(Part of this section was published in the JBIS by Ian Crawford; subsequent modifications and adaptations (and any mistakes) by Michel Lamontagne)

Introduction

An interstellar space vehicle capable of making in situ scientific investigations of a nearby star and accompanying planetary system requires a large array of instruments. This section presents an evaluation of the range of probes and scientific instruments that will be required in order to fulfill its scientific mission. It is important to realize that at this stage in the project, a century before launch any such analysis can only be very preliminary, as decisions regarding the actual complement of scientific instruments carried by an interstellar mission will depend on the following:

(i) The available mass, power, and communications bandwidth budgets; and

(ii) The architecture of the target star and planetary system (especially the number and type of planets present, including any observations of possible biosignatures that may have been made by solar system-based astronomical observations).

The mass budget for scientific instruments and probes referred to in the Icarus Interstellar requirements is 150 tonnes, including about 50 tonnes are reserved for a communication system providing 20 Gbps of bandwidth back to Earth and 3 Gbps towards subprobes, as described in the communications section. The target system is still unknown. A plausible planetary system for an Alpha Centauri mission will serve as a design basis for the exploration target.

Top-level scientific objectives

As discussed by Crawford [1] and Crawford et al. [2], and building on preliminary work by Webb [3] in the context of the Daedalus study, the science objectives of an interstellar probe include:

• Science to be conducted on route: e.g. of the interstellar medium (ISM) encountered en route to the target star, and any number of physical and astrophysical studies which can make use of the vehicle as an observing platform;

• Astrophysical studies of the target stars for the Alpha Centauri system;

• Planetary science studies of any planets in the target system, including moons and large asteroids of interest; and

• Astrobiological/exobiological studies of any habitable (or inhabited) planets which may be found in the target planetary system.

• Flyby study of Proxima Centauri, as defined in the Alpha Centauri Mission chapter.

These broad science areas may themselves be sub-divided into a number of different areas of investigation. These are summarised in Table 1 (taken from Ref. [2]). The remaining sections outline the kinds, and approximate numbers, of sub-probes and accompanying instruments that would be required to address these different scientific questions.

Table 1 - List of scientific objectives

Scientific Areas of Investigation

Outer solar system studies

Heliosphere, Kuiper belt and Oort Cloud

Local interstellar medium

Structure, density, temperature, composition, mag. fields, etc

Astronomical studies

Parallax; other?

Fundamental physics

Gravitational waves, gravity, dark matter, etc

Outer environment of target star system

Astrosphere, dust disk, comets, etc

Target star astrophysics

• Mass, composition, temperature, mag. fields

• Photospheric and coronal activity; long-tem monitoring

• Stellar wind/corona composition

Terrestrial planets

• Mass, density, mag. fields

• Atmospheric composition and structure

• Surface geology

• Internal structure (geophysics)

Giant planets

• Mass, density, magnetic fields

• Atmospheric composition and structure

Asteroids/small bodies

• Numbers, mass, density, composition

• In situ dating of primitive meteorites *

• In situ ressources identification

Astrobiology

Identification of habitable environments in target system

• Search for biomarkers **

• Detection of planetary surfaces

• Life detection below planetary surfaces (incl. oceans)

• Search for evidence of extinct life

• Biochemical characterization of extant life forms ***

• Search for evidence of technological artifacts in target system

* Provides an independent age measurement of the target system.

** Confirmation of biomarkers identified from Earth (if any).

*** Biochemistry, metabolism(s), cellular structure, diversity, evolutionary relationships, etc, of any life forms detected.

Science probes and instruments

Science conducted en route

The main scientific investigations to be conducted en route will be studies of the interstellar medium (ISM) between the Sun and the target star system. The sizes, physical properties, and locations of the low-density cloudlets known to be present in the vicinity of the Sun, and the nature of the lower density inter-cloudlet medium, are of particular interest [5]. Key instruments for these studies would be:

• A dust analyser (to determine both dust masses and compositions)

• Gas analyser(s) (to measure densities, ionisation states, and composition of the gas phase interstellar medium – probably a suite of several instruments in practice)

• Magnetometer(s) and plasma wave instruments

• High-energy (cosmic ray) detectors

These instruments would be similar to those routinely flown on outer Solar System missions (e.g. Cassini; [6]) and proposed for interstellar precursor missions (e.g. [7]). They could be mounted on the main vehicle, but booms (or free-flying sub-probes) may be desirable to insulate them as much as possible from electromagnetic and other interference from the vehicle. Thought will also have to be given to protection from interstellar dust impacts.

