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Getting Down With Gravity

Getting Down With Gravity

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Getting Down With Gravity

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  1. Getting Down With Gravity Shooting the Moon Eric L. Michelsen, PhDTom Murphy, Principal InvestigatorUCSD Center for Astrophysics and Space Science

  2. Topics • Who? • What? • Why? • How? • Where? • When? • Little lost rover Reuben H. Fleet Science Center: Afternoon Scholars

  3. Support • Our research is funded jointly by NASA, and the National Science Foundation. • Which means: You! • APOLLO is based at UCSD, California This presentation is based on one by Tom Murphy, UCSD Reuben H. Fleet Science Center: Afternoon Scholars

  4. What Do We Do? • We trace out the orbit of the moon by measuring the distance to it to 1 mm • thickness of a dime • The moon is ¼ million miles away (~400,000 km) • like measuring the distance around the equator to the thickness of a sheet of paper • Lunar Laser Ranging

  5. Who Am I? • Eric L. Michelsen • PhD from UCSD, June 2010 • Thesis topic: Lunar Laser Ranging • Center for Astrophysics and Space Science (CASS) My One Sentence • The fundamental basis of all science is that thinking (theory) and experiment go hand in hand. Reuben H. Fleet Science Center: Afternoon Scholars

  6. Why Fundamental Research? • We don’t know • But we have some ideas • 80 years ago, physicists studied new subatomic particles: positrons • Today, positrons save 1000s of lives a year through Positron Emission Tomography • Newton’s gravity got us to the moon • Einstein’s General Relativity got us GPS • LLR: Space missions? Asteroids? Particles? Reuben H. Fleet Science Center: Afternoon Scholars

  7. Galileo’s Equivalence Principle • Galileo is said to have dropped different objects from the Leaning Tower of Pisa to see if they fell at the same rate • Galileo concluded that indeed they do Reuben H. Fleet Science Center: Afternoon Scholars

  8. Could We Have Guessed This? Is this one ball or two? • Is it reasonable that heavy objects fall faster? • Thought Experiment • But the ultimate arbiter is real experiment • Thinking (theory) and experiment go hand-in-hand Reuben H. Fleet Science Center: Afternoon Scholars

  9. Einstein’s Remarkable Insight • Thought experiments lead to the 1915 revolution in gravity: General Relativity • Spacetime is a fabric that stretches and twists • Mass-energy bends spacetime, influencing the motion of nearby objects • GR rigorously tested for years • Mercury orbit precession • Deflection of starlight • Slowing of time (vital for Global Positioning System!) • Lunar Laser Ranging Curvature is proportional to mass Reuben H. Fleet Science Center: Afternoon Scholars

  10. Why Isn’t General Relativity Good Enough? • GR and Quantum Mechanics don’t mix • The expansion of the Universe is not following the expected rules • it’s accelerating! • most of the universe is in forms unknown to us • Did Einstein have the last word on gravity? • unlikely Composition of the Universe visible matter and energy, 5% dark matter25% dark energy 70% Reuben H. Fleet Science Center: Afternoon Scholars

  11. How Do We Test Theories of Gravity? • Drop canonballs from the Leaning Tower of Pisa • Laboratory measurements: extremely delicate (e.g., Eot-Wash) • Lunar Laser Ranging (LLR) traces the orbit of the Moon by measuring its distance repeatedly over time • Other orbits: planets • Space missions ($$$) Reuben H. Fleet Science Center: Afternoon Scholars

  12. What Are We Looking For? • New physics! • That is, violations of General Relativity • E.g., what if we could drop the Earth and Moon together onto the sun? • Would they fall at the same rate? • Earth and Moon are different materials: iron vs. silica (rock) • Earth and moon have different gravitational binding energy • Does E = mc2 apply to gravitational energy? Einstein says yes! Reuben H. Fleet Science Center: Afternoon Scholars

  13. The Lunar Tower of Pisa • What if Earth falls more slowly than the Moon? • Earth is sluggish to move (pulled weakly by gravity) • Then Earth orbit is larger than Moon’s • Appears that Moon’s orbit is shifted toward sun farther out Earth orbit sun’s gravity closer in Moon orbit, on average Sun Reuben H. Fleet Science Center: Afternoon Scholars

  14. Why use the Moon? Because it’s our closest, cleanest, biggest example of gravity in action. apogee 55 earth radii perigee Earth (to scale) 65 earth radii Year 2003 orbits.

