Mem. S.A.It. Vol. 83, 382c© SAIt 2012 Memorie della

Probing gravity with the proposed MAGIA andILN lunar missions

M. Garattini1, C. Lops1, S. Dell’Agnello1, A. Boni1, S. Berardi1, C. Cantone1,

G.O. Delle Monache1, N. Intaglietta1, M. Maiello1, M. Martini1, G. Patrizi1,

L. Porcelli1, M. Tibuzzi1, D.G. Currie2, R. Vittori3, G. Bianco4, T. Murphy5,

A. Coradini6, C. Dionisio7, R. March8, G. Bellettini9, and R. Tauraso9

1 Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali di Frascati, Via E. Fermi 40,00044 Frascati (Roma), Italy, e-mail: marco.garattini@lnf.infn.it

2 Department of Physics,University of Maryland (UMD),College Park,MD 20742 USA3 Aeronautica Militare Italiana,Rome,Italy4 Centro di Geodesia Spaziale G.Colombo (ASI-CGS),Matera,Italy5 Physics Department,University of California San Diego(UCSD)6 INAF-Istituto di Fisica dello Spazio Interplanetario (IFSI), via Fosso del Cavaliere 100,

00133, Rome, Italy7 Rheinmetall Italia S.p.A., via Affile 102, 00131 Rome, Italy8 INFN-LNF and CNR-Istituto per le Applicazioni del Calcolo (IAC), viale del Policlinico

137, 00161, Rome, Italy9 INFN-LNF and Department of Mathematics, University of Rome ”Tor Vergata”, via della

Ricerca Scientifica, 00133, Rome, Italy

Abstract. MAGIA (Missione Altimetrica Gravimetrica GeochImica Lunare) is a missionapproved by the Italian Space Agency (ASI) for Phase A study. Using a single large-diameter laser retroreflector, a large laser retroreflector array and an atomic clock onboardMAGIA, we propose to perform several fundamental physics and absolute positioningmetrology experiments: VESPUCCI, an improved test of the gravitational redshift in theEarthMoon system predicted by General Relativity; MoonLIGHT-P, a precursor test ofa second generation Lunar Laser Ranging (LLR) payload for precision gravity Network(ILN). Future ILN geodetic nodes equipped with MoonLIGHT and the Apollo/Lunokhodretroreflectors will become the first realization of the International Moon Reference Frame(IMRF), the lunar analog of the ITRF (International Terrestrial Reference Frame).

Key words. LLR – SLR – Gravitational redshift – Lunar science – ILN – Tests of generalrelativity

Send offprint requests to: M. Garattini

1. Introduction

In 2008 ASI approved for Phase A Study fiveproposals presented in response to the call for

Garattini: Probing Gravity with the Proposed MAGIA and ILN Lunar Mission 383

”Small Missions” issued in 2007. One of theseis MAGIA (Missione Altimetrica GravimetricaGeochImica Lunare), whose PrincipalInvestigator is A. Coradini (INAF-IFSI Rome)and Prime Contractor is Rheinmetall Italiaspa. MAGIA is an altimetry, gravimetric andgeochemical mission consisting of a mainOrbiter in polar orbit, which will release aSubsatellite at the end of the mission. One ofthe LNF-INFN contributions in MAGIA willbe the VESPUCCI (VEga or Soyuz Payloadfor Unified Clock vs. Ccr Investigation) pay-load, with the aim to measure the gravitationalredshift in the Earth-Moon system.

The ILN (International Lunar Network)initiative comes at an opportune time wheninternational space agencies are focusing un-precedented resources on lunar exploration.This will allow the network as a whole to mon-itor geophysical activity over the entire Moon.Each of the lander nodes will carry a core setof ILN defined instruments. One of the instru-ment categories that are being considered forthe ILN, is a new generation of LRRs (LunarLaser ranging). We propose to test on MAGIAspacecraft a new LLR payload, MoonLIGHT-P (Moon Laser Instrumentation for Generalrelativity Highaccuracy Test-Precursor), inte-grating with thermal and optical character-ization at SCF (Space/lunar laser rangingCharacterization Facility) at LNF-INFN.

2. The ASI lunar mission MAGIA

The MAGIA’s scientific goals have been iden-tified in order to avoid overlaps with cur-rently planned, ongoing, or just concluded or-biter or impactor missions: (1) study of themineralogical composition of the Moon bymeans of a VIS/NIR imaging spectrometer; (2)characterization of the Moon polar regions bymeans of concurrent observations with differ-ent instruments, (imaging experiment, altime-ter and thermal); (3) characterization of the lu-nar Gravity field with a two-satellite trackingsimilar to GRACE, with a main Orbiter and asmall Sub-satellite; (4) characterization of thelunar radiation environment by means of a par-ticle radiation monitor; (5) characterization ofthe lunar exosphere; (6) fundamental physics

test of gravity; (7) absolute positioning metrol-ogy measurements; (8) precursor technologicaltest for second generation LLR (Coradini et al.2010).

