Forming Binary Near-Earth Asteroids From Tidal Disruptions

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15 and 25 km s −1 . The models represented the progenitors with 135 particles, and somesimulations produced outcomes more elongated than Geographos, providing a potentialmeans of creating very elongated solar system bodies.Bottke & Melosh (1996a) simulated splitting of contact binaries to explain doubletcraters on <strong>Earth</strong>, Venus and Mars. In this model a binary asteroid is formed during a closeencounter with a planet, and the binary’s separation grows through subsequent encounters.Eventually the binary hits a planet, making a doublet crater. This scenario depends on aconstant refreshing of the NEA binary population via tidal disruptions, and predicts that∼ 15% of NEAs may be binaries at any given time.Simulations of NEA tidal disruption by Richardson et al. (1998) covered a large parameterspace of elongated rotating bodies (constructed with 247 particles) passing <strong>Earth</strong>at various q and v ∞ . The study was designed to quantify disruption and mass loss fortidal encounters, but noted binary formation as an observed outcome and suggested thattidal disruption could explain up to ∼ 15% of the NEAs being binaries. The simulationssparsely covered the parameters of q, v ∞ , progenitor elongation, progenitor spin rate andlong axis alignment. Basic trends in disruption were observed, with increasing disruptionfor closer approaches, slower approach speeds, and faster prograde rotation rates. Moresubtle results were seen as a function of body elongation and long axis alignment at closeapproach.Functionally tidal disruption is an impulsive event that both elongates and torques abody. A spherical body will be elongated and then spun-up during a passage by a planet,whereas a previously elongated body may be reshaped or just spun-up more dramaticallydepending on the long-axis alignment during close approach. In the closest and mostcatastrophic encounters a strengthless body is pulled into a long string of particles, similarto the SL9 event, and numerous clumps form along the string. A less dramatic event willsimply involve a body appearing to have been spun-up to the point that some mass escapes15
off the equator or off one end of a prolate body.Bottke et al. (1999) compared the shape of 1620 Geographos, obtained from delay–Doppler measurements, to that of a tidal encounter outcome. The well-defined shape ofGeographos matched many features of the simulation output, including the cusped ends,an opposed convex side, and a nearly concave side with a large hump. This study wassimilar in approach to Solem & Hills (1996), but matched the high-quality images withhigh-resolution (∼ 500-particle) simulations.Thermal SpinupAn alternative mechanism for rotational disruption is the YORP thermal effect (see Bottkeet al. 2006 for a review). This effect, related to the Yarkovsky effect, relies on re-emissionof absorbed solar radiation to provide a very small thermal thrust. This thrust, over longtimescales, provides a torque which can alter the asteroid’s obliquity and spin rate. Thiseffect depends heavily on the size of the asteroid, and is expected to be quite powerfulfor sub-kilometer bodies (with spin rate doubling timescales on the order of 0.5 Myr)and ineffective for diameters larger than 40 km. Suggested as possibly responsible forthe excess in fast and slow rotators among small asteroids, YORP has been shown in onedramatic case to re-orient spin axes. Vokrouhlický et al. (2003) demonstrated that thecombination of thermal torques from YORP and spin-orbit resonances created a collectionof similar spin periods and obliquities among Koronis family members, matchingobservations perfectly.Despite these detailed studies of the YORP effect, binary creation from thermal spinuphas yet to be studied in detail. Any detailed study relies on various assumptions aboutan asteroid’s shape and thermal properties, as well as the dynamics of a body that hasreached and passed its critical spin frequency. The discovery of binaries in the Main Belt,resembling those in the NEA population, strongly suggest that a thermal force is helping16
Page 1 and 2: AbstractTitle of Dissertation:Formi
Page 3 and 4: Forming Binary Near-Earth Asteroids
Page 5 and 6: PrefaceMuch of the work in this dis
Page 7 and 8: AcknowledgementsI would not have ma
Page 9 and 10: 2.3 Results and Discussion . . . .
Page 11 and 12: List of Tables1.1 Binary NEA proper
Page 13 and 14: 4.8 Percentage of migrated binaries
Page 15 and 16: of the system mass M by way of Kepl
Page 17 and 18: the largest lightcurve amplitudes o
Page 19 and 20: 1.1.3 Binary Main-Belt asteroidsRec
Page 21 and 22: Lightcurve discoveriesA lightcurve
Page 23 and 24: Figure 1.1: (Top) The primary rotat
Page 25 and 26: 1.2.1 CaptureIn this scenario two a
Page 27: ever, understanding the mechanism i
Page 31 and 32: 3 Denotes a secure result with no a
Page 33 and 34: nostic.Recent results have demonstr
Page 35 and 36: another intermediate source of NEAs
Page 37 and 38: 1.5.2 Rubble pilesThe evidence for
Page 39 and 40: Chapter 2Formation of Binary Astero
Page 41 and 42: ∼ 60%, making a bulk density of
Page 43 and 44: shedding mass and distorting prior
Page 45 and 46: Figure 2.3: Snapshots of a tidal di
Page 47 and 48: Figure 2.4: Normalized probability
Page 49 and 50: Figure 2.5: Normalized probability
Page 51 and 52: Figure 2.6: (a) Satellite eccentric
Page 53 and 54: various complications of measuring
Page 55 and 56: etween ω pri and L bin ), with clo
Page 57 and 58: prolate-like), the oblate-like bodi
Page 59 and 60: Figure 2.9: Plots comparing T-PROS
Page 61 and 62: Figure 2.10: Comparison of S-class
Page 63 and 64: 2.3.5 Triples and hierarchical syst
Page 65 and 66: a/R pri based on a simple conversio
Page 67 and 68: Figure 2.13: The eccentricity dampi
Page 69 and 70: Main Belt. Work by Chauvineau & Far
Page 71 and 72: (1992), with a target range of slig
Page 73 and 74: The observations were made from the
Page 75 and 76: peaked period, with a lower signifi
Page 77 and 78: and suggests that it is possibly a
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fit for the period of the lightcurv
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Table 3.2.Orbital, physical and lig
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Figure 3.2: Asteroid lightcurves.70
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Figure 3.4: Asteroid lightcurves.72
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3.4.4 Spin and shape properties amo
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Figure 3.7: The lightcurve amplitud
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Typically observations of eclipse o
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Chapter 4Steady-State Model of the
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values between 0.2-0.6. There is on
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asteroids in the Main Belt is suffi
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Figure 4.2: Percent of surviving NE
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4.1.3 Binary evolutionBasic stabili
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Table 4.1.Lifetimes for binary NEAs
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which means it should have a critic
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Figure 4.4: The total number of bin
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the original eccentricity distribut
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Figure 4.6: Properties of the binar
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Figure 4.8: Binary percentage for m
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Figure 4.9: Effects of tidal evolut
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quickly altered, or involved in bin
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tidal disruption. However, the impl
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Chapter 5ConclusionsA general concl
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small additions to the angular mome
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cently this large pool of observers
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differences in results when a lower
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Table A.1.Number of binaries produc
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Thus the reaccumulated bodies in th
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of 2.2 h observed is significantly
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Benner, L. A. M., Nolan, M. C., Ost
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V7.0:LCREF TAB, 35, 4Harris, A. W.,
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Storrs, A. D., Close, L. M., & Mena
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122Pravec, P., Kušnirá, P., Hicks
Page 143 and 144:
Shepard, M. K., Schlieder, J., Nola
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Forming Binary Near-Earth Asteroids From Tidal Disruptions

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