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Trojan Binary Asteroid Systems As Future Mission Targets Trans. JSASS Aerospace Tech. Japan Vol. 8, No. ists27, pp. Pk_39-Pk_44, 2010 Original Paper Trojan Binary Asteroid Systems as Future Mission Targets By Julie BELLEROSE1) and Hajime YANO1),2) 1) JAXA Space Exploration Center (JSPEC), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan 2) Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan (Received July 17th, 2009) To date, the Jupiter-Sun Lagrangian points are populated with almost 4500 asteroids, for which their formation and history are still debated. In the current work, we look at rationales for a mission to Jovian Trojan asteroids, and discuss the scientific benefits to investigate binary systems and contact binary systems. We summarized the dynamics for a solar sail mission, which is currently thought to go along the Europa Jupiter System Mission (EJSM), and we show a case study of the contact binary Hektor, and its moon S/2006, which offer the most suitable conditions for spacecraft operations. Trojans asteroids offer many opportunities, and we list some of the targets in time. KeyKey Words Words:: JovianJovian Trojan Trojan Asteroids, asteroids, Binary binary Systems, systems, C contactontact Binary, binary, solarSolar sailSail dynamics, Dynamics, Jupiter Jupiter system System 2 5) Nomenclature to date − . To give a few examples, (617) Patroclus and its companion Menoetius were found to be a low density EKBO : Kuiper Belt Objects asteroid system, while (624) Hektor is a fairly dense system EJSM : Europa Jupiter System Mission 3,4). These two systems have sizes between 140 km and 225 R3BP : Restricted Three Body Problem km, while Hektor is more than 300 km in diameter with a [x,y,z] : Position of S/C in R3BP satellite of 15 km. Mann et al. (2007) 2) have discussed the ΩJup : Orbit rate of Jupiter properties of suspected binary systems located at L4 and L5. µJup : Specific gravity Jupiter The Trojan asteroids formation and evolution are still rJA : Position w.r.t Jupiter debated, as they may be tied to the formation of the giant r : Position w.r.t. the Sun SA planets, or tied to capture of some of the Edgeworth-Kuiper SRP : Solar radiation Pressure [ , , ] Belt Objects (EKBO) in the process of possible migration gx gy gz : Acceleration due SRP 6) 7) [x˜,y˜,z˜] : Position S/C in Hill appr. In their earlier work, Marzani et al. show that small 3 Ω = sqrtµsun/a : Orbit rate of the asteroid Trojans (with diameter < 1 km) are more stable at L5 due ast 8) µsun : Specific gravity Sun to gas drag. However, Gomes et al. have shown that aast : Asteroid distance to Sun planet migration tends to destabilize asteroids at this partic- U˜ : Asteroid potential ular region. Morbidelli has argued that their high inclina- [xˆ,yˆ,zˆ] : Position asteroid orbiter tions could only be explained from captures 9). Hence, it ωast : Asteroid spin rate is not clear if these asteroids were captured from outer re- aSRP : SRP, asteroid frame gions like the Kuiper Belt, or if they are remnants from the time of the giant planets formation. The work from Marzani 1. Introduction et al. 10), in Asteroids III, and more recently, Yano et al. 11) Trojan asteroids are orbiting the Sun in a 1:1 resonance also give a more complete description of these ques- with Jupiter, separated in two distinct regions, L4 and L5. tions and theories. Knowing more about the density, com- At the time of writing this paper, from the Minor Planet position, surface features such as impact history and space Center (MPC) (http://www.cfa.harvard.edu/iau/mpc.html), weathering of these objects can help validating these theo- we count 2798 at L4 and 1747 at L5. However, there are ries, and better complete our knowledge of the solar system only a fraction of them that have been more thoroughly formation. observed and have been given an estimated size. So far, Since Jovian Trojan asteroids are so remote, a solar pow- most of the spectrally observed Trojans have been classi- ered sail mission has now been proposed as part of the fied D-type, with a few being P-type 1). D-type asteroids coming international mission Europa Jupiter System Mis- are believed to be made of primordial, icy, and organic com- sion (EJSM) internationally worked on by NASA, ESA and 10,12) pounds. Among the Trojan asteroid population, sizes vary JAXA . A mission going to the L4 Trojan asteroid 13) from a few kilometers to 100’s of kilometers, and spin rate population would benefit from Jupiter’s gravity assist . may be only a few hours to 100’s of hours. They orbit with Hence, we look especially at suspected contact binary aster- an average inclination of 11 and 15 , respectively for oid systems located in this region and we discuss the benefit ∼ ◦ ∼ ◦ each L4 and L5 population, with