  15. Weak Gravity Acceleration Law Newtonian scalar perturbations modify magnitude vector perturbations modify direction & magnitude velocity terms ~ 1/r2 acceleration term ~ 1/r • Used to model the Solar System Reuben H. Fleet Science Center: Afternoon Scholars

  16. How Does Lunar Laser Ranging Work? • Outbound pulse starts out 3.5 meters in diameter, 2 cm thick • Each pulse has 300 quadrillion (3 x 1017) photons • Atmosphere causes pulse to diverge by 1 arc-second or more • At the moon, pulse is ~ 2 km across, still only 2 cm thick • Only ~1 in 30 million photons (= ~10 billion photons) in the 2 km disc hit the breadbox-sized reflector Moon Travels “faster than light” from our earth viewpoint • Return pulse expands ~8 arcseconds due to corner-cube diffraction • Return pulse on earth is about 15 km across • About 3 of the returning photons hit the 3.5 m mirror, ~ 1 detected • APOLLO launches 20 pulses per second • Round trip time is about 2.5 seconds • There are ~50 pulses in-flight at any time Atmosphere Earth Telescope Not to scale

  17. Right Back At Ya • Mirrors don’t work for us • Reflection misses the Earth Close up at mirror • Retroreflectors send light back exactly the way it came in • Back into our telescope Close up at reflector Reuben H. Fleet Science Center: Afternoon Scholars

  18. Lunar Retroreflector Arrays Corner cubes Apollo 11 retroreflector array Apollo 14 retroreflector array Apollo 15 retroreflector array Reuben H. Fleet Science Center: Afternoon Scholars

  19. 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, Lunakhod 1 (lost for 39 years, now found!) • Luna 21, Lunakhod 2 • Similar in size to A11, A14 • Lunokhod 2 doesn’t work in sunlight Reuben H. Fleet Science Center: Afternoon Scholars

  20. LLR Through the Decades Previously 100 meters • From U-Texas, France, and Italy Uncertainty (cm) APOLLO 1mm Reuben H. Fleet Science Center: Afternoon Scholars

  21. The Next Big Thing In LLR APOLLO offers 10x improvements in LLR with1 millimeter range precision by: • Using a 3.5 meter telescope • Operating at 20 pulses/sec • 12 simultaneous detectors • other stations have only 1 • Advanced detector technology • 25 ps (3.5 mm resolution) Reuben H. Fleet Science Center: Afternoon Scholars

  22. Apache Point Observatory, NM New Mexico • Southern NM (Sunspot) • 3.5 meter telescope • That’s big! • High-grade research telescope • 9,200 ft (2800 m) elevation • Great “seeing”: 1 arcsec • 7-university consortium • U Washington • U Chicago • Johns Hopkins • Princeton • NMSU • Colorado • U Virginia Reuben H. Fleet Science Center: Afternoon Scholars

  23. Equipment Mounted on Telescope Reuben H. Fleet Science Center: Afternoon Scholars

  24. First Light: July 24, 2005 Reuben H. Fleet Science Center: Afternoon Scholars

  25. World’s Biggest Laser Pointer Reuben H. Fleet Science Center: Afternoon Scholars

  26. Hunting for Airplanes Reuben H. Fleet Science Center: Afternoon Scholars

  27. Optical System Reuben H. Fleet Science Center: Afternoon Scholars

  28. APOLLO Laser • Nd:YAG mode-locked, cavity-dumped • Frequency-doubled to 532 nm (green) • 90 ps pulse width • 1/10 of a billionth of a second • 20 pulses per second • 2.3 watt average power • Less than a night light • 1 gigawatt peak power! • 1 billion watts • Beam expanded to 3.5 meters • Less of an eye hazard • Less damaging to optics • Negligible diffraction Reuben H. Fleet Science Center: Afternoon Scholars

  29. Catching Some Rays APD array APD APD lenslet array • Several photons per pulse require multiple detectors to time-tag each photon • 44 array of .03 mm circular detectors on 0.1 mm centers • Lenslet array in front recovers all the light, eliminating gaps between APDs • Focused image is formed at lenslet 0.1 mm Image plane on lenslet array Reuben H. Fleet Science Center: Afternoon Scholars

  30. 16-channel APD Electronics APD Package daughter board (magnified) daughter boards Reuben H. Fleet Science Center: Afternoon Scholars

  31. Gimme Some Latitude • APOLLO needs to know range to ~10m in advance geocentric latitude geodetic latitude Reuben H. Fleet Science Center: Afternoon Scholars

  32. Example Data From an Early Run Return photons from reflector width is < 0.5 m 2150 photons in 14,000 shots Randomly-timed background photons (bright moon) Reuben H. Fleet Science Center: Afternoon Scholars

  33. Killer Returns Apollo 15 November 19, 2011 Apollo 11 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 2344 photons in 5000 shots 369,817,674,951.1  0.7 mm Reuben H. Fleet Science Center: Afternoon Scholars

  34. Lunokhod 1: Little Lost Rover • Soviet rover landed 17 November, 1970 • Attempt to blunt Apollo 11 victory in race to the moon • Operated on surface for 10 months • Parked during lunar nights to allow ranging attempts • Soviets and French both got returns December 1970, on first lunar night • But both failed in later attempts, even after end of mission • Americans never convincingly found it • A 1976 report states that Soviets found L1 again in May 1974 • Claims regular observations thereafter • No substantiation • APOLLO tried occasionally, beginning April 2008 • In hindsight, position was far off: no chance of success reflector Lost for 39 years Reuben H. Fleet Science Center: Afternoon Scholars

