science and geodesy on the Moon

Transcription

1 MGF-3SC: a 3-spacecraft 3 mission for science and geodesy on the Moon A. Coradini (1), M. Fermi (2), E. Perozzi (3), G. Vulpetti (2), F. Sansò (4), M. Gregnanin (2), M. Verdino (2), M. Mazzolena (2), V. Iafolla (1), S. Casotto (6), S. Dell Agnello (5) 1. CNR/INAF/IFSI - Rome Italy 2. Galileian Plus s.r.l Rome - Italy 3. Telespazio SpA Rome - Italy 4. Politecnico di Milano/Polo di Como - Como - Italy 5. INFN - Frascati (Rome) - Italy 6. University of Padova - Padova - Italy

2 Acknowledgements: The ideas presented in this work have been maturated in the frame of the scientific studies funded by the Italian Space Agency within the program Italian Vision for Moon Exploration ( )

3 Presentation Structure: 1. Introduction 2. Mission Purposes 3. Multi-Satellite Configuration 4. System Features 5. Critical Problems 6. Conclusions and Recommendations

4 Introduction Despite of the significant efforts made in the lunar investigation, many questions remain still open: What is the internal structure of the Moon? Is it the Moon differentiated? Is it the Moon Geophysically Active? What are the main events characterising lunar history? What is the Geochemical Evolution of the Moon? What is the Chemical / Mineralogical Composition of the Moon? What is the geomorphology structure of the Moon surface? What are the resources? What is the Interaction Surface / Environment? The different lunar missions that are now in place or under development all try to reply to some of them. In this presentation we will try to asses what could be measured from orbit

5 Mission Purposes: Gravity and figure The Moon is an ancient body, whose primordial differentiation stopped about 3.2 (TBC!) billion years ago, when most of the lunar mare basalt was formed; another important phenomenon of the lunar evolution was the large scale bombardment that happened about 3.5 billion years ago. The Moon, after accretion, underwent a new extensive phenomenon of melting (Lunar Magma Ocean), that gave rise to the highlands, anorthositic in composition. Measurements of the depth of the anorthositic crust are needed to infer the extension of the original ocean (250 - to 1000 km deep, according to different models). Therefore detailed gravimetric measurements are needed, with a resolution that allow to identify the highlands depth at global scale

6 Mission Purposes: Geochemistry Estimates mineralogical composition of the crust and mantle are needed On the Moon is present a significant crustal asymmetry (KREEP, mare basalts) that reflects very early differentiation processes A similar asymmetry is also present between the near and the far sides Therefore: geochemical mineralogical measurements are needed

7 Mission Purposes: Selenodesy Accurate gravity field recovery combined with Lunar Laser Ranging should allow the establishment of a well defined Moon-centered reference system tied with the Earth-Centered Reference System. This is a goal in itself not only because it has to be compliant with the internal structure of the Moon, but also because it allows precise orbit determination and navigation of orbiting satellites. This is needed for any extended exploration and exploitation activities. In the proposed mission, the goal is to retrieve the Lunar Gravity field up to degree 120 and order 120 with an accuracy of 1 mgal (10-5 m/s 2 ) implying a resolution on ground of about 90 km.

8 Mission Purposes: Fundamental Physics A. High-Precision measurements of Gravitational Redshift B. Direct measurement of the Moon s barycenter vectorposition in the International Celestial Reference Frame (ICRF) C. Testing 2 nd generation retro-reflectors (in orbit) for very high precision measurements (to be performed in a subsequent mission) between Earth-bound laser stations and Moonbound retro-reflectors.

9 Mission Purposes: Fundamental Physics Gravitational theories, including General Relativity, and other theories based on the Einstein-Hayashi-Shirafuji (EHS) Lagrangians (which consider claimed space-time torsion at different levels) entail different amounts of gravitational redshift and relate to other important phenomena too. The above objectives require not only the new-generation retro-reflectors in progress at INFN-LNF in Frascati (Rome), but also high-precision & high-stability clocks onboard satellites. The choice of the clock system, though, is strongly affected by its overall mass.

10 Mission Purposes: Technological/Utilitarian goals Support for Future Navigation on the Moon Navigation on the Moon surface requires the availability of a selenocentric reference system. One of the most cost effective way to support navigation on the moon should be through the Lunar Galileo Navigation satellite System whose orbits have to be known in the selenocentric reference system. Lunar Galileo constellation designed at Utah University Accurate lunar gravity field is a preliminary need to maintain such constellation and establish a Global Lunar Positioning Service (GLPS)

11 Multi-Satellite Configuration The proposed multi-satellite configuration consists of three satellites: 1. The Geochemical Low Moon Orbiter thrusted satellite (Y1) 2. The co-orbiting probe-satellite free falling in the lunar gravity field (Y2) 3. The TLC satellite in an eccentric orbit (Y3)

12 Multi-Satellite Configuration Geochemical-Payload Satellite (Y1) Geochemistry. Among the main goals, we have identified the geological/geochemical evolution of the Moon, the identification of the presence of water and recognition of the distribution of lunar resources. Gravity-Field Payload. This requires a tri-axes accelerometer, a high stability clock, two X-band X micro wave systems: the first acts as inter-satellite link with Y2, and the second is devoted to tracking Y1 from Y3

