APOLLO

1. APOLLO Testing Gravity via Laser Ranging to the Moon Tom Murphy (UCSD)

2. 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. Q2C3

3. Newtonian piece Simplified (Conservative) PPN Equations of Motion Q2C3

4. Relativistic 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  1013; SEP to 4104 • time-rate-of-change of G • fractional change < 1012 per year • geodetic precession • to  0.5% • 1/r2 force law • to 1010 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 Q2C3

5. LLR through the decades Previously 200 meters APOLLO Q2C3

6. APOLLO: 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 Q2C3

7. The APOLLO Collaboration UCSD: Tom Murphy (PI) Eric Michelsen U Washington: Eric Adelberger Erik Swanson Apache Point Obs. Russet McMillan Harvard: Chris Stubbs James Battat Humboldt State: C. D. Hoyle Northwest Analysis: Ken Nordtvedt JPL: Jim Williams Slava Turyshev Dale Boggs CfA/SAO: Bob Reasenberg Irwin Shapiro John Chandler Lincoln Lab: Brian Aull Bob Reich Q2C3

8. Q2C3 Photo by NASA

9. Lunar Retroreflector Arrays Corner cubes Apollo 11 retroreflector array Apollo 14 retroreflector array Apollo 15 retroreflector array Q2C3

10. The 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: • 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 Q2C3

11. 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 • APOLLO gets 8–10 < 1 hour sessions per lunar month • 7-university consortium (UW NMSU, U Chicago, Princeton, Johns Hopkins, Colorado, Virginia) Q2C3

12. APOLLO 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 Q2C3

13. Catching 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 • 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 Q2C3

14. Differential 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 Q2C3

15. APOLLO Random Error Budget Ignoring retro array, APOLLO system has 104 ps (16 mm) error per photon Q2C3

16. Laser Mounted on Telescope Q2C3

17. Laser Illumination of Telescope Q2C3

18. Gigantic Laser Pointer Q2C3

19. Out the Barn Door Q2C3

20. Blasting the Moon Q2C3

21. Breaking All Records • 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 Q2C3

22. Killer Returns Apollo 11 Apollo 15 2007.11.19 red curves are theoretical profiles: get convolved with fiducial to make lunar return which array is physically smaller? 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 • 2344 photons in 5000 shots • 369,817,674,951.1  0.7 mm • 1 detection with 8 photons Q2C3

23. Sensing the Array Size & Orientation 2007.10.28 2007.10.29 2007.11.19 2007.11.20 Q2C3

24. 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 median = 1.8 mm 1.1 mm recent shaded  recent results Q2C3

25. Residuals 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 15 mm individual error bars:   1.5 mm Q2C3

26. Residuals Run-to-Run We can get 1 mm range precision in single “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 1.16 mm 2269 photons; 3k shots Apollo 15 reflector 2008.02.18 1.73 mm 901 photons; 2k shots 0.66 mm 8457 photons; 10k shots 1.45 mm 1483 photons; 3k shots Q2C3

27. Residuals Against JPL Model APOLLO data points processed together with 16,000 ranges over 38 years shows consistency with model 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 is about 8 mm   3 for this fit plot redacted: no agreement from JPL to make public Q2C3

28. APOLLO Impact on Model If APOLLO data is down-weighted to 15 mm, we see what the model would do without APOLLO- quality data 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 plot redacted: no agreement from JPL to make public Q2C3

29. Summary & 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 Q2C3