The LEDA mission: Exploration opportunities prompted by a return to the Moon

Adv. Space Res. Vol. 19, No. 10, pp. 16294635,

1991

8 1997 COSPAR. Published by Elscvier Science Ltd. All fights reserved

Printed in Great Britain 0273-117707 $17.00 + 0.00 PII: SO273-1177(97)00378-5

THE LEDA MISSION: EXPLORATION OPPORTUNITIES PROMPTED BY A RETURN TO THE MOON M. Novara and D. Kassing System Studies Division, ESAIESTEC, Postbus 299,220O AG, Noordw& The Netherlands

ABSTRACT The main objective of the proposed LEDA (Lunar European Demonstration Approach) mission is for Europe to soft-land a spacecraft on the lunar surface and to operate it in situ, using mostly European means, technologies and capabilities. The spacecraft shall carry a payload to undertake investigations pertinent to the future phases of the ESA programme of lunar exploration and utilisation. Technology demonstration is the main objective of LEDA, but the mission shall also provide the scientific return compatible with the accomplishment of its primary objective. This must be accomplished within a total cost of 350 MECU to the European Space Agency (ESA). A LEDA Assessment Study has been conducted by ESA together with ASI, CNES and DLR, with some support from European industry in critical areas, during 1994- 1995. This study has shown that it is within the capabilities of ESA, working together with other European agencies, to undertake a lander mission to the Moon early in the 2 I st Century, and to demonstrate safe and effective landing and surface operations. The Assessment Study has defined a baseline mission, to be used for further analysis in future preparatory activities. The baseline mission is presented in this paper, including an overview of the envisaged exploration opportunities and of the reference scientific payload. 0 1997 COSPAR. Published by Elsevier Science Ltd. WHY TO THE MOON? The first International Lunar Workshop held in Beatenberg, Switzerland, in June 1994 defined the overall objectives for a staged, but evolutionary Moon Programme. About 140 representatives from space agencies, scientific institutions and industry from around the world considered plans for the implementation of internationally coordinated programmes for robotic and human lunar exploration. It was agreed that the time is right, scientifically, technologically and financially, to initiate the first phase involving Moon orbiters and landers with roving robots to prepare for “science of the Moon” (illuminating the history of the Earth-Moon system), “from the Moon” (for astronomical projects) and “on the Moon” (biological reactions to low gravity and the unique radiation environment). Details are given in ESA publications (Battrick and Barron, 1992, and Balsiger et a/., 1994). Actually, the enthusiasm in Beatenberg about the rich opportunities offered by the exploration and utilisation of the Moon was the point of departure for LEDA. ESA’s study of a “Lunar European Demonstration Approach”. The study includes a series of in-house and external activities to define an exploration mission consisting of a spacecraft that soft-lands on the lunar surface after having been orbited by Europe’s ARlANE 5 launcher in the year 2002. The spacecraft shall carry a payload consisting of a rover, a robotic arm. a soil processing test facility and a number of instruments for in situ measurements of the lunar environment. This must be accomplished within the budget of a medium-size mission. A summary of the LEDA mission is given in Table I, and a pictorial view in Figure I. A range of mission options, landing sites, spacecraft/rover design concepts and technologies are being assessed at present by a working team of experts from ESA. the French Centre National d’Etudes Spatiales (CNES), the Agenzia Spaziale Italiana (ASI) and the German Aerospace Research Establishment (DLR). 1629

M. Novm andD. Karsing

630

Table 1. LEDA Mission Summary

Objectives

-

Europe to soft-land a spacecraft on the lunar surface using ARIANE

5

Carry a payload to undertake investigations pertinent to future phases of ESA programme Budget of a medium size mission Spacecraft

-

Mass: 3347 kg in GTO, 1005 kg on Moon surface Size: diameter 4. I m, height 2 m Propulsion: 8 x 400 N, 8 x IO N thrusters, pulsed-mode operation for thrust modulation during descent Power: 300 W bus power from 4.5 m2 GaAs fixed solar panels (207 W/m’), I6 kg NiH, batteries (60 Wh/kg) Thermal Control: passive + active (radiator louvres), 5 kg RHUs (100 W,) GNC: 3-axis stabilisation, coarse sun sensor, Inertial Measurement Unit, radar altimeter, Doppler radar, camera vision system Data: 8 Gbit MMU (video sequence storage) Communications: 20-W S-band transponder, omni antennas for orbit and landing, 0.5 m high-gain antenna for surface operations Landing: 4 legs, 0.5 m stroke, 5 m/s vertical speed, ~5 g landing shock Payload mass: 200 kg Payload may include rover, robotic arm, soil processing test facility

Payload

In situ measurement payload: soil characterisation, imaging, operational environment evaluation Launch & Orbit

