52 research outputs found
Near-Infrared Photometry of Irregular Satellites of Jupiter and Saturn
We present JHKs photometry of 10 Jovian and 4 Saturnian irregular satellites,
taken with the Near-InfraRed Imager (NIRI) at the 8-m Gemini North Observatory
on Mauna Kea, Hawaii. The observed objects have near-infrared colors consistent
with C, P and D-type asteroids, although J XII Ananke and S IX Phoebe show weak
indications of possible water features in the H filter. The four members of the
Himalia-family have similar near-infrared colors, as do the two members of the
Gallic family, S XX Paaliaq and S XXIX Siarnaq. From low resolution normalized
reflectance spectra based on the broadband colors and covering 0.4 to 2.2
microns, the irregular satellites are identified as C-type (J VII Pasiphae, J
VI Himalia and S IX Phoebe), P-type (J XII Ananke and J XVIII Themisto) and
D-type (J IX Carme and J X Sinope), showing a diversity of origins of these
objects.Comment: Accepted by ApJ Letters (emulateapj, 8pages, including 4 figures);
Typos corrected, references adde
The Albedo Distribution of Near Earth Asteroids
The cryogenic WISE mission in 2010 was extremely sensitive to asteroids and
not biased against detecting dark objects. The albedos of 428 Near Earth
Asteroids (NEAs) observed by WISE during its fully cryogenic mission can be fit
quite well by a 3 parameter function that is the sum of two Rayleigh
distributions. The Rayleigh distribution is zero for negative values, and
follows for positive x. The peak
value is at x=\sigma, so the position and width are tied together. The three
parameters are the fraction of the objects in the dark population, the position
of the dark peak, and the position of the brighter peak. We find that 25.3% of
the NEAs observed by WISE are in a very dark population peaking at , while the other 74.7% of the NEAs seen by WISE are in a moderately dark
population peaking at . A consequence of this bimodal distribution
is that the Congressional mandate to find 90% of all NEAs larger than 140 m
diameter cannot be satisfied by surveying to H=22 mag, since a 140 m diameter
asteroid at the very dark peak has H=23.7 mag, and more than 10% of NEAs are
darker than p_V = 0.03.Comment: 7 pages LaTex, 4 figures, accepted for publication in the
Astronomical Journa
The Complex History of Trojan Asteroids
The Trojan asteroids provide a unique perspective on the history of Solar
System. As a large population of small bodies, they record important
gravitational interactions and dynamical evolution of the Solar System. In the
past decade, significant advances have been made in understanding physical
properties, and there has been a revolution in thinking about the origin of
Trojans. The ice and organics generally presumed to be a significant part of
Trojan compositions have yet to be detected directly, though low density of the
binary system Patroclus (and possibly low density of the binary/moonlet system
Hektor) is consistent with an interior ice component. By contrast, fine-grained
silicates that appear to be similar to cometary silicates in composition have
been detected, and a color bimodality may indicate distinct compositional
groups among the Trojans. Whereas Trojans had traditionally been thought to
have formed near 5 AU, a new paradigm has developed in which the Trojans formed
in the proto-Kuiper Belt, and they were scattered inward and captured in the
Trojan swarms as a result of resonant interactions of the giant planets.
Whereas the orbital and population distributions of current Trojans are
consistent with this origin scenario, there are significant differences between
current physical properties of Trojans and those of Kuiper Belt objects. These
differences may be indicative of surface modification due to the inward
migration of objects that became the Trojans, but understanding of appropriate
modification mechanisms is poor and would benefit from additional laboratory
studies. Many open questions remain, and the future promises significant
strides in our understanding of Trojans. The time is ripe for a spacecraft
mission to the Trojans, to turn these objects into geologic worlds that can be
studied in detail to unravel their complex history.Comment: Chapter for Asteroids IV book (UA Press), accepted for publication,
33 pages, 10 figure
Photometry of Irregular Satellites of Uranus and Neptune
We present BVR photometric colors of six Uranian and two Neptunian irregular
satellites, collected using the Magellan Observatory (Las Campanas, Chile) and
the Keck Observatory, (Manua Kea, Hawaii). The colors range from neutral to
light red, and like the Jovian and the Saturnian irregulars (Grav et al. 2003)
there is an apparent lack of the extremely red objects found among the Centaurs
and Kuiper belt objects.
