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APOLLO: Next-Generation Lunar Laser Ranging

APOLLO: Next-Generation Lunar Laser Ranging. Tom Murphy UCSD. The APOLLO Collaboration. UCSD: Tom Murphy (PI) Eric Michelsen Adam Orin Eric Williams Philippe LeBlanc Evan Million. U Washington: Eric Adelberger C. D. Hoyle Erik Swanson. Harvard: Chris Stubbs James Battat. Funding:

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APOLLO: Next-Generation Lunar Laser Ranging

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  1. APOLLO: Next-Generation Lunar Laser Ranging Tom Murphy UCSD

  2. The APOLLO Collaboration UCSD: Tom Murphy (PI) Eric Michelsen Adam Orin Eric Williams Philippe LeBlanc Evan Million U Washington: Eric Adelberger C. D. Hoyle Erik Swanson Harvard: Chris Stubbs James Battat Funding: initially NASA Code U now split: 60% Code S 40% NSF grav. phys. Northwest Analysis: Ken Nordtvedt Close Associates JPL: Jim Williams Slava Turyshev Dale Boggs Jean Dickey Lincoln Lab: Brian Aull Bob Reich

  3. 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) < 2.510-5 (Cassini) • (–1) < 1.110-4 (LLR)

  4. 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: |  | ≤ 410-4 • 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)

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

  6. LLR through the decades Previously 100 meters APOLLO

  7. 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

  8. Lunar Retroreflector Arrays Corner cubes Apollo 11 retroreflector array Apollo 14 retroreflector array Apollo 15 retroreflector array

  9. 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)

  10. APOLLO Laser • Nd:YAG mode-locked, cavity-dumped • 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

  11. Catching All the Photons • Several photons per pulse necessitates multiple “buckets” to time-tag each • Avalanche Photodiodes (APDs) respond only to first photon • Lincoln Lab prototype APD arrays are perfect for APOLLO • 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

  12. Laser Mounted on Telescope

  13. First Light: July 24, 2005

  14. First Light: July 24, 2005

  15. Blasting the Moon

  16. APOLLO Random Error Budget

  17. 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)

  18. APOLLO Superlatives • More lunar return photons in 10 minutes than the McDonald station gets in three years • best single run: >2500 photons in 10,000 shots (8 minutes) • Peak rates of >0.6 photons per shot (12 per second) • compare to typical 1/500 for McDonald, 1/100 for France • Range with ease at full moon • 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! • APD array is essential • Centimeter precision straight away • Millimeter-capable beginning April 2006

  19. Future Directions • LLR tests gravity on our doorstep • Although additional “doorstep” opportunities via lunar landing missions • sparse arrays, transponders • There’s also a back yard: the solar system • Interplanetary laser ranging offers another order-of-magnitude • 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

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