-

Shared ARIANE 5 into GTO (58% of launch mass capability) Manoeuvres to LLO: perigee, mid-course, lunar orbit iir_iection(total AV = 1730 m/s) Duration about 80 days from launch to landing (including lunar orbital phase) Lunar polar orbit at 100 x 100 km altitude, period 2 hours Orbit lowering to I5 x 100 km, l-2 orbits prior to landing, for site survey Descent & landing (AV = 2000 m/s) in Moon South Pole region, 83-85” S, 20” E-20” W

Operations

-

Communications: S-band (2,076/2,255 MHz) Data volume: 44 kb/s to 4.4 Mb/s, on-board I :4 video data compression, 2 ESA 15-m ground stations ESOC operations centre Operational lifetime: 4 lunar days on Moon surface No orbital relay, direct communications to Earth (<73% of time)

Programmatics

-

ESA cost <350 MECU Launch possible in 200 l-2002 time frame after development programme of 4 to 5 years Based on European capability alone (except RHUs) ESA provides shared ARIANE 5 launch Payload contributed by National Agencies

X

k

(2.0 m*)

Z

.~, I@.

Y

w

Radiator

XI

Tanks

8 Engines

Fig. 1. The LEDA spacecraft

LANDING SITE SELECTION The Moon has been extensively visited during the 1960s and 1970s by automatic and piloted missions. Nevertheless, there are plenty of sites which were not, or only summarily investigated, and are deemed to be of high interest to the scientific community. For example: Polar areas of the Moon are hardly known. No lander missions were ever flown at high or polar latitudes, and even orbiter data are scanty for those areas (the CLEMENTINE imagery is limited to a resolution of 100 m, and no altimetry data were obtained). Scientific interest for these areas is manifold, ranging from the search for water ice, residue from cometary impacts, which may have survived in permanently shadowed areas which are likely to exist near the South Pole, to the possibility of installing infrared interferometry devices in these same shadowed, and thus extremely cold, areas (Foing, 1995). Access to unique geological features, such as the Aitken basin (extending to a large portion of the South Polar region) is also a highly regarded opportunity. Research of the Moon volcanism in the past, the lunar radiometric age, and the heat flow is a new field of investigation not previously undertaken, and for which the lava-flooded areas to be found at medium/high latitude (up to 70”) are ideal sites. Beside the scientific and exploratory interest, technical and operational considerations on the choice of the landing site have to be made. The South Polar region, which is of the highest interest, is not very well known, but it appears to be considerably rougher, in general, than lower-latitude sites. Such morphology has several consequences: The risk of not finding a suitable landing site (obstacle-free) is higher. Both the sun and the Earth will be visible from the landing site at a very low elevation, thus sunlight and communications to Earth will be very much impacted by local features (peaks, valleys). On the other hand, imaging from the CLEMENTINE mission suggests the possibility that a small area on the rim of the 20km diameter South Pole crater may be in permanent sunlight, at least during the Austral summer. Operations of a mobile payload (a rover system) on the surface will be constrained by the difficult terrain to

1632

M. Nowa and D. K&q

overcome (slope, size of boulders), and also by the fact that sight of the lander may be lost very soon after moving away from it. In particular, it has to be reminded that the regions around the poles (roughly, beyond 83” latitude) disappear periodically from sight to an observer on Earth (the so-called optical libration phenomenon). This phenomenon lasts for a half of the lunar day (of about 14 Earth days) at the pole itself Given the fact that the lunar night also has a comparable duration of 14 Earth days, the resulting pattern is that the coincidence of the periods during which Earth can be seen from a polar region during the local daytime occur with an annual periodicity. During only about 4 months per year the best conditions occur, and this seems to determine the maximum duration of a surface mission to the poles, as well as to impose a “landing window” (at the beginning of the 4-month period). Given that a launch window constraint exists for a shared launch to GTO, a waiting time in lunar orbit for phasing is thus necessary. The low elevation of the Sun also dictates that solar generators are required to cope with the full 180” azimuth variation of sunlight direction in order to receive sufficient energy (while at lower latitudes a simple flat, horizontal solar array may receive enough). Also, long shadows reduce visibility, imposing for example that a vision-based landing takes place only around the local noon (thus losing surface operations time in the preceding morning). The fact that no high-resolution mapping of polar regions is available to a sufficient level of detail is also a concern. Imaging to a resolution better than IO m is required in order to select the areas which are of highest exploratory interest. A Digital

