232 research outputs found
Detection of Contact Binaries Using Sparse High Phase Angle Lightcurves
We show that candidate contact binary asteroids can be efficiently identified
from sparsely sampled photometry taken at phase angles >60deg. At high phase
angle, close/contact binary systems produce distinctive lightcurves that spend
most of the time at maximum or minimum (typically >1mag apart) brightness with
relatively fast transitions between the two. This means that a few (~5) sparse
observations will suffice to measure the large range of variation and identify
candidate contact binary systems. This finding can be used in the context of
all-sky surveys to constrain the fraction of contact binary near-Earth objects.
High phase angle lightcurve data can also reveal the absolute sense of the
spin.Comment: 4 pages, 4 figures, 1 table. Accepted for publication in ApJ
A ring as a model of the main belt in planetary ephemerides
We assess the ability of a solid ring to model a global perturbation induced
by several thousands of main-belt asteroids. The ring is first studied in an
analytical framework that provides an estimate of all the ring's parameters
excepting mass. In the second part, numerically estimated perturbations on the
Earth-Mars, Earth-Venus, and Earth-Mercury distances induced by various subsets
of the main-belt population are compared with perturbations induced by a ring.
To account for large uncertainties in the asteroid masses, we obtain results
from Monte Carlo experiments based on asteroid masses randomly generated
according to available data and the statistical asteroid model. The radius of
the ring is analytically estimated at 2.8 AU. A systematic comparison of the
ring with subsets of the main belt shows that, after removing the 300 most
perturbing asteroids, the total main-belt perturbation of the Earth-Mars
distance reaches on average 246 m on the 1969-2010 time interval. A ring with
appropriate mass is able to reduce this effect to 38 m. We show that, by
removing from the main belt ~240 asteroids that are not necessarily the most
perturbing ones, the corresponding total perturbation reaches on average 472 m,
but the ring is able to reduce it down to a few meters, thus accounting for
more than 99% of the total effect.Comment: 18 pages, accepted in A&
Constraining multiple systems with GAIA
GAIA will provide observations of some multiple asteroid and dwarf systems.
These observations are a way to determine and improve the quantification of
dynamical parameters, such as the masses and the gravity fields, in these
multiple systems. Here we investigate this problem in the cases of Pluto's and
Eugenia's system. We simulate observations reproducing an approximate planning
of the GAIA observations for both systems, as well as the New Horizons
observations of Pluto. We have developed a numerical model reproducing the
specific behavior of multiple asteroid system around the Sun and fit it to the
simulated observations using least-square method, giving the uncertainties on
the fitted parameters. We found that GAIA will improve significantly the
precision of Pluto's and Charon's mass, as well as Petit Prince's orbital
elements and Eugenia's polar oblateness.Comment: 5 pages, accepted by Planetary and Space Science, Gaia GREAT-SSO-Pis
Mass and density of B-type asteroid (702) Alauda
Observations with the adaptive optics system on the Very Large Telescope
reveal that outer main belt asteroid (702) Alauda has a small satellite with
primary to secondary diameter ratio of 56. The secondary revolves around
the primary in 4.9143 0.007 days at a distance of 1227 24 km,
yielding a total system mass of (6.057 0.36) 10 kg.
Combined with an IRAS size measurement, our data yield a bulk density for this
B-type asteroid of 1570 500 kg~m.Comment: In press, ApJ 2011. 6 pages, 4 figure
SIM_EXPLORE: Software for Directed Exploration of Complex Systems
Physics-based numerical simulation codes are widely used in science and engineering to model complex systems that would be infeasible to study otherwise. While such codes may provide the highest- fidelity representation of system behavior, they are often so slow to run that insight into the system is limited. Trying to understand the effects of inputs on outputs by conducting an exhaustive grid-based sweep over the input parameter space is simply too time-consuming. An alternative approach called "directed exploration" has been developed to harvest information from numerical simulators more efficiently. The basic idea is to employ active learning and supervised machine learning to choose cleverly at each step which simulation trials to run next based on the results of previous trials. SIM_EXPLORE is a new computer program that uses directed exploration to explore efficiently complex systems represented by numerical simulations. The software sequentially identifies and runs simulation trials that it believes will be most informative given the results of previous trials. The results of new trials are incorporated into the software's model of the system behavior. The updated model is then used to pick the next round of new trials. This process, implemented as a closed-loop system wrapped around existing simulation code, provides a means to improve the speed and efficiency with which a set of simulations can yield scientifically useful results. The software focuses on the case in which the feedback from the simulation trials is binary-valued, i.e., the learner is only informed of the success or failure of the simulation trial to produce a desired output. The software offers a number of choices for the supervised learning algorithm (the method used to model the system behavior given the results so far) and a number of choices for the active learning strategy (the method used to choose which new simulation trials to run given the current behavior model). The software also makes use of the LEGION distributed computing framework to leverage the power of a set of compute nodes. The approach has been demonstrated on a planetary science application in which numerical simulations are used to study the formation of asteroid families
The Resolved Asteroid Program - Size, shape, and pole of (52) Europa
With the adaptive optics (AO) system on the 10 m Keck-II telescope, we
acquired a high quality set of 84 images at 14 epochs of asteroid (52) Europa
on 2005 January 20. The epochs covered its rotation period and, by following
its changing shape and orientation on the plane of sky, we obtained its
triaxial ellipsoid dimensions and spin pole location. An independent
determination from images at three epochs obtained in 2007 is in good agreement
with these results. By combining these two data sets, along with a single epoch
data set obtained in 2003, we have derived a global fit for (52) Europa of
diameters (379x330x249) +/- (16x8x10) km, yielding a volume-equivalent
spherical-diameter of 315 +/- 7 km, and a rotational pole within 7 deg of [RA;
Dec] = [257,+12] in an Equatorial J2000 reference frame (ECJ2000: 255,+35).