This work will exploit passive,maintenance-free laser retroreflectors andan atomic clock on the Orbiter. With MAGIAwe propose to perform the following ex-periments in the Earth-Moon system: (1)VESPUCCI: a significant improvement ofthe measurement of the gravitational redshiftin the trans-lunar flight and in Moon orbits.This will be illustrated in Section 2.1; (2)MoonLIGHT-P: a technological test of thepayload for second generation LLR. The im-portance of a precursor test will be explainedin Section 2.2. The retroreflectors will betracked by the ILRS (Pearlman et al. 2002).If MAGIA will be approved, the performanceof the two retroreflector payloads will becharacterized at the dedicated ”Satellite/lunarlaser ranging Characterization Facility (SCF)”of INFN-LNF (Dell’Agnello et al. 2008).For the atomic clock analysis we consider aclock stability in the range between 10−13 and2 × 10−14 (Section 2.1).

2.1. VESPUCCI: improvedmeasurement of the gravitationalredshift

The measurement of the gravitational redshift(GRS) is an important test of the Local PositionInvariance (LPI) of General Relativity (GR)and of any metric theory of gravity (Will 2006).The most accurate GRS measurement in space,|α| < 2 × 10−4, was performed in 1976 bythe satellite Gravity ProbeA (GP-A) (Vessotet al. 1980), which employed a space-bornehydrogen-maser clock, reached a maximum or-bital height of 10,000 km and took data forabout 2 h. The trans-lunar flight of MAGIAand the period the orbiter will spend aroundthe Moon (∼7 months) offer the possibility toperform an experimental test of the gravita-tional redshift in the Earth-Moon system basedon Corner Cube Retroreflector-Array (CCR-A), Corner Cube Retroreflector-Moon (CCR-M) and an onboard atomic clock with relative

384 Garattini: Probing Gravity with the Proposed MAGIA and ILN Lunar Mission

frequency stability of 10−13 or less. Positionson the ground and space clocks will be ref-erenced to the ITRF (Fermi, M., et al. 2010).MAGIA is suited to improve significantly theGP-A measurement for the following rea-sons: (1) the high-precision ISA (Italian SpringAccelerometer) (Iafolla, V., Peron, R. 2010)and radio science (RS) payloads which ensurethat the systematic error due to the Dopplershift background will be kept under control;(2) lots of data will be acquired in a region be-tween the Earth and the Moon where the twogravity fields can be considered simple point-like potentials, thus greatly simplifying thephysics analysis; (3) MAGIA will navigate twogravity potential wells experiencing the high-est possible variation of GRS in the EarthMoonsystem; (4) MAGIA positioning with respect tothe ITRF is achieved with two complementarytechniques: SLR/LLR tracking by the ILRS,which includes the Space Geodesy Center ofASI in Matera, Italy (ASI-CGS), providingvery accurate and absolute distance determina-tion with respect to the ITRF, and mission ra-dio telemetry from the ASI-CGS (and possibly,other stations). Our goal is to improve the bestdirect limit by GP-A, |α| < 2× 10−4. As shownin fig.1, GRS will increase away from the Earthand slightly decrease near the Moon, up to thenominal MAGIA altitude of 100 Km over theMoon surface.

Fig. 1. GRS variation vs. MAGIA distance fromthe geocenter, assuming point-like potentials. Theinset shows a blow-out of the end point of the curvevery near the Moon.

The expected statistical error on α for aPOD (Precise Orbit Determination) accuracy

of 10 m and for three different clock accura-cies: (1) |α| < 8 × 10−5 for a clock accuracy of3.3×10−13, a factor 2.5 improvement over GP-A; (2) |α| < 2 × 10−5 for a clock accuracy of10−13, a factor 10 improvement over GP-A; (3)|α| < 1×10−5 for a clock accuracy of 5×10−14,a factor 20 improvement over GP-A.

The current baseline design of CCR-Ais shown in fig.2. The arrays will be madeof solid, fused silica CCRs, with a diameteridentical to the Apollo mission lunar reflec-tors. Other optical and mechanical array pa-rameters will be optimized and finalized inlater phases of the mission, if approved. Themounting scheme and the choice materialswill inherit from the Apollo/LAGEOS(LAserGEodinamics Satellite) payloads and will becharacterized at INFN-LNF.