The instruments required to study the structure of the interstellar medium should have the highest priority for cruise-phase science. If the probe is also to be used as a platform for detecting ‘exotic’ (e.g. dark matter) particles then additional instrumentation may be required (the Alpha Magnetic Spectrometer [8], currently mounted on the ISS, could provide an example, although in practice an innovative new instrument would probably need to be designed). If the starship was to be used as a platform for optical or radio parallax studies, appropriate instrumentation (i.e. optical or radio telescopes) needs to be defined and a allowance made in the appropriate mass budget.

Table 1 also identifies possible studies of the Solar System’s heliosphere, Kuiper Belt and Oort Cloud as possible targets for investigation. These three areas of investigation are rather different. The instrumentation described above for the study of the ISM will also be sufficient for heliospheric studies, and indeed the boundary between the heliosphere and the local ISM is of compelling scientific interest [5]. It is not considered here that Kuiper belt objects (KBO) should be a high priority for an interstellar explorer, partly because precursor missions are likely to have made in situ observations of these objects, and partly because refining the trajectory to pass close a KBO may overly complicate the flight profile. The Oort cloud is so thinly populated that it is unlikely that an interstellar probe will pass close enough to an Oort cloud comet to make in situ observations, and in any case the vehicle will probably be travelling at or close to its cruise velocity, making such observations difficult (and potentially dangerous).

Studies of target stars(s)

Studies of the target star(s) will be of compelling astrophysical interest, but it must be noted that for these very nearby stars a lot can, and will, be learned from astronomical observations made from the Solar System. In situ studies should be designed to complement these studies, and obtain information that cannot be obtained astronomically (e.g. in situ measurements of stellar wind and magnetic field, high-spatial resolution of stellar photospheres, active regions and coronae, and ultra-high precision measurements of mass, radii and luminosity.

The choice of suitable instruments should be guided by instruments on existing solar missions like Ulysses, SOHO and STEREO [9, 10, 11], but should probably include:

• Magnetometers

• Charged particle (stellar wind) detectors

• X-Ray, UV and visible imaging systems

• An instrument to measure total stellar luminosity to high precision (essentially the stellar ‘solar constant’) and any variability therein.

Most of these instruments could be mounted on the main spacecraft bus rather than on sub-probes, and some of them (excluding the imaging instruments) could probably be the same as those used during the cruise phase to study the ISM. That said, STEREO [11] has demonstrated the value of making simultaneous observations of solar (here stellar) activity from different angles, so at least one sub-probe dedicated to stellar observations in addition to instruments on the main bus would be desirable (and at least two such stellar-physics sub-probes of a binary star such as alpha Centauri is the target).

Planetary science

The requirements for planetary probes will depend on the number and types of planets present in the target system. It is anticipated that, at least for planets of Moon-size and larger, by the time an interstellar probe is launched this information will have been provided by Solar System-based astronomical observations. In what follows, we identify some generic instrumentation for different modes of planetary science investigations (although closely related, specifically astrobiology-related investigations are discussed in Section 3.4).

Planetary orbiters

Planetary orbiter shown deploying an atmospheric lander and in close up. The small antenna will provide communication with the landers while the large antenna provides high bandwidth communication back to the probe carrier or the main starship itself. Three landers are shown, holding within their fairing any number of configuration of instruments, including possibly small dirigibles or drone like flying instrument platforms. A small utility drone is seen as well, able to carry out maintenance on the vehicle to increase its operating lifetime.

Planetary orbiters provide an efficient means of mapping planets, determining surface and atmospheric composition, and making top-level inferences about their geological evolution. In general planetary mapping orbiters should be placed in polar orbits to maximize surface coverage, and this will either require each planet’s polar axis to be identified before orbital insertion, or each orbiter to carry sufficient fuel to effect orbital plane changes. The instrumentation required on planetary orbiters will depend somewhat on the nature of the planet (e.g. gas giant or rocky planet), but orbiters designed for giant planets should also be able to explore orbiting moons (as for the Galileo and Cassini missions). Instrumentation will also depend on the presence or absence of an atmosphere, and the extent to which a planet may be totally cloud covered (like Venus and Titan). These may be difficult to determine in advance, so a flexible instrument suite would be desirable.