  35. The Lunokhod Reflectors • 14 triangular CCRs, 11 cm side length • At 532 nm, expect brightness between A11/A14 and A15 • However, L2 was once similar in strengthto A15 • But now L2 is 1/10th the strength of A15 • So we expected L1 to be similar in strength to L2, at best • Or maybe lack of returns meant L1, if found, would be weaker than L2 Reuben H. Fleet Science Center: Afternoon Scholars

  36. Enter Lunar Reconnaissance Orbiter • The Lunar Reconnaissance Orbiter (LRO) helped three ways: • LROC imaging (March 2010) found the rover and provided coordinates • LOLA altimetry fixed the site radius • A corner-cube-reflector array on LRO prompted APOLLO to develop a wide-gate capability, making L1 searches easier (~80 m window vs usual 10 m) Reuben H. Fleet Science Center: Afternoon Scholars

  37. We found it, and it’s strong! Each range measurement is a slice intersecting the lunar surface in a circle, centered on the sub-earth point Range measurements at different librations allow us to pinpoint the reflector Our observations through June 2010 constrain the position to about 0.1 m Our lunar radius is ~1 m larger than LOLA! Why? “X” Marks the Spot laser pulses to earth sub-earth point Reflector found April, 2010 Reuben H. Fleet Science Center: Afternoon Scholars

  38. APOLLO Superlatives • APOLLO beats previous records by far • Best 1-hour night (Oct 17 2008): • 66,000 photons • more than Texas station in 38 years • four years-worth of French station (who held previous records) • Detection rates up to 0.5 - 1 photons per pulse (10 - 20 per second) • As high as 12 detected in a single pulse • Range with ease at full moon • Current stations can’t fight the background • Our data precision exceeds the JPL model • That’s expected: the model is based on data! • Precisely located the 39-year-lost rover Lunokhod 1 • Substantially improves gravity and lunar interior science Reuben H. Fleet Science Center: Afternoon Scholars

  39. Science on a Budget • APOLLO to date: $2 million • Ground experiment • Allows repairs and upgrades • Previous LLR claims 0.1% accuracyon “gravitomagnetism” • APOLLO aims to make that 0.01% • Gravity Probe B: $1 billion • Space mission: implies high risk • Targeting 1% accuracy on gravitomagnetism • Sad failure: Unexpected noise in measurement • no results • no hope of repair Reuben H. Fleet Science Center: Afternoon Scholars

  40. Project Status and Plans Operating 7-9 nights per month ~1 hour each Frequently achieve 1 mm precision in a night Sufficient data for 10x tighter bounds on gravity parameters in coming years Eliminate remaining sources of systematic error Earth tides (crustal deformation, few mm out of ~0.5 m) Atmospheric delay (several mm out of 2 m) Telescope thermal expansion (~3 mm) First photon bias (~2 mm) Acquire in-project solar system model Rapid analysis => rapid debugging of problems Reuben H. Fleet Science Center: Afternoon Scholars

  41. The End • Questions? Reuben H. Fleet Science Center: Afternoon Scholars

  42. New Ideas • Finding Lunokhod 1 • Reflector is 1 m above surface • We can detect size of reflectors from earth • Like resolving .5m/400 Mm • Push benchtop detector ~1”, see histogram jump Reuben H. Fleet Science Center: Afternoon Scholars

  43. Abstract The APOLLO Lunar Laser Ranging operation measures the orbit of the moon to 1 mm. It provides 10x improvements in precision over previous laser ranging, as a new test of all major aspects of gravity. APOLLO is an example of a world-class, high-precision physics experiment that operates on a comparatively small budget. The keys to APOLLO’s improvements are large aperture, multiple photon detector array, high laser output, and good seeing at the telescope site. We present an overview of APOLLO’s design, operation, and history, as well as examples of its theoretical implications. We provide a rare, behind-the-scenes look at the reality of a modern physics experiment, including its successes, some gaffes, and unexplained results. Reuben H. Fleet Science Center: Afternoon Scholars

  44. What’s Happening Where Lunokhod 1 Apollo 15 Lunokhod 2 Apollo 11 Apollo 14 • 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, Lunokhod 1 (lost 39 years, now located by LRO and APOLLO) • Luna 21, Lunokhod 2 • Similar in cross-section to A11, A14 • Lunokhod 2 doesn’t work in sunlight Reuben H. Fleet Science Center: Afternoon Scholars

  45. Sensing the Array Size & Orientation 2007.10.28 2007.10.29 2007.11.19 2007.11.20 Reuben H. Fleet Science Center: Afternoon Scholars

  46. APOLLO Return Rates • APOLLO firmly into multi-photon-per-pulse regime • French station record ~0.1; Texas ~0.02 • Some shots get 12 photons (out of 12 working detector elements) • Having a detector array is crucial: multiple buckets for photons (relative to pre-APOLLO record) Reuben H. Fleet Science Center: Afternoon Scholars