Elevation Model (DEM),

by either stereoscopic imaging, laser or microwave altimetry, is

required in order to assess the safest landing site conditions. If the mission is required to include the search for particular chemical species which are only rarely found on the surface (e.g. water ice), then remote sensing from orbit is needed to pinpoint the location of such species (e.g. neutron detection for the search of hydrogen). Otherwise, the probability of finding them within the limited surface mobility range of the LEDA mission, expected to be of a few tens of km, is unacceptably low. The recently announced NASA Lztmr Prospecfor mission. planned for a 1997 launch, may provide Moon geochemical information crucial for the LEDA mission planning. If no other orbital missions to the Moon can be flown to obtain the necessary high-resolution mapping beforehand, LEDA will be required to carry out its own remote sensing. This will have to be limited to the latitude of specific interest, since a global mapping would require a major re-design of the spacecraft (i.e. Moon-pointing, instead of Sun-pointing attitude). The 83”-85” latitude band is currently considered as a reference, this being the lowest at which access to permanently shadowed areas and to Aitken basin features appears possible. The landing site would be in any case as close as possible to 0” longitude (nearside) for best Earth visibility conditions. Should the South Pole landing site turn out to be too risky from the operational point of view, a less demanding site at latitudes lower than 70” could be chosen. It would allow the validation of technologies and operational capabilities required to perform complex teleoperated or automated robotic tasks. Landing on the farside of the Moon (the regions not visible from Earth) would require a Moon-orbiting data relay satellite and is financially not realistic for Phase 1 of the Moon Programme. The main characteristics and constraints associated with some landing sites are summarised in Table 2. It is therefore clear that, in order to cope with such a challenging landing site as the South Pole region, a thorough assessment of the required technologies for survival and operations is a necessary condition. before committing to the mission. Whether LEDA will be able to tackle the hurdles posed by the prime landing area near the South Pole, or a safer approach should be taken in a first mission, is a central issue of the current investigations. PAY LOAD The need to face a large velocity increment to land on the surface of the Moon implies that the mass available for the useful payload (scientific, exploratory, or technology demonstration) is limited, and therefore choices have to be made, and maximum performance of the embarked hardware is required. The current Model Payload has a total

1633

TheLEDA MissiontotheMoon

target mass of 200 kg.

Table 2. Comparison

of lunar landing site characteristics

Thermal Environment

Earth Visibility

Polar Regions (SO”-90” N/S)

-230”/-40” C

HighLatitude Regions (70”-80” N/S)

(Energy)

Landing Opportunities

Exploration Opportunities

Intermittent (50% 100% of the time)

Very low elevation (affected by topography)

14/28-day intervals 4 months per year

Geology

- 160”/+60” c

Direct

Low elevation

14/28-day intervals from polar orbit

Geology Volcanism Environment

Mapping (Clementine)

Equatorial Regions (O”-IO” N/S)

-160°/+1300 C

Direct

Near zenith

Geology Environment Historical

APOLLO

(no shadows)

Continuous from equatorial orbit

Limb/Farside (>80” E/W)

Function latitude

Function of latitude

Function of latitude

Geology Environment Access to EM-quiet cone

Function of latitude

of

Intermittent/ Indirect

Sunlight

Environment Water ice

Required Data Base Mapping Thermal Chemical

A rover system, with a mass in the order of 150 kg, is regarded as the primary LEDA payload, essentially because: reach capability to a few tens of km from a safe landing site is necessary, in order to access more difficult but interesting terrain surface mobility is one of the most important technology demonstrations to be achieved in preparation of subsequent phases of a Moon programme. The main mission requirements for the rover system include exploration, in situ measurement support, technology demonstration, and information and education. The instruments carried by the rover have been defined as a reference for the study, and a typical composition is shown in Table 3. Beside the rover system, and according to the available payload mass. additional hardware may be installed on the lander itself and operated in a stationary mode. Candidates include: Instruments for geochemistry, environmental measurements and imaging. Some of them could be back-ups to those carried on the rover, if mass allocation is available. A robotic manipulator, which may be used in support of other payload items (e.g. pick-up/deploy tasks, soil sample feeding), but which would primarily be flown in order to demonstrate the technology for further missions, in such tasks as structure assembly. The manipulator would operate a drilling unit, able to reach down to 0.2 m under the surface, for thermal probe installation. A soil processing test facility, aimed at evaluating the critical technologies (e.g. melt handling in hypogravity) needed for such future applications as oxygen production from lunar soil. The application of virtual reality and telepresence to Moon surface payload operations is important for exploration, in-situ measurement, technology demonstration, information coverage. It provides a powerful user interface to operations of rover, robot, scientific instruments. It offers a “sensory” feedback (images of environment and

1634

M. Novan mdD. Kassjng

spacecraft, rover and robot states, instrument measurements), on-line or off-line (stored). It grants an interaction capability (commanding at various levels), on-line (immediate execution) or off-line (planning in simulated environment, validation, later execution). In terms of public information and education, such telepresence involvement can be offered on a worldwide scale (from scientists and experts to students and general public).

