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Astronomical Considerations Relating to the Choice of Target Star Project Icarus: Astronomical Considerations RelangJBIS, to the Vol. Choice 63, pp. of 419-425Target Star, 2010 PROJECT ICARUS: ASTRONOMICAL CONSIDERATIONS RELATING TO THE CHOICE OF TARGET STAR I.A. CRAWFORD Department of Earth and Planetary Sciences, Birkbeck College London, Malet Street, London, WC1E 7HX, UK. In this paper we outline the considerations required in order to select a target star system for the Icarus interstellar mission. It is considered that the maximum likely range for the Icarus vehicle will be 15 light-years, and a list is provided of all known stars within this distance range. As the scientific objectives of Icarus are weighted towards planetary science and astrobiology, a final choice of target star(s) cannot be made until we have a clearer understanding of the prevalence of planetary systems within 15 light-years of the Sun, and we summarize what is currently known regarding planetary systems within this volume. We stress that by the time an interstellar mission such as Icarus is actually undertaken, astronomical observations from the solar system will have provided this information. Finally, given the high proportion of multiple star systems within 15 light- years (including the closest of all stars to the Sun in the α Centauri system), we stress that a flexible mission architecture, able to visit stars and accompanying planets within multiple systems, is desirable. This paper is a submission of the Project Icarus Study Group. Keywords: Interstellar travel; nearby stars; extrasolar planets; astrobiology 1. INTRODUCTION The Icarus study is tasked with designing an interstellar space 2. STARS WITHIN 15 LIGHT-YEARS OF THE SUN vehicle capable of making in situ scientific investigations of a nearby star and accompanying planetary system [1, 2]. This Within 15 light-years of the Sun there are approximately 56 paper outlines the considerations which will feed into the choice stars, in 38 separate stellar systems. The number is approximate of the target star, the choice of which will be constrained by a for several reasons. Firstly, at the outer boundary the errors on number of factors. Foremost among these are the design crite- the distances can amount to a few tenths of a light-year, which ria specified by the Project’s Terms of Reference (ToR) docu- could mean that some stars notionally just beyond 15 light- ment [1]. The most relevant ToR relating to the selection of the years might actually be closer (and vice versa). Secondly, not target star(s) are: all stars within this volume may yet have been discovered, although this is only likely for the very dimmest red or brown ToR#3: “The spacecraft must reach its stellar destination dwarfs. Thirdly, perhaps surprisingly, there are still slight dis- within as fast a time as possible, not exceeding a century crepancies between the catalogues of nearby stars. Probably and ideally much sooner”; and the most authoritative recent compilation, and the one on which ToR#5: “The spacecraft propulsion must be mainly fusion the number of 56 stars is based, is the RECONS (Research based” Consortium on Nearby Stars) list of the one hundred nearest star systems [3]. Taken together, these imply a maximum realistic range of about 15 light-years from the Solar System. This would imply The known stars within 15 light-years are listed in Table 1. an interstellar cruise velocity of 15% of the speed of light (i.e. Of these 56 stars, there is one star of spectral type A (Sirius); 0.15c), which is probably close to the upper end of what is one F star (Procyon); 2 G stars (α Centauri A and τ Ceti); five K feasible with a fusion-based propulsion system extrapolated stars; 41 M stars (red dwarfs); 3 white dwarfs; and three prob- from current knowledge. Moreover, given that ToR#3 adds the able brown dwarfs (the latter all members of multiple systems – mission should ‘ideally’ be completed in less than 100 years, it there are no currently known free-floating brown dwarfs within follows that the ideal target would actually be significantly this volume; these would be difficult to detect but in principle closer than 15 light-years. could exist). For the purposes of this study we will therefore assume