doorstep” opportunities via lunar landing missions
sparse arrays, transponders
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