Table 3. Potential complement of rover-mounted instruments Rover-Mounted

Instruments

Mass

Geochemical

GPR

Ground Penetrating Radar

TAP

Thermal Array Probes (2x)

CPM

Complex Permittivity Meter

APX

Alpha-Proton-X-ray

Analysis

Imaging

Environmental Measurements

Data Rate

Electronics

SensoJ Unit Geophysical Analysis

Power

5.0 0.15

10.0 0.2

I/S

1.0

0.05 Is

3.0

1.S

O.O16/min

0.25

0.3

32 / sample

Spectrometer

GRS

Gamma-Ray Spectrometer

0.5

0.4

I .o

8 / sample

NED

Neutron Detector

0. I

0.2

0.2

0.032 / sample

PCS

Panoramic Camera System

0.75

0.75

4.0

5 12/ image

GUI

Close-up lmager

0. I

0.2

4.0

512/image

EGA

Evolved Gas Analyze1

0.5

0.2

8.0

1000 / sample

Radiation Dose Monitors

0.2

0.25

2.0

0.4 / meas.

RDM

(2x) Total


The constraints to be faced are essentially: the communication time delay (I 0 s), which requires interaction at sufficiently high level, and implementation of sufficient on-board autonomy, with no “immersive” displays for the user -

limited telemetry bandwidth (300 kb/s): only critical for image data, requiring on-board data compression, low

-

refresh rate (but compatible with dynamics of scene and level of interaction!) rover operational limitations (power, control): very low speeds (cl km/h), long pauses may cause boredom and fatigue of the operator.

Possible user interfaces to the payload in a telepresence mode include: “Augmented Reality” (virtual image of predicted rover position in real video image) for teleoperation fully simulated scenes (virtual rover in virtual world, alternative view points, widgets) for planning, supervision of autonomous execution off-line replays of (condensed) real data in virtual environment (for analysis, planning, training, validation of new algorithms, simulation of fUtLJJeoperations, research, teaching, entertainment, etc.) “virtual dome” from real images for users to move around and explore (“hot spots” for interaction). A long-ranging result of telepresence applications is the possibility of winning and maintaining public support, stimulating multi-disciplinary participation in lunar exploration and utilisation (exhibits, contests). Media products could be eventually derived from the development and operation tools and scientific data made available by a lander/rover mission, e.g. “Moon Rover rides” (like flight simulator, static or mobile) for education (schools,

The LEDA Miuion

museums)

to the Moon

and entertainment.

CONCLUSIONS The LEDA Assessment Study has shown that it is within the capabilities of ESA, together with other European agencies, to undertake a lander mission to the Moon e&y in the 21 st Century, and to demonstrate safe and effective landing and surface operations. A number of mission options and of critical technologies, for which either a development activity in Europe, or the procurement from a foreign source is demanded (e.g. radioisotope systems), have not been finalised at this stage. The relevant information on the various options and technologies provided by the LEDA study allows the management of ESA and of the National Agencies to generate programmatic directives and to initiate the necessary agreements on cooperative undertakings.

ACRONYMS APX ASI CNES CPM GUI DEM DLR EGA EM ESA GNC GPR GRS GTO HG LEDA LLO MECU NASA MMU NED PCS RDM TAP

Alpha-Proton-X-ray Spectrometer Agenzia Spaziale ltaliana Centre National d’Etudes Spatiales Complex Permittivity Meter Close-up Imager Digital Elevation Model Deutsche Forschungsanstalt fir Luft- und Raumfahrt Evolved Gas Analyzer Electra-Magnetic European Space Agency Guidance, Navigation and Control Ground Penetrating Radar Gamma-Ray Spectrometer Geostationary Transfer Orbit High Gain Lunar European Demonstration Approach Low Lunar Orbit Million of European Currency Units National Air and Space Administration Mass Memory Unit Neutron Detector Panoramic Camera System Radiation Dose Monitor Radioisotope Heating Unit Thermal Array Probe

REFERENCES Balsiger, H., M. C. E. Huber, P. Lena, and B. Battrick, International Lunar Workshop - Towards a World Straregy for the Exploration and Utilisation of Our Natural Satellite, ESA-SP- 1170 (1994). Battrick, B., and C. Barron, Mission to the Moon - Europe ‘s Priorities jbr the Scientr$c Exploration and Utilisation of the Moon, ESA-SP- 1150 (1992). Foing, B. H., Astronomy and Space Sciencefrom Station Moon, Journal of The British Interplanetary Society, Vol. 48, pp. 67-70 (1995).