Typical instruments for planetary orbiters would include:

• High-resolution optical imaging system

• High-resolution imaging UV-VIS-IR mapping spectrometer(s)

• X-Ray fluorescence or gamma-ray spectrometer (airless bodies only)

• Laser altimeter (airless bodies or planets with transparent atmospheres)

• Synthetic aperture radar (primarily for mapping cloud covered planets)

• Magnetometer

Planetary atmospheric probes (giant planets)

Here the principal interest will be in making in situ measurements of the chemical composition of the atmosphere. A good model would be the Galileo entry probe [12], although if long-term monitoring were considered desirable balloons or aircraft might be used. Principle instruments would be:

• Mass spectrometer(s)

• Nephelometer

• Thermometers, barometers, etc

Line-of-sight communication with an orbiter, or with the main vehicle, will be required for data transfer. Doppler tracking of the signal will permit measurements of wind speed at different depths.

Planetary atmospheric probes (rocky/icy planets)

To a first approximation the kinds of instruments required to probe the atmospheres of solid planets are the same as required for giant planets (see above), but in this case balloons and/or aircraft would appear to be especially desirable. This is because, in addition to measuring atmospheric parameters, they could assist in surface exploration (e.g. by obtaining high-resolution images and spectra data of the surface, something which would be especially important if the surface was obscured from orbital investigation by clouds or hazes). In this case, in addition to the instruments designed to characterize the atmosphere (listed above) additional instruments for surface observations could include:

• High-resolution multi-spectral imaging system

• UV-VIS-IR mapping spectrometer(s)

Planetary probes (rocky/icy planets – penetrators)

From a planetary science point of view, much of the information we would like to obtain about solid planets will require in situ measurements made by contact instruments at the surface. Obtaining some of this information will require sophisticated instruments to be soft-landed on the planetary surface, and many will require rover-facilitated mobility (as discussed below). However, a lot of valuable top-level geophysical and geochemical information can be obtained by suitably instrumented penetrators, which would be dropped from orbit and embed themselves a few metres below the surface. Penetrators are likely to be especially efficient at emplacing network geophysical instruments (e.g. seismometers and heat-flow probes), as several penetrators could be targeted at each planet of interest. This mode of deploying instruments will be most effective on airless bodies, but might be adapted for planets with atmospheres as well.

Examples of suitable penetrator studies in clued the MoonLITE [13] and LunarEX [14] concepts proposed for lunar exploration, and a similar concept proposed for Europa [15]. As demonstrated in these studies, examples of the kind of scientific instruments which could be efficiently deployed using penetrators include:

• Seismometers

• Heat-flow-probes

• In situ geochemical sensors (e.g. mass spectrometers and X-ray fluorescence spectrometers)

At least intermittent line-of-sight communication with an orbiting satellite will be required for data downlink from penetrators.

Planetary probes (rocky/icy planets – soft landers/rovers)

Some important planetary science investigations, especially those relating to geology and astrobiology, will require larger and more complex instrumentation than can plausibly be emplaced by penetrators. Moreover, many of these investigations would benefit from mobility which implies the ability to land rovers on planetary surfaces. A good state-of-the-art rover, that is equipped with appropriate instrumentation, is NASA’s Mars Science Laboratory [16, 17].

Examples of the kinds of instruments required for surface geological and environmental investigations include:

• Multi-spectral imaging system

• Weather station (e.g. temperature, humidity, wind speed, etc)

• Rock abrasion tool and/or rock splitter

• High-resolution close-up (‘hand lens’) imager

• In situ geochemical instruments (e.g. XRF spectrometer and/or Raman-LIBS)

• Mass spectrometer(s) [Note that different mass spectrometers may be required for geochemical and astrobiological investigations, as the former will mostly be concerned with trace element concentrations in rock samples, whereas the latter will be concerned with identifying complex organic molecules; we return to the latter in Section 3.4]

• In situ rock dating capability (using some combination of the above)

• Drill (for sub-surface sampling; depth TBD, but ideally several metres)

• Geophysical package (e.g. seismometer, heat-flow probe).