a 3. SCIENTIFIC CRITERIA FOR maximum range of 15 light-years. ToR #4 states that “the THE CHOICE OF TARGET STAR spacecraft must be designed to allow for a variety of target stars”, from which it follows that the mission cannot be predi- The scientific objectives of an interstellar vehicle such as Icarus cated on one particular target. Thus all stars within the maxi- have been described by Webb [5] and Crawford [6], and for mum range need to be assessed as possible targets, although the Icarus specifically as a trade study conducted within Icarus wording of ToR#3 implies that, scientific considerations being Module 14 [7]. Briefly, these scientific objectives can be di- equal, closer stars would be preferred. vided into the following broad categories: 1 I.A. Crawford TABLE 1: List of Star Systems Within 15 Light-Years of the Sun [3]. GJ is Each Star’s Number in the Gliese-Jahreiß Catalogue [4]; l, b are Each Star’s Galactic Longitude and Latitude, Respectively. Dist. GJ Popular name Spectural Distance lb Order Type (lt-yrs) (deg) (deg) 1 551 Proxima Cen M5.5V 4.2 313.9 -01.9 559 α Cen A G2V 4.4 315.7 -00.7 α Cen B K0V 2 699 Barnard’s Star M4V 6.0 031.0 +14.1 3 406 Wolf 359 M6V 7.8 244.1 +56.1 4 411 Lalande 21185 M2V 8.3 185.1 +65.4 5 244 α CMa A (Sirius) A1V 8.6 227.2 -08.9 α CMa B DA2 6 65 Luyten 726-8 A M5.5V 8.7 175.5 -75.7 Luyten 726-8 B M6V 7 729 Ross 154 M3.5V 9.7 011.3 -10.3 8 905 Ross 248 M5.5V 10.3 110.0 -16.9 9 144 ε Eri K2V 10.5 195.8 -48.1 10 887 Lacaille 9352 M1.5V 10.7 005.1 -66.0 11 447 Ross 128 M4V 10.9 270.1 +59.6 12 866 EZ Aqr A M5V 11.3 047.1 -57.0 EZ Aqr B M? EZ Aqr C M? 13 280 α CMi A(Procyon) F5IV-V 11.4 213.7 +13.0 α CMi B DA 14 820 61 Cyg A K5V 11.4 082.3 -05.8 61 Cyg B K7V 15 725 Struve 2398 A M3V 11.5 089.3 +24.2 Struve 2398 B M3.5V 16 15 (A) GX And M1.5V 11.6 116.7 -18.4 (B) GQ And M3.5V 17 845 ε Ind A K5Ve 11.8 336.2 -48.0 ε Ind B T1 ε Ind C T6 18 1111 DX Can M6.5V 11.8 197.0 +32.4 19 71 τ Cet G8V 11.9 173.1 -73.4 20 1061 M5.5V 12.0 251.9 -52.9 21 54.1 YZ Cet M4.5V 12.1 149.7 -78.8 22 273 Luyten’s Star M3.5V 12.4 212.3 +10.4 23 Teegarden’s Star M7V 12.5 160.3 -37.0 24 SCR1845-6357 A M8.5V 12.6 331.5 -23.5 SCR1845-6357 B T 25 191 Kapteyn’s Star M1.5V 12.8 250.5 -36.0 26 825 AX Mic M0V 12.9 003.9 -44.3 27 860 Kruger 60 A M3V 13.1 104.7 +00.0 Kruger 60 B M4V 28 DEN J1048-3956 M8.5V 13.2 278.7 +17.1 29 234 Ross 614 A M4.5V 13.3 212.9 -06.2 Ross 614 B M8V 30 628 Wolf 1061 M3.0V 13.8 003.4 +23.7 31 35 Van Maanen’s Star DZ7 14.1 121.9 -57.5 2 Project Icarus: Astronomical Considerations Relating to the Choice of Target Star TABLE 1 (continued). Dist. GJ Popular name Spectural Distance lb Order Type (lt-yrs) (deg) (deg) 32 1 M3V 14.2 343.6 -75.9 33 473 Wolf 424 A M5.5V 14.3 288.8 +71.4 Wolf 424 B M7V 34 83.1 TZ Ari M4.5 14.5 147.7 -46.5 35 687 M3V 14.8 098.6 +32.0 36 3622 LHS 292 M6.5V 14.8 261.0 +41.3 37 674 M3V 14.8 343.0 -06.8 38 1245 V1581 Cyg A M5.5V 14.8 078.9 +08.5 V1581 Cyg B M6.0V V1581 Cyg C M? (1)Science to be conducted on route, e.g. of the local a planetary system will greatly increase the scientific priority of a interstellar medium (LISM), and physical and potential target star. This is because there are many aspects of astrophysical studies which could make use of the Icarus planetary science which can only be addressed by in situ measure- vehicle as an observing platform; ments, including the landing of scientific instruments on planetary (2)Astrophysical studies of the target star itself, or stars if a surfaces [6]. In addition to the intrinsic planetary science interest in multiple system is selected; such objects, the habitability of any such planets will be of compel- ling scientific interest. This will be especially true if astronomical (3)Planetary science studies of any planets in the target observations from the Solar System indicate that any may actually system, including moons and large asteroids of interest; be inhabited. In the latter case, definitive proof of the existence of (4)Astrobiological/exobiological studies of any habitable indigenous life, and studies of its underlying biochemistry, cellular (or inhabited) planets or moons which may be found in structure, ecological diversity and evolutionary history will require the target planetary system.
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