13 Multi-Satellite Configuration Probing Satellite (Y2) Small satellite endowed with omni-directional antenna, highstability clock, three axes accelerometer and X-band microwave transponder for high precision inter-satellite link Apart from the action of non-gravitational forces, the satellite will free fall in the gravity field of the Moon No active attitude system should be onboard: the Y2 shape will be designed accordingly

14 Multi-Satellite Configuration Telecom & Laser Satellite (Y3) This satellite should have two main purposes: 1. Microwave tracking to the satellites Y1 and Y2; Laser retroreflectors for high precision tracking from the Earth 2. Providing communications (scientific data, telemetry and telecommands) with the control center for all the constellation 3. Therefore, Y1/Y2 communicates with Y3 via a much simpler and lighter system

15 System Features Orbital Configuration Y1 and Y2 should revolve about Moon on quasi-frozen orbit; Y3 s orbit should be chosen chiefly for maximizing inter-satellite visibility All orbits below ~800 km have to be very accurately designed also due to the big uncertainties in the current knowledge of the lunar harmonics

16 System Features Orbital Configuration Low-altitude polar orbits are better for observing the lunar surface but require high insertion delta-v (upper bold curves) Medium-altitude eccentric orbits maximize ground visibility and require lower insertion delta-v (lower curves) delta-v (km/s) 1 0,8 0,6 0,4 0,2 0 Moon H 0 =100 H 0 =3000 H 0 = distance from the Moon (km) delivering the satellites onto different orbits allow implementing a staging strategy leading to propellant saving and thus to higher payload mass L1

17 System Features Inter-Satellite Visibility Satellites Y1-Y2 could be designed to act in a SST GRACE-like configuration so to probe the farside gravitationally for a half of the mission time dedicated to lunar geodesy package For a fraction of the time Y1-Y2 spend over the farside, all three satellites should be visible to each other for improving orbit determination

18 System Features Visibility from Earth Orbiter Ground Station visibility 100x100 polar orbiter Fucino station 100x2000 polar orbiter Fucino station Daily Average Coverage Time Daily Average Access Duration 7.7 h /day 1.4 h 9.9 h /day 3.85 h

19 System Features Force Field Models and Time Scales Any satellite orbiting the Moon can be dynamically viewed as a test-body in the restricted 4-body problem with the additional Moon finite-size gravity and the light s radiation pressure from Sun, Moon and Earth (in decreasing magnitude of non-conservative accelerations). If appropriate, even the influence from Jupiter, Venus and Mars can be taken in to account in very precise orbit determination. One needs six reference frames for dealing with the full problem: (1) ICRF, (2) selenocentric quasi-inertial frame, (3) selene body fixed reference frame, (4) the satellite-fixed frames. Additional intermediate frames should be considered too, for allowing reference frames transformations. A challenging problem is to synchronize the time scales from each frame, including the proper time of each satellite of the science configuration. Although, in principle, this is an iterative process, however it should be possible to begin sufficiently with the current knowledge of the lunar gravity.

20 Priorities and Critical Problems Geochemical Payload Instrument Weight The highest priority experiments are: Multispectral cameras γ,, X, UV and IR spectrometers radio and radar measurements Multispectral Camera + Sodium filter Spectrometer IR um 6.0 kg Spectrometer IR um 5.0 kg Spectrometer X, γ, neutrons 10.0 kg Spectrometer UV - FUV 5.0 kg Radar Sounder 15.0 kg Radiometer 10.0 kg Thermal MIR 7-14 µm TOTAL 2.8 kg kg

21 Critical Problems Lunar-Gravity Payload: ISA accelerometers 1/2 Three-axis accelerometer 10 sensitivity 10 g / Hz ^ mass 6 kg power 8 W

22 Critical Problems Lunar-Gravity Payload: ISA accelerometers 2/2 ( simplified example) r A Φ m c Sun a = C S Sun R ˆ

23 Critical Problems Astrodynamics The main problem that will affect the long-term evolution of the three satellites in the lunar-field environment should come from the uncertainties in the current lunar harmonics beyond 8 x 8. A satellite orbit computed almost frozen for years taking the LP165P harmonics set might exhibit a significantly different behaviour; for instance, the uncontrolled satellite may knock earlier against the t lunar mountains or maria. Anyway, the days before such an event may be precious for getting information about the harmonics due to the very low altitude the satellite is spanning. The experience and results gained by the Japanese SELENE may help us to avoid subtle design mistakes

24 Conclusions and Recommendations Feasibility Preliminary evaluations, also based on previous ESA studies such as MORO, induce us to be confident of its feasibility. Nevertheless, open problems still remain: e.g., laser ranging capabilities from the Earth to lunar orbiting satellites for fundamental physics experiment and for achieving precise orbit determination have to be confirmed. However, microwave tracking appears to be a valid alternative to Laser Ranging. Further studies are needed. Geochemical an Gravity field recovery experiments should be run in two different mission phases. Capability to really decouple lunar harmonics should be feasible adopting a GRACE-like configuration.

25 Conclusions and Recommendations Recommendations The study of the Moon asks for an integrated approach that could allow developing well-focused (in terms of science and payload integration) and dedicated high-technology satellites In addition to the determination of high-precision/accuracy lunar gravity field models, the astrodynamics of Moon-orbiting satellites might find answers to a few questions from fundamental-physics