As was the case for penetrators, at least intermittent line-of-sight communication with an orbiting satellite will be required for data downlink from surface landers.

Cubesats

Over 1000 1.3kg cubesats may be carried by various sub probes for an array of information gathering tasks. This technology should be mature by the time the interstellar probe flies, and miniature instruments should be able to complement the work done by the larger probes.

Astrobiology

Requirements for astrobiological investigations are closely related to those of planetary science. In particular they will likely require the soft-landing of rover-facilitated mobile instruments. Presumably dedicated astrobiology instruments would only be targeted for those planets identified in advance as being good candidates for habitability (e.g. possessing liquid water and a life-friendly atmospheric composition), or for which atmospheric biomarkers have already been detected spectroscopically (either from Earth or from previously deployed orbiting spacecraft). In the event that biosignatures have been detected from Earth prior to the launch of the interstellar probe it is likely that this would dominate the entire scientific investigation of the target system, and that the scientific payload would be tailored to its further investigation (possibly at the expense of some of the other scientific objectives outlined above). Even in the absence of the prior detection of actual biosignatures, it is likely that astronomical observations from the Solar System will have identified potentially habitable planets in the target system, if such exist, and that this information will also inform the particular choice of astrobiology experiments to be included in the payload.

However, in the absence of such prior information, we here outline generic astrobiology investigations that might be suitable for the investigation of potentially habitable planets in the target system. Good models for astrobiological instrument suites include the Viking biology package [18] (although it would be possible to design a more sophisticated package today, and ideally more tailor-made to the specific target environment to the extent that this can be known in advance), the Pheonix Lander high-resolution microscope [19], and the SAM [20] and Urey [21] instruments designed for Mars Science Laboratory and ExoMars, respectively.

Appropriate instruments would include items (1)-(8) identified in Section 3.3.5 above as being required for planetary science investigations, but with the addition of the following:

• High-resolution microscope

• Mass spectrometer(s) for detection and characterization of complex organic molecules and carbon (and perhaps other element) isotope ratios

• Measurements of temperature, pH and redox state (ORP) of liquid water (or any other liquids) found in the vicinity of the landing site

• ‘Wet’ biology experiments to identify and characterize active metabolism and metabolic products (e.g. loosely based on the Viking biology package, but with updated (and ideally specially tailored) experimental protocols)

• Some kind of metagenomic analysis might be desirable, but probably impossible without precise knowledge of indigenous genetic processes.

If the target planet is wholly or partially covered in liquid water then, given the importance of liquid water to biology as we understand it, it will be desirable to deploy some of these instruments on a mobile floating platform (i.e. a boat) and to obtain and analyse water samples. In such a situation, a water (liquid) sampler would replace the drill specified earlier.

Number of probes required and implications for payload mass

The number of probes required to conduct a thorough investigation of the target planetary system will depend on number and types of planets present. In practice this is likely to be determined before the launch of the interstellar mission by astronomical observations made from the Solar System. Such observations will also inform the relative balance between giant planet and rock planet investigations, and the weighting given to astrobiological investigations.

Using the hypothetical solar system described in chapter 2 section 5, we can determine the following number of probes:

• Stellar orbiters: 2-4

• Planetary orbiters: 6-8 (say one per planet; may need more)

• Giant planet entry probes/balloons: 2-4 (at least one per giant planet)

• Rocky planet atmospheric vehicles/balloons: 6-8 (ideally at least two for each rocky planet with an atmosphere)

• Planetary soft landers: 6-8 (ideally at least two for each rocky planet; at least half should be equipped for astrobiological investigations and at least two designed to function on water)

• Low mass penetrators for simple in situ geochemical and geophysical studies of airless planets, moons and asteroids: 30-50

• 1000+Cubesats, to be used for a wide array of tasks and functions.

We still have to add the mass of instruments required for cruise phase science discussed in Section 3.1. The basic measurements of fields and particles are similar to those already made by outer solar system spacecraft such as Cassini, so allowing an additional 2.0 tonnes (i.e. approximately the total mass of the Cassini Orbiter) for these instruments would appear to be conservative. However, if it is desired to make use of the vehicle as a platform for more exotic studies during the cruise phase (e.g. dark matter searches or astronomical observations) additional mass must be allowed for these instruments. As a baseline we here add another 8 tonnes for these unspecified instruments (this may also be conservative, but note that the Magnetic Spectrometer on the ISS, which might act as a proxy for an advanced exotic particle detector, has a mass of 6.7 tonnes [8]).