The Uranian irregulars can be divided into three possible dynamical families,
but the colors collected show that two of these dynamical families, the Caliban
and Sycorax-clusters, have heterogeneous colors. Of the third possible family,
the 168-degree cluster containing two objects with similar average inclinations
but quite different average semi-major axis, only one object (U XXI Trinculo)
was observed. The heterogeneous colors and the large dispersion of the average
orbital elements leads us to doubt that they are collisional families. We favor
single captures as a more likely scenario. The two neptunians observed (N II
Nereid and S/2002 N1) both have very similar neutral, sun-like colors. Together
with the high collisional probability between these two objects over the age of
the solar system (Nesvorny et al. 2003, Holman et al. 2004), this suggests that
S/2002 N1 be a fragment of Nereid, broken loose during a collision or cratering
event with an undetermined impactor.Comment: 13 pages (including 3 figures and 2 tables). Submitted to ApJ Letter
NEOWISE: Observations of the Irregular Satellites of Jupiter and Saturn
We present thermal model fits for 11 Jovian and 3 Saturnian irregular
satellites based on measurements from the WISE/NEOWISE dataset. Our fits
confirm spacecraft-measured diameters for the objects with in situ observations
(Himalia and Phoebe) and provide diameters and albedo for 12 previously
unmeasured objects, 10 Jovian and 2 Saturnian irregular satellites. The
best-fit thermal model beaming parameters are comparable to what is observed
for other small bodies in the outer Solar System, while the visible, W1, and W2
albedos trace the taxonomic classifications previously established in the
literature. Reflectance properties for the irregular satellites measured are
similar to the Jovian Trojan and Hilda Populations, implying common origins.Comment: 17 pages, 3 figures, accepted for publication in Astrophysical
Journa
Detection of Earth-impacting asteroids with the next generation all-sky surveys
We have performed a simulation of a next generation sky survey's (Pan-STARRS
1) efficiency for detecting Earth-impacting asteroids. The steady-state
sky-plane distribution of the impactors long before impact is concentrated
towards small solar elongations (Chesley and Spahr, 2004) but we find that
there is interesting and potentially exploitable behavior in the sky-plane
distribution in the months leading up to impact. The next generation surveys
will find most of the dangerous impactors (>140m diameter) during their
decade-long survey missions though there is the potential to miss difficult
objects with long synodic periods appearing in the direction of the Sun, as
well as objects with long orbital periods that spend much of their time far
from the Sun and Earth. A space-based platform that can observe close to the
Sun may be needed to identify many of the potential impactors that spend much
of their time interior to the Earth's orbit. The next generation surveys have a
good chance of imaging a bolide like 2008TC3 before it enters the atmosphere
but the difficulty will lie in obtaining enough images in advance of impact to
allow an accurate pre-impact orbit to be computed.Comment: 47 pages, 16 figures, 2 table
The Short Rotation Period of Nereid
We determine the period, p = 11.52 \pm 0.14 h, and a light curve peak-to-peak
amplitude, a = 0.029 \pm 0.003 magnitudes, of the Neptunian irregular satellite
Nereid. If the light curve variation is due to albedo variations across the
surface, rather than solely to the shape of Nereid variations, the rotation
period would be a factor of two shorter. In either case, such a rotation period
and light curve amplitude, together with Nereid's orbital period, p=360.14
days, imply that Nereid is almost certainly in a regular rotation state, rather
than the chaotic rotation state suggested by Schaefer and Schaefer (1988,2000)
and Dobrovolskis (1995).
Assuming that Nereid is perfectly spherical, the albedo variation is 3%
across the observed surface. Assuming a uniform geometric albedo, the observed
cross sectional area varies by 3%. We caution that the lightcurve found in this
paper only sets limits on the combination of albedo and physical irregularity
and that we cannot determine the orientation of Nereid's spin axis from our
data.Comment: Accepted by ApJ Letters, 11 pages (incl. 1 figure
The NEO Surveyor Near Earth Asteroid Known Object Model
The known near-Earth object (NEO) population consists of over 32,000 objects,
with a yearly discovery rate of over 3000 NEOs per year. An essential component
of the next generation of NEO surveys is an understanding of the population of
known objects, including an accounting of the discovery rate per year as a
function of size. Using a near-Earth asteroid (NEA) reference model developed
for NASA's NEO Surveyor (NEOS) mission and a model of the major current and
historical ground-based surveys, an estimate of the current NEA survey
completeness as a function of size and absolute magnitude has been determined
(termed the Known Object Model; KOM). This allows for understanding of the
intersection of the known catalog of NEAs and the objects expected to be
observed by NEOS. The current NEA population is found to be complete
for objects larger than 140m, consistent with estimates by Harris & Chodas
(2021). NEOS is expected to catalog more than two thirds of the NEAs larger
than 140m, resulting in of NEAs cataloged at the end of its 5 year
nominal survey (Mainzer et al, 2023}, making significant progress towards the
US Congressional mandate. The KOM estimates that of the currently
cataloged objects will be detected by NEOS, with those not detected
contributing to the final completeness at the end its 5 year mission.
This model allows for placing the NEO Surveyor mission in the context of
current surveys to more completely assess the progress toward the goal of
cataloging the population of hazardous asteroids.Comment: 27 pages, 18 figures, 3 tables. Accepted for publication in Planetary
Science Journal (PSJ
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