Towards Advanced Lunar Laser Ranging

1. Towards Advanced Lunar Laser Ranging Slava G. Turyshev with special thanks to Richard T. Baran, Dale H. Boggs, William M. Folkner, Gary Gutt, Ruwan P. Somawardhana, Robert Spero, James G. Williams Jet Propulsion Laboratory, California Institute of Technology 4800 Oak Grove Drive, Pasadena, CA 91009 USA “Theory and Model for the New Generation of the Lunar Laser Ranging Data”the First LLR workshop, Bern, Switzerland, 16-19 February 2010

2. LUNAR LASER RANGING SCEINCE Lunar Laser Ranging Lunar laser ranging (LLR) begun over 40 year ago… Laser Ranges between observatories on the Earth and retroreflectors on the Moon started by Apollo in 1969 and continue to the present McDonald 2.7 m • 4 reflectors are ranged: • Apollo 11, 14 & 15 sites • Lunakhod 2 Rover • Historically LLR conducted primarily from 3 observatories: • McDonald (Texas, USA) • OCA (Grasse, France) • Haleakala (Hawaii, USA) • New LLR stations: • Apache Point (NM, USA) • Matera (Matera, Italy) • South Africa, former OCA LLR equipment

3. ADVANCED LUNAR LASER RANGING EXPERIMENT Excellent Legacy of the Apollo Program The Apollo 11 retroreflector initiated a shift from analyzing lunar position angles to ranges. Today LLR is the only continuing experiment since the Apollo-Era. Apollo 11 Edwin E. Aldrin, Apollo 11 Apollo 14

4. French-built retroreflector array Lunokhod Rover (USSR, 1972) LUNAR LASER RANGING SCEINCE Lunar Retroreflectors Beginning of the laser ranging technology. Today, laser ranging has many applications: • Satellite laser ranging, communication systems, metrology, 3-D scanning, altimetry, etc. Apollo 15

5. LUNAR LASER RANGING SCEINCE Historical Accuracy of LLR Schematics of the lunar laser ranging experiment • Raw ranges vary by ~1,000s km • Present range accuracy ~1.5cm RMS of data fit today 1.5 cm ~ 3.610-11 big telescope, fat laser pulse Solution parameters include: • Dissipation: tidal & solid / fluid core mantle boundary (CMB); • Dissipation related coefficients for rotation & orientation terms; • Love numbers k2, h2, l2; • Correction to tilt of equator to the ecliptic – approximates influence of CMB flattening; • Number of relativity parameters. Near-Term Goal 1 mm ~ 2.410-12 small telescope, narrow laser pulse big telescope, narrow laser pulse

6. Future Lunar Laser Ranging • Future LLR sites & corner cube retro-reflectors (CCRR): • A wider spread of future LLR site locations would improve the determination of three-dimensional rotation and tides to the benefit of lunar science. • New CCRRs could give strong signals with no spread of laser pulse. • As of Feb 1, 2010 – no astronauts on the moon…, so instruments must have low mass and allow for robotic deployment… • Time scales (no transponders!): • Important time scales for lunar science observations span 1/2 month to decades. • A useful position for a new array (or laser transponder) could be determined with one to several months of tracking, though spans of years give the highest accuracy. • Data spans of years are optimum for most lunar science applications and that would include continued accurate tracking of the four existing lunar retroreflector arrays.

7. A Next Generation of Lunar Laser Ranging • The next-generation of the LLR experiment would rely on either • the new sets of laser retroreflector arrays on the Moon. • Improving the efficiently of LLR science: • Old reflectors are degraded (perhaps due to dust and/or radiation) • The current distribution of the retroreflectors is not optimal: • A geographic distribution of new instruments on the Moon wider than the current distribution would be a great benefit • Better sensitivity to physical librations and lunar tides • The accuracy of the lunar science parameters would increase several times. • Improvements in LLR Science: • Properties of the lunar interior, including liquid core & solid inner core can be determined from lunar rotation, orientation, and tidal response. • Earth geophysics/geodesy: positions & rates for the Earth stations, Earth rotation, precession rate, nutation, tidal influences on the orbit. • Improvements are also expected in several tests of general relativity.

8. Large Hollow Corner Cube for LLR • A 170mm diameter, hollow CC, retro reflector system has been modeled using a combination of thermal, mechanical, and optical diffraction physics models. • The optical power returned by one 170mm hollow CC is approximately the same as the power returned by 100 Apollo solid corner cubes. The velocity-aberration leads to a desired pattern for the returned light. Two basic design parameters for the corner cube are available to create this desired pattern: aperture diameter and dihedral angle error.

9. Summary • LLR determines • physical librations, tides and retroreflector array locations. • Lunar physical librations and tides give LLR sensitivity to mantle and core properties: • determine core is fluid and its moment of inertia; • determine Love numbers and find low tidal Qs; • sense strong dissipation at and flattening of core/mantle boundary. • Future goals: • continue to collect quality LLR data; • improve fits and solution parameter results, and add new parameters. • Future possibilities: • seek inner core; • find Lunokhod 1; • establish new LLR sites on the Moon.