Using the average of all mass determinations available forEuropa, we derive a
density of 1.5 +/- 0.4, typical of C-type asteroids. Comparing our images with
the shape model of Michalowski et al. (A&A 416, 2004), derived from optical
lightcurves, illustrates excellent agreement, although several edge features
visible in the images are not rendered by the model. We therefore derived a
complete 3-D description of Europa's shape using the KOALA algorithm by
combining our imaging epochs with 4 stellar occultations and 49 lightcurves. We
use this 3-D shape model to assess these departures from ellipsoidal shape.
Flat facets (possible giant craters) appear to be less distinct on (52) Europa
than on other C-types that have been imaged in detail. We show that fewer giant
craters, or smaller craters, is consistent with its expected impact history.
Overall, asteroid (52) Europa is still well modeled as a smooth triaxial
ellipsoid with dimensions constrained by observations obtained over several
apparitions.Comment: Accepted for publication in Icaru
Nanoclay Modification of Shape Memory Polyurethane
Effect of nanoclay modification on the properties of polytetramethylene oxide-based polyurethane was examined. Nanoclay was dispersed in polyurethane wherein the clay content was varied from 1 to 5 wt.%.
The nanocomposites were characterized by thermal, FTIR, XRD and thermo-mechanical analyses and
their shape memory properties were evaluated. Morphology was examined by TEM analysis. Bending test
was adopted for the evaluation of shape memory property. Increase in clay content resulted an increase
in transition temperature. Tensile strength and modulus increased proportional to nanoclay content. The
elongation decreased with clay content. Intercalated structure of clay in the PU matrix was observed
from XRD studies, which was confirmed by TEM analysis. Modulus ratio showed a decreasing trend
with nanoclay content. This resulted in decreased shape recovery characteristics. Highest shape recovery of
92% was observed for PU with 1 wt.% clay content. Moderate nanoclay leveling is conducive to deriving
mechanically stronger PU without loss of shape memory characteristics
Physical Properties of (2) Pallas
We acquired and analyzed adaptive-optics imaging observations of asteroid (2)
Pallas from Keck II and the Very Large Telescope taken during four Pallas
oppositions between 2003 and 2007, with spatial resolution spanning 32-88 km
(image scales 13-20 km/pix). We improve our determination of the size, shape,
and pole by a novel method that combines our AO data with 51 visual
light-curves spanning 34 years of observations as well as occultation data.
The shape model of Pallas derived here reproduces well both the projected
shape of Pallas on the sky and light-curve behavior at all the epochs
considered. We resolved the pole ambiguity and found the spin-vector
coordinates to be within 5 deg. of [long, lat] = [30 deg., -16 deg.] in the
ECJ2000.0 reference frame, indicating a high obliquity of ~84 deg., leading to
high seasonal contrast. The best triaxial-ellipsoid fit returns radii of a=275
km, b= 258 km, and c= 238 km. From the mass of Pallas determined by
gravitational perturbation on other minor bodies [(1.2 +/- 0.3) x 10-10 Solar
Masses], we derive a density of 3.4 +/- 0.9 g.cm-3 significantly different from
the density of C-type (1) Ceres of 2.2 +/- 0.1 g.cm-3. Considering the spectral
similarities of Pallas and Ceres at visible and near-infrared wavelengths, this
may point to fundamental differences in the interior composition or structure
of these two bodies.
We define a planetocentric longitude system for Pallas, following IAU
guidelines. We also present the first albedo maps of Pallas covering ~80% of
the surface in K-band. These maps reveal features with diameters in the 70-180
km range and an albedo contrast of about 6% wrt the mean surface albedo.Comment: 16 pages, 8 figures, 6 table
The Origin of (90) Antiope From Component-Resolved Near-Infrared Spectroscopy
The origin of the similary-sized binary asteroid (90) Antiope remains an
unsolved puzzle. To constrain the origin of this unique double system, we
recorded individual spectra of the components using SPIFFI, a near-infrared
integral field spectrograph fed by SINFONI, an adaptive optics module available
on VLT-UT4. Using our previously published orbital model, we requested
telescope time when the separation of the components of (90) Antiope was larger
than 0.087", to minimize the contamination between components, during the
February 2009 opposition. Several multi-spectral data-cubes in J band (SNR=40)
and H+K band (SNR=100) were recorded in three epochs and revealed the two
components of (90) Antiope. After developing a specific photometric extraction
method and running an error analysis by Monte-Carlo simulations, we
successfully extracted reliable spectra of both components from 1.1 to 2.4 um
taken on the night of February 21, 2009. These spectra do not display any
significant absorption features due to mafic mineral, ices, or organics, and
their slopes are in agreement with both components being C- or Cb- type
asteroids. Their constant flux ratio indicates that both components' surface
reflectances are quite similar, with a 1-sigma variation of 7%. By comparison
with 2MASS J, H, K color distribution of observed Themis family members, we
conclude that both bodies were most likely formed at the same time and from the
same material. The similarly-sized system could indeed be the result of the
breakup of a rubble-pile proto-Antiope into two equal-sized bodies, but other
scenarios of formation implying a common origin should also be considered.Comment: 46 pages, 1 table, 11 figures accepted for publication to Icaru
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