APOLLO NextGeneration Lunar Laser Ranging Tom Murphy UCSD

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 Le. Blanc 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 n n The Post-Newtonian Parameterization (PPN) describes deviations from GR The main parameters are and n n 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: n n ( – 1) < 2. 5 10 -5 (Cassini) ( – 1) < 1. 1 10 -4 (LLR)

Relativistic Observables in the Lunar Range n Lunar Laser Ranging provides a comprehensive probe of gravity, boasting the best tests of: n n n n Weak Equivalence Principle: a/a 10 -13 Strong Equivalence Principle: | | ≤ 4 10 -4 time-rate-of-change of G: ≤ 10 -12 per year geodetic precession: 0. 35% 1/r 2 force law: 10 -10 times force of gravity gravitomagnetism (frame-dragging): 0. 1% Equivalence Principle (EP) Violation n n 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 cos. D (D is lunar phase angle)

Equivalence Principle Signal n Sluggish orbit If, for example, Earth has greater inertial mass than gravitational mass (while the moon does not): n n Nominal orbit: Moon follows this, on average n n 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: cos. D signal

Previously 100 meters LLR through the decades APOLLO

APOLLO: the next big thing in LLR n APOLLO offers order-of-magnitude improvements to LLR by: n n n n 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 n The Apache Point Observatory’s 3. 5 meter telescope n Southern NM (Sunspot) n 9, 200 ft (2800 m) elevation n Great “seeing”: 1 arcsec n n Flexibly scheduled, high-class research telescope 7 -university consortium (UW, U Chicago, Princeton, Johns Hopkins, Colorado, NMSU, U Virginia)

APOLLO Laser n n n n Nd: YAG mode-locked, cavitydumped Frequency-doubled to 532 nm (green) 90 ps pulse width (FWHM) 115 m. J per pulse 20 Hz repetition rate 2. 3 Watt average power GW peak power!! Beam is expanded to 3. 5 meter aperture n n Less of an eye hazard Less damaging to optics

Catching All the Photons n Several photons per pulse necessitates multiple “buckets” to time-tag each n n Lincoln Lab prototype APD arrays are perfect for APOLLO n n Avalanche Photodiodes (APDs) respond only to first photon 4 4 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 n More lunar return photons in 10 minutes than the Mc. Donald station gets in three years n n Peak rates of >0. 6 photons per shot (12 per second) n n n 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! n n compare to typical 1/500 for Mc. Donald, 1/100 for France Range with ease at full moon n n best single run: >2500 photons in 10, 000 shots (8 minutes) APD array is essential Centimeter precision straight away n Millimeter-capable beginning April 2006

Future Directions n LLR tests gravity on our doorstep n n n There’s also a back yard: the solar system Interplanetary laser ranging offers another order-of-magnitude n n n Although additional “doorstep” opportunities via lunar landing missions n sparse arrays, transponders Measure via Shapiro delay Measure strong equivalence principle as Sun falls toward Jupiter Multi-task laser altimeters as asynchronous transponders n incredible demonstration to MESSENGER: 24 million km 2 -way link Piggyback on optical communications/navigation Other methods for probing local spacetime n n n Weak equivalence principle tests Solar-induced curvature via interferometric angular measurements Clocks in space to test Lorentz invariance/SME