some having inclinations of exploring these Jovian Trojan binary systems in more de- reaching 50 . tails. As the mission could integrate solar sail technology, ∼ ◦ Among this asteroid population, a few binary asteroid we give an overview of the basics on the dynamics of Tro- systems and contact binary systems have been discovered jans and on the dynamics of solar sail spacecraft at these Copyright© 2010 by the Japan Society for Aeronautical and Space Sciences1 and ISTS. All rights reserved. Pk_39 Trans. JSASS Aerospace Tech. Japan Vol. 8, No. ists27 (2010) systems. Finally, we look at Hektor as a case study, and a same line of thoughts, looking at differences in processes target opportunities in time. such as space weathering is primordial; the effects may be 2. Jovian Trojan Binary Asteroid System Exploration less in outer regions even if the composition is similar. It may even be different for each of the binary components. By exploring binary asteroid systems at Jovian Trojan lo- By exploring these systems, we can also make a direct cor- cations, we can directly verify their composition, and thus relation with data at 50 AU since, by the time of arrival to verify if they are both from common origins. Their forma- Lagrangian regions, the New Horizons mission to the Pluto- tion may also be tied to captures of EKBO, or may be a Charon system would have returned data from this remote 3,5) product of fission or dynamical friction . Binary may system 14). Having data from New Horizons, links may be also show a direct view of their internal composition if made between the two families of small bodies and minor formed through fission, and, of course, provide two target planets. This will greatly enhance our knowledge of the opportunities at once instead of only one. solar system evolution, on the scale from 1 AU to 50 AU. From current interest in sending a spacecraft to L4, Ta- Since Trojan asteroids are of large size, exploring the 2) ble 1 lists some of these suspected contact binaries at L4 , small satellites, which is still kilometers in size, may of- along with their nominal diameter (D), rotation period (P), fer better and safer conditions for orbiting or performing and asteroid spectral class, also including Hektor which is proximity operations compared to approaching the main believed to be a contact binary itself with a small moon dis- body. These rationales for exploring secondary satellites 3) covered in 2006 . From the list shown in Table 1, only have been discussed in Bellerose and Yano (2009) 15), also Hektor, Antilochus, and Neoptolemus have been classified discussing other binary system populations, and we give a D-type, while the other ones are yet to be determined. In more comprehensive approach with the Hektor case study addition, the rotation period which factor in the proximity presented next. We start by giving an overview of the dy- operations are only available for Hektor, 1999 YY2, Poly- namics related to Trojan asteroids, from far to near fields poites, Antilochus, Automedon, and Neoptolemus, being for close operations. 6.9 hrs, 7 hrs, 43 hrs, 31.5 hrs, 10.2 hrs, and 12 hrs. Al- though of similar type, fast rotation periods may indicate 3. Dynamics at Trojan Asteroids a higher strength body while slow rotating ones may be 3.13.1. SolarSolar Sailsail Explorationexploration loosely packed. There has been a few studies and mission proposals to Trojans in this last decade. Yano et al. (2004) 16) discussed Table 1. Ecliptic crossing timeline for Jovian Trojan suspected binary con- a solar powered sail flyby at the L4 Lagrangian point, and 2) tact asteroids at L4 (targets listed in Mann et al. (2007) ). Empty cells robotic exploration missions are currently being studied, indicate unresolved data to date. such as PARIS 17) (ESA) and SHOTPUT 18) (JPL) target- Trojans D (km) P (hrs) Class 19) 2146 Stentor 77 ing both Hektor, and ILion (NASA) to investigate Trojan 624 Hektor 360 6.9 D asteroids at L5. JAXA is now currently looking at the feasi- 9694 Lycomedes 78 bility of exploring Trojan asteroids at L4, as part of the next 4068 Menestheus 78 possible flagship mission to the Jovian system, in collabo- 1999 YY2 47 7 10,12) 3709 Polypoites 87 43 ration with NASA and ESA , as represented in Fig. 1. 9431 1996 PS1 59 1583 Antilochus 99 31.5 D 11668 Balios 47 2001 CE21 33 1868 Thersites 80 1999 XW261 44 2920 Automedon 111 10.2 1869 Philoctetes 34 1998 VA50 45 1647 Menelaus 37 1999 XJ55 38 2260 Neoptolemus 77 12 D 4834 Thoas 91 2002 EZ13 36 4833 Meges 95 If these Trojan asteroids are mostly composed of rocky material, and of similar composition to giant planets, they may indicate an outward migration of giants as the aster- oids may have formed from rocky material inside the snow Fig.
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