Fig. 2. Sketches of CCR-A: (a) exploded viewof the CCR showing the aluminum and Poly-ChloroTriFluoroEthylene (KEL-F) assembly rings;(b) full array.

2.2. MoonLIGHT-P: a precursor forsecond generation lunar laserranging

MoonLIGHT-P is developed by the Universityof Maryland and INFN-LNF for NASA’sLunar Science Sortie Opportunities (LSSO)program (Currie et al. 2006, 2009), for twoASI studies and, currently, for the ILN. ThePrimary Investigator (PI) of LSSO was D. G.Currie and Co-PI was S. DellAgnello; Italiancollaborators participated at no cost for NASA.This project is known to NASA as LunarLaser Ranging Retroreflector Array for the21st Century (LLRRA21).

The first generation LLR based on theretroreflector payloads deployed with Apolloand Lunokhod missions has provided numer-

Garattini: Probing Gravity with the Proposed MAGIA and ILN Lunar Mission 385

ous precision tests of gravity (Williams et al.2004) and unique measurements of lunar plan-etary science (Williams et al. 2006). In partic-ular, LLR currently gives the best overall testof General Relativity with a single experiment,as shown by tab.3, included also the major im-provements expected from second generationLLR with a total range accuracy of 1 mm and0.1 mm:

Fig. 3. Expected physics reach of firstGen. LLR and with Second Gen. LLR (withMoonLIGHT/LLRRA21).

The deployment of our large, singleretroreflector on MAGIA will allow for testingtwo critical instrumental effects: the thermalperturbation of the optical performance due tothe Sun and the laser ranging return at lunardistances for an orbiting target, which is moredifficult than for a payload on the surface.

Finally, MoonLIGHT-P, a precur-sor test of CCR-M on MAGIA, willstrengthen the Italian contribution to theInternational Lunar Network (ILN, see alsohttp://iln.arc.nasa.gov/) in the areas of fun-damental physics and lunar science. TheILN was formed by space agencies fromnine countries (including ASI) to establish anetwork of standardized payloads composedby a set of common core instruments to bedeployed with robotic missions. The ILNselected the following core instruments: (1)seismometer, (2) electromagnetic sounding,(3) heat flow probe, (4) CCR (Morgan,Dell’Agnello et al. 2009). The preliminaryspecs for the ILN CCR are fully compatiblewith our MoonLIGHT/LLRRA21 payload.

3. New generation of lunar laserranging

In 2007 a new station capable of mm-classrange accuracy started operations: APOLLO,the Apache Point Apache Point ObservatoryLunar Laser-ranging Operation, funded jointlyby National Science Foundation (NSF) andNASA (Murphy et al. 2008). Following this,the largest source of error is now closely linkedto the retroreflector arrays on the lunar surface,which are particularly affected by the lunar li-brations.

The motivation for a sparse, distributed ar-rays of single, very large CCRs (10 cm di-ameter) on the lunar surface is to remove theperturbation of the geometric librations of theMoon from LLR. Currently the librations ofthe Apollo and Lunokhod retroreflectors arethe dominant contribution to the LLR error.Our goal is to improve by a factor at least100 this contribution, from cm level down to0.1 mm. This new approach has been devel-oped by UMD and INFN-LNF with thermal,optical and orbital simulations and is now be-ing validated at INFN-LNF with the SCF-Test(Section 4) of a 100 mm diameter CCR fundedby NASA for LSSO. The general concept ofthe second generation of LLR is to considera number (notionally eight) large single CubeCorner Retroreflectors spread over tens of me-ters, unaffected by the libration and, conse-quently, by increased spread of the return laserpulse. The return from each of the CCRs willbe registered separately and can be identifiedby comparison with the nominal lunar orbitand earth rotational parameters. This is shownschematically in fig.4.

We currently envision the use of 100 mmCCRs composed of T19 SupraSil I. This is thesame material used in LLRA 20th and bothLAGEOS satellites. This will be mounted inan aluminum holder that is thermally shieldedfrom the Moon surface, in order to maintaina relatively constant temperature through thelunar day and night. It is also isolated fromthe CCR, by two coassial ”gold cans”, sothe CCR receives relatively little thermal in-put due to the high temperature of the lu-nar day and the low temperature of the lu-

386 Garattini: Probing Gravity with the Proposed MAGIA and ILN Lunar Mission

Fig. 4. Concept of the 2nd generation of LunarLaser Ranging.

nar night. Actual hardware prototype and themounting of the CCR inside the housing areshown in fig.5. KEL-F rings could be usedfor this mounting (its used in LAGEOS) dueto its good insulating, low out-gassing andnon-hygroscopic properties. The CCR and thethermal shield have been provided by LSSOfunds. Mechanical design and construction ofthe housing, rings and SCF-testing has beenprovided by INFN-LNF.

