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Near-Earth Binaries and Triples
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Title: Near-Earth Binaries and Triples: Origin and Evolution of Spin-Orbital Properties
Authors: Julia Fang, Jean-Luc Margot

In the near-Earth asteroid population, binary and triple systems have been discovered with mutual orbits that have significant eccentricities as well as large semi-major axes. All known systems with eccentric orbits and all widely-separated primary-satellite pairs have rapidly rotating satellites. Here we study processes that can elucidate the origin of these spin-orbital properties. Binary formation models based on rotational fissioning can reproduce asynchronous satellites on orbits with high eccentricities and a wide range of separations, but do not match observed properties. We explore whether any evolutionary mechanisms can link the spin and orbital parameters expected from post-fission dynamics to those observed today. We investigate four processes: tidal torques, radiative perturbations (BYORP), close planetary encounters, and Kozai oscillations. We find that a combination of post-fission dynamics and tidal evolution can explain nearly all the spin-orbit properties in a sample of nine well-characterised near-Earth binaries and triples. The other mechanisms may act but are not required to explain the observed data. Lastly, we describe evolutionary pathways between observed spin-orbital states including synchronous and circular, asynchronous and circular, and asynchronous and eccentric configurations.

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RE: Double Asteroids
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Title: Binary Asteroid Encounters with Terrestrial Planets: Timescales and Effects
Authors: Julia Fang, Jean-Luc Margot

Many asteroids that make close encounters with terrestrial planets are in a binary configuration. Here we calculate the relevant encounter timescales and investigate the effects of encounters on a binary's mutual orbit. We use a combination of analytical and numerical approaches with a wide range of initial conditions. Our test cases include generic binaries with close, moderate, and wide separations, as well as seven well-characterised near-Earth binaries. We find that close approaches (<10 Earth radii) occur for almost all binaries on 1-10 million year timescales. At such distances, our results suggest substantial modifications to a binary's semi-major axis, eccentricity, and inclination, which we quantify. Encounters within 30 Earth radii typically occur on sub-million year timescales and significantly affect the wider binaries. Important processes in the lives of near-Earth binaries, such as tidal and radiative evolution, can be altered or stopped by planetary encounters.

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Posts: 131433
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Binary Asteroid Systems
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Title: Binary Asteroid Systems: Tidal End States and Estimates of Material Properties
Authors: Patrick A. Taylor, Jean-Luc Margot

The locations of the fully despun, double synchronous end states of tidal evolution are derived for spherical components. With the exception of nearly equal-mass binaries, binary asteroid systems are in the midst of lengthy tidal evolutions, far from their fully synchronous tidal end states. Calculations of material strength indicate that binaries in the main belt with 100-km-scale primary components are consistent with being made of monolithic or fractured rock as expected for binaries likely formed from sub-catastrophic impacts in the early solar system. To tidally evolve in their dynamical lifetime, near-Earth binaries with km-scale primaries or smaller must be much weaker mechanically than their main-belt counterparts even if formed in the main belt prior to injection into the near-Earth region. Small main-belt binaries with primary components less than 10 km in diameter, depending on their ages, could either be as strong as large main-belt binaries or as weak as near-Earth binaries because the inherent uncertainty in the age of a binary system can affect the calculation of material strength by orders of magnitude. Several other issues are considered, though these typically affect the calculation of material strength by no more than a factor of two. We also find indirect evidence within all three groups of binary asteroids that the inter-component separation may evolve via another mechanism(s) with the binary YORP effect being a likely candidate.

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Posts: 131433
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Binary Asteroids with Large Separations
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Title: Rotation Periods of Binary Asteroids with Large Separations - Confronting the Escaping Ejecta Binaries Model with Observations
Authors: D. Polishook, N. Brosch, D. Prialnik

Durda et al. (2004), using numerical models, suggested that binary asteroids with large separation, called Escaping Ejecta Binaries (EEBs), can be created by fragments ejected from a disruptive impact event. It is thought that six binary asteroids recently discovered might be EEBs because of the high separation between their components (~100 > a/Rp > ~20). However, the rotation periods of four out of the six objects measured by our group and others and presented here show that these suspected EEBs have fast rotation rates of 2.5 to 4 hours. Because of the small size of the components of these binary asteroids, linked with this fast spinning, we conclude that the rotational-fission mechanism, which is a result of the thermal YORP effect, is the most likely formation scenario. Moreover, scaling the YORP effect for these objects shows that its timescale is shorter than the estimated ages of the three relevant Hirayama families hosting these binary asteroids. Therefore, only the largest (D~19 km) suspected asteroid, (317) Roxane, could be, in fact, the only known EEB. In addition, our results confirm the triple nature of (3749) Balam by measuring mutual events on its lightcurve that match the orbital period of a nearby satellite in addition to its distant companion. Measurements of (1509) Esclangona at different apparitions show a unique shape of the lightcurve that might be explained by color variations.

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Asteroid 90 Antiope
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90 Antiope is an asteroid discovered on October 1, 1866 by Robert Luther. The 90th asteroid to be discovered, it is named after Antiope from Greek mythology, though it is disputed as to whether this is Antiope the Amazon or Antiope the mother of Amphion and Zethus.

