Forming Binary Near-Earth Asteroids From Tidal Disruptions

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Chapter 1Introduction1.1 Scientific Motivation1.1.1 Why binary asteroids are interestingThe discovery and observation of binary asteroids over the past decade has provided newways to study the properties of Solar System bodies. Before the 1993 discovery of Ida’scompanion Dactyl in the main asteroid belt by the Galileo mission, there were only unconfirmeddetections and theoretical speculation about binary asteroids. The inventory ofbinary minor planets has now eclipsed 100, there are known triple and even quadruplesystems, and detailed radar shape models are being produced for some nearby binaries(see Richardson & Walsh 2006 for a review). Of particular interest for this dissertationare binaries in the Main-Belt and near-Earth populations.The reason astronomers have searched for binary asteroids since the discovery of (1)Ceres is that binary asteroids can provide extensive insight into the physical properties ofthe bodies. They can also provide information about past and present dynamics affectingsmall bodies in the inner Solar System. A binary system typically allows for estimation1
of the system mass M by way of Kepler’s Third Law,P 2 = 4π 2 a 3 /GM (1.1)with period P and mean separation a typically directly measured, and G is the gravitationalconstant. In most situations component sizes and shapes can be modeled allowingfor a rare measurement of asteroid density. When coupled with a spectroscopic estimateof asteroidal composition, estimates on porosity and internal structure can be made.The dynamical mechanisms affecting various populations of small bodies can also bestudied by way of binary asteroids. Binaries have been discovered in nearly every dynamicalpopulation of minor planets, with large numbers among trans-Neptunian objects (orKuiper Belt Objects), one binary Jovian Trojan, two binary Centaurs, and a plethora ofbinary Main Belt and near-Earth asteroids. Adding to this inventory are recent discoveriesof complex systems such as the additional satellites of Pluto, a triple KBO 2003 EL 61 ,and a triple Main-Belt asteroid (MBA) system (87) Sylvia. Since each population boastsdramatically different dynamical, collisional and thermal environments, the existence anddetailed study of binaries will be quite valuable.1.1.2 Binary near-Earth asteroidsThe known binary near-Earth asteroids (NEAs) have been discovered from a combinationof lightcurve and radar observations. Lightcurve studies, where an asteroid’s magnitudeis plotted over time, have been conducted on a large number of bodies. However, certainorbital properties (asynchronous orbit and favorable geometry) are needed to make anunambiguous assessment of the state of the binary (Weidenschilling et al. 1989; Pravec& Hahn 1997). The secondaries must be large enough (∼20% of the primary) for theirnonsynchronous periods to be observed above any noise in the lightcurve of the primaries.Observations must also capture occultations/eclipses over multiple revolutions of the sec-2
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: 4.8 Percentage of migrated binaries
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 and 28: ever, understanding the mechanism i
Page 29 and 30: off the equator or off one end of a
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
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a/R pri based on a simple conversio
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Figure 2.13: The eccentricity dampi
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Main Belt. Work by Chauvineau & Far
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(1992), with a target range of slig
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The observations were made from the
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peaked period, with a lower signifi
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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
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Shepard, M. K., Schlieder, J., Nola
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Forming Binary Near-Earth Asteroids From Tidal Disruptions

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