Fig. 5. Views of current design of theMoonLIGHT/LLRRA21 CCR: (a) fully assembled;(b) exploded view with its internal mountingelements and outer aluminum housing.

4. Thermal and optical tests inFrascati

SCF (Satellite/lunar laser rangingCharacterization Facility), at LNF/INFNin Frascati, Italy, is a cryostat where we areable to reproduce the space environment:cold (77 K with Liquid Nitrogen), vacuum,

and the Sun spectra. The SCF includes a Sunsimulator (www.ts-space.co.uk), that providesa 40 cm diameter beam with close spectralmatch to the AM0 standard of 1 Sun in space(1366.1 W/m2), with a uniformity better than±5% over an area of 35 cm diameter. Next tothe cryostat we have an optical table, wherewe can reproduce the laser path from Earthto the Moon, and back, studying the FarField Diffraction Pattern (FFDP) coming backfrom the CCR to the laser station, useful tounderstand how good is the optical behaviorof the CCR.

The SCF-Test (Dell’Agnello et al. 2011) isa new test procedure to characterize and modelthe detailed thermal behavior and the opticalperformance of laser retroreflectors in spacefor industrial and scientific application, neverbefore been performed. We perform an SCF-Test on the MoonLIGHT CCR to evaluate thethermal and optical performance in space envi-ronment. About thermal measurements we useboth an infrared (IR) camera and temperatureprobes, which give a real time measurementsof all the components of the CCR and its hous-ing. In particular we look at the temperaturedifference from the front face to the tip, study-ing how the FFDP changes during the differentthermal phases. This is the best representativeof the thermal distortion of the return beam tothe Earth. Various configurations and designsof the CCR and the housing have been and arebeing tested in the SCF Facility, with the solarsimulator, the temperature data recording, theinfrared camera and the measurement of theFar Field Diffraction Pattern (FFDP).

In fig.6 is shown theMoonLIGHT/LLRRA-21 flight CCR FFDPintensity variation at Moon velocity aberra-tions (2V/c) during key points of the SCF-Test:(1) in air, (2) in vacuum, (3) during chamber’sshields cooling, (4) Sun on orthogonal to theCCR’s face with the housing temperaturecontrolled at T=310 K, (5) Sun on at 30◦of inclination (no break-thru), (6) Sun on at−30◦ of inclination (break-thru), (7) Sun onorthogonal with the housing temperature leftfloating. From this graph we can deduce thatthe intensity decreases during no orthogonallighting of the CCR, in particular when the

Garattini: Probing Gravity with the Proposed MAGIA and ILN Lunar Mission 387

Sun enters in the housing cavity during thebreak-thru phase. This effect is due to a strongincrease of the ”Tip-Face” thermal gradientduring this two phase of the test. When thehousing temperature is left floating, the inten-sity slightly increases because the ”Tip-Face”gradient is reducing.

Fig. 6. MoonLIGHT/LLRRA-21 flight CCR FFDPintensity variation at Moon velocity aberrations(2V/c) during tests.

5. Conclusions

The Phase A study was concluded inDecember 2008 with the final review andthe full MAGIA Proposal was submitted toASI. The MAGIA collaboration is now await-ing the decision of the new ASI managementand the new National Space Plan. In the mean-time, the work of the INFN-LNF group on thedevelopment of the MoonLIGHT-P prototypecontinued in 2010-2011 in the frameworkof the ILN and with an R&D experimentapproved by INFN for the period 2010-2012,called MoonLIGHT-ILN.

Acknowledgements. In support of the research atFrascati, we wish to acknowledge the support of theItalian INFN-LNF, granted since the LSSO project,MoonLIGHT-M(anned) (Currie 2006, DellAgnello2007). We also wish to thank the support of ASIduring the 2007 lunar studies and the 2008 Phase

A study for MAGIA. We warmly thank SylvieEspinasse, formerly at ASI, now at ESA, for en-couraging the lunar science applications of our workwithin the ILN. We wish to acknowledge the sup-port of the University of Maryland via the NASALSSO program (Contract NNX07AV62G) to in-vestigate Lunar Science for the NASA MannedLunar Surface Science and the LUNAR consor-tium (http://lunar.colorado.edu), headquartered atthe University of Colorado, which is funded bythe NASA Lunar Science Institute (via CooperativeAgreement NNA09DB30A) to investigate conceptsfor astrophysical observatories on the Moon.

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