The following tables summarizes these probes and instruments, as determined for the Firefly vehicle in the Icarus Interstalla project:

It is possible to arrive at an evaluation of the mass of the scientific payload using analogy rather than the mass breakdown shown in table 2. Taking SOHO (mass 1.85 tonnes [22]) as a model for a stellar orbiter; Cassini (mass 2.125 tonnes [6]) as a high-performance planetary orbiter; the Galileo Entry Probe (mass 0.339 tonnes [12]) as a typical planetary atmospheric probe); Mars Science Laboratory (mass 0.90 tonnes [17]) as a high performance planetary rover with astrobiology capability); and the LunarEX penetrators (mass 0.040 tonnes, including descent modules [14]) as typical of this type of delivery system, we arrive at a total scientific payload mass in the range of 26 to 38 tonnes for the lower and upper limits for the number of probes given above. Adding 17 tonnes for the probe carriers, and 38 tonnes of propellant, this brings the mass to about 105 tonnes, and the match with the values in table 2 is good.

Subprobes propellant

Using a deltaV of 25 km/s for the probe carriers and 5 km/s average deltaV for the sub probes themselves, 36 tonnes of propellant would be required for the mission.

An alternative to carrying propellant for the sub probes all the way from the Solar System to Alpha Centauri would be to carry propellant production units. These would be designed to extract propellant from a suitable small asteroid or cometary body using in-situ production methods. However, finding a suitable propellant source might be difficult and moving in too close to a larger planetary body looking for usable moons might use more fuel than it saves.

Proxima flyby secondary mission

A Proxima Centauri flyby would resemble the original Daedalus mission quite closely. The encounter phase will only last a few hours. A subset of instruments, as well as a number of sub probes deployed before the encounter phase will be required. A mass budget of about 30 tonnes can be allocated for this task. Taking Firefly as a reference, the mass of the main vehicle, including the deceleration propellant, is about 7000 tonnes when the Proxima vehicle separates from the main mission. Putting the 30 tonnes of the scientific instruments at about 0,4% of the mass of the vehicle at this point. This is basically less than the margin of error of the calculations for the design, and we can therefore exclude the mass of these instruments from the 150 tonnes target mass for the payload. The flyby mission is truly a freeby for a alpha Centauri mission project.

Conclusions and recommendations

• We have presented some considerations on the selection of probes and instruments that would be required to make a scientifically useful exploration of a planetary system orbiting a nearby star. In practice, the scientific payload will be tailored to both the capabilities of the launch vehicle and the architecture of the particular planetary system to be explored. Information on the latter is likely to be provided by astronomical observations from the Solar System long before an interstellar probe is constructed. Nevertheless, it is possible to consider a generic scientific payload able to address the top-level science requirements identified in Ref. [2]. This has led to the following conclusions:

• Based on existing spacecraft and instruments, the dry mass of probes required to perform a minimally useful exploration of a target system containing 1-2 stars and 6-8 planets (such as may be appropriate for number of nearby star systems, including alpha Centauri A/B [24]) will probably be in the range of 26-38 tonnes. While this might be reduced by improved technology (especially the likely miniaturization of instruments over the coming century), it seems best to use this figure in order to be conservative (and any mass savings made as a result of improved technology would be better used to increase the range of science that could be performed rather than reducing the scientific payload mass).

• Allowance must be made for transporting the scientific probes to the locations in orbit about, or on the surfaces of, planets to be investigated. This will require propulsion systems and fuel requirements that would increase the total mass required for science probes by a factor of 2.5, bringing it into the range 65-95 tonnes. Using in-situ fuel production might allow the use of most of the propellant mass budget for additional probes and instruments.

• Adding an estimated 10 tonnes for cruise phase science brings the total to 75-105 tonnes. 50 tonnes of communication equipment rounds this up to the target of 150 tonnes. The instruments for a secondary flyby mission to Proxima Centauri might mass up to 30 tonnes, a tiny number compared to the mass of the starship.

References

[1] Crawford, I.A., "The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight", JBIS, 62, 415-421, (2009).

