229 research outputs found
Testing the inversion of asteroids' Gaia photometry combined with ground-based observations
We investigated the reliability of the genetic algorithm which will be used
to invert the photometric measurements of asteroids collected by the European
Space Agency Gaia mission. To do that, we performed several sets of simulations
for 10 000 asteroids having different spin axis orientations, rotational
periods and shapes. The observational epochs used for each simulation were
extracted from the Gaia mission simulator developed at the Observatoire de la
C\^{o}te d'Azur, while the brightness was generated using a Z-buffer standard
graphic method. We also explored the influence on the inversion results of
contaminating the data set with Gaussian noise with different values.
The research enabled us to determine a correlation between the reliability of
the inversion method and the asteroid's pole latitude. In particular, the
results are biased for asteroids having quasi-spherical shapes and low pole
latitudes. This effect is caused by the low lightcurve amplitude observed under
such circumstances, as the periodic signal can be lost in the photometric
random noise when both values are comparable, causing the inversion to fail.
Such bias might be taken into account when analysing the inversion results, not
to mislead it with physical effects such as non-gravitational forces. Finally,
we studied what impact on the inversion results has combining a full lightcurve
and Gaia photometry collected simultaneously. Using this procedure we have
shown that it is possible to reduce the number of wrong solutions for asteroids
having less than 50 data points. The latter will be of special importance for
planning ground-based observations of asteroids aiming to enhance the
scientific impact of Gaia on Solar system science.Comment: Accepted in MNRA
Asteroids seen by JWST-MIRI: Radiometric Size, Distance and Orbit Constraints
Infrared measurements of asteroids are crucial for the determination of
physical and thermal properties of individual objects, and for the
understanding of the small-body populations in the solar system as a whole. But
standard radiometric methods can only be applied if the orbit of an object is
known, hence its position at the time of the observation. We present MIRI
observations of the outer-belt asteroid 10920 and an unknown object, detected
in all 9 MIRI bands in close proximity to 10920. We developed a new method
"STM-ORBIT" to interpret the multi-band measurements without knowing the
object's true location. The method leads to a confirmation of radiometric
size-albedo solution for 10920 and puts constraints on the asteroid's location
and orbit in agreement with its true orbit. Groundbased lightcurve observations
of 10920, combined with Gaia data, indicate a very elongated object (a/b >=
1.5), with a spin-pole at (l, b) = (178{\deg}, 81{\deg}), and a rotation period
of 4.861191 h. A thermophysical study leads to a size of 14.5 - 16.5 km, a
geometric albedo between 0.05 and 0.10, and a thermal inertia in the range 9 to
35 Jm-2s-0.5K-1. For the newly discovered MIRI object, the STM-ORBIT method
revealed a size of 100-230 m. The new asteroid must be on a very
low-inclination orbit and it was located in the inner main-belt region during
JWST observations. A beaming parameter {\eta} larger than 1.0 would push the
size even below 100 meter, a main-belt regime which escaped IR detections so
far. These kind of MIRI observations can therefore contribute to formation and
evolution studies via classical size-frequency studies which are currently
limited to objects larger than about one kilometer in size. We estimate that
MIRI frames with pointings close to the ecliptic and only short integration
times of a few seconds will always include a few asteroids, most of them will
be unknown objects.Comment: 17 pages, 10 figures, 4 tables, accepted for A&A publication on Nov
22, 202
Shape models and physical properties of asteroids
Despite the large amount of high quality data generated in recent space
encounters with asteroids, the majority of our knowledge about these objects
comes from ground based observations. Asteroids travelling in orbits that are
potentially hazardous for the Earth form an especially interesting group to be
studied. In order to predict their orbital evolution, it is necessary to
investigate their physical properties. This paper briefly describes the data
requirements and different techniques used to solve the lightcurve inversion
problem. Although photometry is the most abundant type of observational data,
models of asteroids can be obtained using various data types and techniques. We
describe the potential of radar imaging and stellar occultation timings to be
combined with disk-integrated photometry in order to reveal information about
physical properties of asteroids.Comment: From Assessment and Mitigation of Asteroid Impact Hazards boo
Large halloween asteroid at lunar distance