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Title:
A Giant Crater on 90 Antiope?
Authors: P. Descamps, F. Marchis, T. Michalowski, J. Berthier, J. Pollock, P.Wiggins, M. Birlan, F. Colas, F. Vachier, S. Fauvaud, M. Fauvaud, J.-P. Sareyan, F. Pilcher, D.A. Klinglesmith

Mutual event observations between the two components of 90 Antiope were carried out in 2007-2008. The pole position was refined to lambda0 = 199.5±0.5 eg and beta0 = 39.8±5 deg in J2000 ecliptic coordinates, leaving intact the physical solution for the components, assimilated to two perfect Roche ellipsoids, and derived after the 2005 mutual event season (Descamps et al., 2007). Furthermore, a large-scale geological depression, located on one of the components, was introduced to better match the observed lightcurves. This vast geological feature of about 68 km in diameter, which could be postulated as a bowl-shaped impact crater, is indeed responsible of the photometric asymmetries seen on the "shoulders" of the lightcurves. The bulk density was then recomputed to 1.28±0.04 gcm^-3 to take into account this large-scale non-convexity. This giant crater could be the aftermath of a tremendous collision of a 100-km sized proto-Antiope with another Themis family member. This statement is supported by the fact that Antiope is sufficiently porous (~50%) to survive such an impact without being wholly destroyed. This violent shock would have then imparted enough angular momentum for fissioning of proto-Antiope into two equisized bodies. We calculated that the impactor must have a diameter greater than ~17 km, for an impact velocity ranging between 1 and 4 km/s. With such a projectile, this event has a substantial 50% probability to have occurred over the age of the Themis family.

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Posts: 131433
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RE: Double Asteroids
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Study Puts Solar Spin on Asteroids, their Moons
Asteroids with moons, which scientists call binary asteroids, are common in the solar system. A longstanding question has been how the majority of such moons are formed.
In this week's issue of the journal Nature, a trio of astronomers from Maryland and France say the surprising answer is sunlight, which can increase or decrease the spin rate of an asteroid.

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Posts: 131433
Date:
Binary asteroids
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La fragmentation par rotation à l'origine des astéroïdes binaires
Deux chercheurs du laboratoire CASSIOPEE (INSU-CNRS - Observatoire de la Côte d'Azur, Université de Nice) en collaboration avec un chercheur de l'Université de Maryland (USA) viennent de trouver par simulation numérique l'origine des astéroïdes binaires qui constituent 15% des deux populations d'astéroïdes, celle située entre Mars et Jupiter dans la Ceinture principale et celle croisant la trajectoire de la Terre. Un effet thermique est connu pour entraîner une augmentation de la vitesse de rotation d'un astéroïde. Lorsque celui-ci est un agrégat, l'accélération de sa rotation provoque un déplacement de matière des pôles vers l'équateur et un échappement de cette matière à l'équateur. Cette matière va de nouveau s'agréger pour former un satellite de l'astéroïde avec les propriétés observées. Ces travaux sont publiés dans la revue Nature du 10 juillet 2008.

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Posts: 131433
Date:
RE: Double Asteroids
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Asteroids with moons, which scientists call binary asteroids, are common in the solar system. A longstanding question has been how most such moons are formed. In the journal Nature, a trio of astronomers say the surprising answer is sunlight, which can increase or decrease the spin rate of an asteroid. Their findings match observations and give information important for deflecting threatening asteroids away from Earth.

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Title:  (42355) Typhon-Echidna: Scheduling Observations for Binary Orbit Determination
Authors: W.M. Grundy, K.S. Noll, J. Virtanen, K. Muinonen, S.D. Kern, D.C. Stephens, J.A. Stansberry, H.F. Levison, J.R. Spencer

We describe a strategy for scheduling astrometric observations to minimise the number required to determine the mutual orbits of binary transneptunian systems. The method is illustrated by application to Hubble Space Telescope observations of (42355) Typhon-Echidna, revealing that Typhon and Echidna orbit one another with a period of 18.971 ± 0.006 days and a semimajor axis of 1628 ± 29 km, implying a system mass of (9.49 ± 0.52) x 10^17 kg. The eccentricity of the orbit is 0.526 ± 0.015. Combined with a radiometric size determined from Spitzer Space Telescope data and the assumption that Typhon and Echidna both have the same albedo, we estimate that their radii are 76 +14/-16 and 42 +8/-9 km, respectively. These numbers give an average bulk density of only 0.44 +0.44/-0.17 g cm^-3, consistent with very low bulk densities recently reported for two other small transneptunian binaries.

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Title: Main Belt Binary Asteroidal Systems With Eccentric Mutual Orbits
Authors: F. Marchis, P. Descamps, J. Berthier, D. hestroffer, F. vachier, M. Baek, A. Harris, D. Nesvorny

Using 8m-10m class telescopes and their Adaptive Optics (AO) systems, we conducted a long-term adaptive optics campaign initiated in 2003 focusing on four binary asteroid systems: (130) Elektra, (283) Emma, (379) Huenna, and (3749) Balam. The analysis of these data confirms the presence of their asteroidal satellite. We did not detect any additional satellite around these systems even though we have the capability of detecting a loosely-bound fragment (located at 1/4 x RHill) ~40 times smaller in diameter than the primary. The orbits derived for their satellites display significant eccentricity, ranging from 0.1 to 0.9, suggesting a different origin. Based on AO size estimate, we show that (130) Elektra and (283) Emma, G-type and P-type asteroids respectively, have a significant porosity (30-60% considering CI-CO meteorites as analogues) and their satellite's eccentricities (e~0.1) are possibly due to excitation by tidal effects. (379) Huenna and (3749) Balam, two loosely bound binary systems, are most likely formed by mutual capture. (3749) Balam's possible high bulk density is similar to (433) Eros, another S-type asteroid, and should be poorly fractured as well. (379) Huenna seems to display both characteristics: the moonlet orbits far away from the primary in term of stability (20% x RHill), but the primary's porosity is significant (30-60%).

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