[2] Crawford, I.A., Long, K., Obousy, R., Swinney, R., Tziolas, A.C., "Project Icarus: Scientific Objectives", Project Icarus Internal Report, (2011).

[3] Webb, G.M., "Project Daedalus: Some Principles for the Design of a Payload for a Stellar Flyby Mission", Project Daedalus: Final Report, JBIS, S149-S161, (1978).

[4] Crawford, I.A., ''Project Icarus: A Review of Local Interstellar Medium Properties of Relevance for Space Missions to the Nearest Stars'', Acta Astronautica, 68, 691-699, (2011). [http://arxiv.org/abs/1010.4823].

[5] Crawford, I.A., ''Project Icarus: A Review of Local Interstellar Medium Properties of Relevance for Space Missions to the Nearest Stars'', Acta Astronautica, 68, 691-699, (2011).

[6] http://www.esa.int/esaMI/Cassini-Huygens/SEMY182VQUD_0.html

[7] McNutt, R.L., Andrews, G.B., McAdams, J., Gold, R.E., Santo, A., Oursler, D., Heeres, K., Fraeman, M. and Williams, B., ‘Low Cost Interstellar Probe’, Acta Astronautica, 52, 267-279, (2003).

[8] http://ams.nasa.gov/

[9] http://sci.esa.int/science-e/www/area/index.cfm?fareaid=11

[10] Fleck, B., Müller, D., Haugan, S., Sánchez Duarte, L., Siili, T. and Gurman, J. B., ‘10 years of SOHO’, ESA Bull., 126, 24-32, (2006).

[11] Harrison, R.A., Davies, J.A., Rouillard, A.P., Davis, C.J., Eyles, C.J.;, Bewsher, D., Crothers, S.R., Howard, R.A., Sheeley, N.R., Vourlidas, A., Webb, D.F., Brown, D.S. and Dorrian, G.D., ‘Two Years of the STEREO Heliospheric Imagers: Invited Review’, Solar Phys., 256, 219-237, (2009).

[12] http://en.wikipedia.org/wiki/Galileo_(spacecraft)#Galileo.27s_atmospheric_entry_probe

[13] Crawford, I.A. and Smith, A., ''MoonLITE: A UK-Led Mission to the Moon'', Astronomy and Geophysics, 49, 3.11-3.14, (2008).

[14] Smith, A., et al., "LunarEX: A Propoasal to Cosmic Vision," Experimental Astronomy, 23, 711-740, (2009).

[15] Gowen, R. et al., "Penetrators for in situ sub-surface investigations of Europa," Adv. Space Res., 48, 725-742, (2011).

[16] http://msl-scicorner.jpl.nasa.gov/

[17] http://en.wikipedia.org/wiki/Mars_Science_Laboratory

[18] Brown, F.S., Adelson, H.E., Chapman, M. C., Clausen, O.W., Cole, A.J., Cragin, J.T., Day, R.J., Debenham, C.H., Fortney, R.E. and Gilje, R.I, ‘The Biology Instrument for the Viking Mars Mission’, Rev. Sci. Instruments, 49, 139-182, (1978).

[19] Staufer, U., Parrat, D., Gautsch, S., Pike, W.T., Marshall, J., Blaney, D., Mogensen, C. T. and Hecht, M, ‘The PHOENIX Microscopy Experiments’, Fourth International Conference on Mars Polar Science and Exploration, Davos, Switzerland, abstract No. 8097, (2006).

[20] http://msl-scicorner.jpl.nasa.gov/Instruments/SAM/

[21] Aubrey, A.D., Chalmers, J.H., Bada, J.L., Grunthaner, F.J., Amashukeli, X., Willis, P., Skelley, A.M., Mathies, R.A., Quinn, R.C., Zent, A.P., Ehrenfreund, P., Amundson, R., Glavin, D. P.; Botta, O., Barron, L., Blaney, D.L., Clark, B.C., Coleman, M., Hofmann, B.A., Josset, J.-L., et al., ‘The Urey Instrument: An Advanced In Situ Organic and Oxidant Detector for Mars Exploration’, Astrobiology, 8, 583-595, (2008).

[22] http://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory

[23] Crawford, I.A., ''Project Icarus: Astronomical Considerations Relating to the Choice of Target Star'', J. Brit. Interplanet. Soc., 63, 419-425, (2010).

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