The near-Earth asteroid (NEA) 2015 TB had a very close encounter with Earth at 1.3 lunar distances on October 31, 2015. We obtained 3-band mid-infrared observations of this asteroid with the ESO VLT-VISIR instrument covering approximately four hours in total. We also monitored the visual lightcurve during the close-encounter phase. The NEA has a (most likely) rotation period of 2.939 ± 0.005 h and the visual lightcurve shows a peak-to-peak amplitude of approximately 0.12 ± 0.02 mag. A second rotation period of 4.779 ± 0.012 h, with an amplitude of the Fourier fit of 0.10 ± 0.02 mag, also seems compatible with the available lightcurve measurements. We estimate a V-R colour of 0.56 ± 0.05 mag from different entries in the MPC database. A reliable determination of the object's absolute magnitude was not possible. Applying different phase relations to the available R-/V-band observations produced H = 18.6 mag (standard H-G calculations) or H = 19.2 mag and H = 19.8 mag (via the H-G procedure for sparse and low-quality data), with large uncertainties of approximately 1 mag. We performed a detailed thermophysical model analysis by using spherical and partially also ellipsoidal shape models. The thermal properties are best explained by an equator-on (± 30°) viewing geometry during our measurements with a thermal inertia in the range 250-700 J m s K (retrograde rotation) or above 500 J m s K (prograde rotation). We find that the NEA has a minimum size of approximately 625 m, a maximum size of just below 700 m, and a slightly elongated shape with a/b 1.1. The best match to all thermal measurements is found for: (i) thermal inertia Γ = 900 J m s K; D = 644 m, p = 5.5% (prograde rotation with 2.939 h); regolith grain sizes of 50-100 mm; (ii) thermal inertia Γ = 400 J m s K; D = 667 m, p = 5.1% (retrograde rotation with 2.939 h); regolith grain sizes of 10-20 mm. A near-Earth asteroid model (NEATM) confirms an object size well above 600 m (best NEATM solution at 690 m, beaming parameter η = 1.95), significantly larger than early estimates based on radar measurements. In general, a high-quality physical and thermal characterisation of a close-encounter object from two-week apparition data is not easily possible. We give recommendations for improved observing strategies for similar events in the future. © ESO, 2017.The research leading to these results has received funding from the European Union's Horizon 2020 Research and Innovation Programme, under Grant Agreement No. 687378. Funding from Spanish grant AYA-2014-56637-C2-1-P is acknowledged. Hungarian funding from the NKFIH grant GINOP-2.3.2-15-2016-00003 is also acknowledged. R.D. acknowledges the support of MINECO for his Ramon y Cajal Contract.Peer Reviewe
(16) Psyche: A mesosiderite-like asteroid?
Asteroid (16) Psyche is the target of the NASA Psyche mission. It is
considered one of the few main-belt bodies that could be an exposed
proto-planetary metallic core and that would thus be related to iron
meteorites. Such an association is however challenged by both its near- and
mid-infrared spectral properties and the reported estimates of its density.
Here, we aim to refine the density of (16) Psyche to set further constraints on
its bulk composition and determine its potential meteoritic analog.
We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large
program (ID 199.C-0074). We used the high angular resolution of these
observations to refine Psyche's three-dimensional (3D) shape model and
subsequently its density when combined with the most recent mass estimates. In
addition, we searched for potential companions around the asteroid. We derived
a bulk density of 3.99\,\,0.26\,gcm for Psyche. While such
density is incompatible at the 3-sigma level with any iron meteorites
(7.8\,gcm), it appears fully consistent with that of
stony-iron meteorites such as mesosiderites (density
4.25\,cm). In addition, we found no satellite in our images
and set an upper limit on the diameter of any non-detected satellite of
1460\,\,200}\,m at 150\,km from Psyche (0.2\%\,\,R, the
Hill radius) and 800\,\,200\,m at 2,000\,km (3\%\,\,).
Considering that the visible and near-infrared spectral properties of
mesosiderites are similar to those of Psyche, there is merit to a
long-published initial hypothesis that Psyche could be a plausible candidate
parent body for mesosiderites.Comment: 16 page
The equilibrium shape of (65) Cybele: primordial or relic of a large impact?
Cybele asteroids constitute an appealing reservoir of primitive material
genetically linked to the outer Solar System, and the physical properties of
the largest members can be readily accessed by large telescopes. We took
advantage of the bright apparition of (65) Cybele in July and August 2021 to
acquire high-angular-resolution images and optical light curves of the asteroid
with which we aim to analyse its shape and bulk properties. 7 series of images
acquired with VLT/SPHERE were combined with optical light curves to reconstruct
the shape of the asteroid using the ADAM, MPCD, and SAGE algorithms. The origin
of the shape was investigated by means of N-body simulations. Cybele has a
volume-equivalent diameter of 263+/-3km and a bulk density of
1.55+/-0.19g.cm-3. Notably, its shape and rotation state are closely compatible
with those of a Maclaurin equilibrium figure. The lack of a collisional family
associated with Cybele and the higher bulk density of that body with respect to
other large P-type asteroids suggest that it never experienced any large
disruptive impact followed by rapid re-accumulation. This would imply that its
present-day shape represents the original one. However, numerical integration
of the long-term dynamical evolution of a hypothetical family shows that it is
dispersed by gravitational perturbations and chaotic diffusion over Gyrs of
evolution. The very close match between Cybele and an equilibrium figure opens
up the possibility that D>260km small bodies from the outer Solar System all
formed at equilibrium. However, we cannot rule out an old impact as the origin
of the equilibrium shape. Cybele itself is found to be dynamically unstable,
implying that it was recently (<1Ga) placed on its current orbit either through
slow diffusion from a relatively stable orbit in the Cybele region or, less
likely, from an unstable, JFC orbit in the planet-crossing region.Comment: 19 pages, 14 figures, 4 tables, accepted for publication in A&
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