80 research outputs found
First albedo determination of 2867 Steins, target of the Rosetta mission
We present the first albedo determination of 2867 Steins, the asteroid target
o f the Rosetta space mission together with 21 Lutetia. The data were obtained
in polarimetric mode at the ESO-VLT telescope with the FORS1 instrument in the
V and R filters. Observations were carried out from Jun e to August 2005
covering the phase angle range from 10.3 deg. to 28.3 deg., allowing the
determination of the asteroid albedo by the well known experimenta l
relationship between the albedo and the slope of the polarimetric curve at th e
inversion angle. The measured polarization values of Steins are small,
confirming an E-type cla ssification for this asteroid, as already suggested
from its spectral propertie s. The inversion angle of the polarization curve in
the V and R filters is resp ectively of 17.3 +/-1.5deg. and 18.4+/-1.0 deg.,
and the corresponding sl ope parameter is of 0.037+/-0.003 %/deg and
0.032+/-0.003 %/deg. On the basis of its polarimetric slope value, we have
derived an albedo of 0.45 +/-0.1, that gives an estimated diameter of 4.6 km,
assuming an absolute V ma gnitude of 13.18 mag.Comment: 4 pages, 4 figures, letter accepted for pubblication on A&
25 years of asteroid investigations by Kharkiv asteroid group
In the middle of the 1970s physical investigations of asteroids became quite active in the USA and partly in Europe (Italy, Austria, Sweden). It was evident that asteroids are the bodies of great interest first of all from the point of view of cosmogonic problems of the Solar System. That was the principal reason for the beginning of asteroid studies at our Institute (Astronomical Observatory, at that time). The most important results of asteroid study obtained by our group during the last 25 years are presented
Albedo heterogeneity on the surface of (1943) Anteros
We have investigated the effect of rotation on the polarization of scattered
light for the near-Earth asteroid (1943) Anteros using the Dual Beam Imaging
Polarimeter on the University of Hawaii's 2.2 m telescope. Anteros is an L-type
asteroid that has not been previously observed polarimetrically. We find weak
but significant variations in the polarization of Anteros as a function of
rotation, indicating albedo changes across the surface. Specifically, we find
that Anteros has a background albedo of p_v = 0.18 +/- 0.02 with a dark spot of
p_v < 0.09 covering < 2% of the surface.Comment: Accepted to Icarus, 15 pages, 3 fig
Physical properties of ESA Rosetta target asteroid (21) Lutetia: Shape and flyby geometry
Aims. We determine the physical properties (spin state and shape) of asteroid
(21) Lutetia, target of the ESA Rosetta mission, to help in preparing for
observations during the flyby on 2010 July 10 by predicting the orientation of
Lutetia as seen from Rosetta.
Methods. We use our novel KOALA inversion algorithm to determine the physical
properties of asteroids from a combination of optical lightcurves,
disk-resolved images, and stellar occultations, although the latter are not
available for (21) Lutetia.
Results. We find the spin axis of (21) Lutetia to lie within 5 degrees of
({\lambda} = 52 deg., {\beta} = -6 deg.) in Ecliptic J2000 reference frame
(equatorial {\alpha} = 52 deg., {\delta} = +12 deg.), and determine an improved
sidereal period of 8.168 270 \pm 0.000 001 h. This pole solution implies the
southern hemisphere of Lutetia will be in "seasonal" shadow at the time of the
flyby. The apparent cross-section of Lutetia is triangular as seen "pole-on"
and more rectangular as seen "equator-on". The best-fit model suggests the
presence of several concavities. The largest of these is close to the north
pole and may be associated with large impacts.Comment: 17 pages, 5 figures, 3 tables, submitted to Astronomy and
Astrophysic
Migration of Jupiter-family comets and resonant asteroids to near-Earth space
We estimated the rate of comet and asteroid collisions with the terrestrial
planets by calculating the orbits of 13000 Jupiter-crossing objects (JCOs) and
1300 resonant asteroids and computing the probabilities of collisions based on
random-phase approximations and the orbital elements sampled with a 500 yr
step. The Bulirsh-Stoer and a symplectic orbit integrator gave similar results
for orbital evolution, but sometimes give different collision probabilities
with the Sun. A small fraction of former JCOs reached orbits with aphelia
inside Jupiter's orbit, and some reached Apollo orbits with semi-major axes
less than 2 AU, Aten orbits, and inner-Earth orbits (with aphelia less than
0.983 AU) and remained there for millions of years. Though less than 0.1% of
the total, these objects were responsible for most of the collision probability
of former JCOs with Earth and Venus. Some Jupiter-family comets can reach
inclinations i>90 deg. We conclude that a significant fraction of near-Earth
objects could be extinct comets that came from the trans-Neptunian region.Comment: Proc. of the international conference "New trends in astrodynamics
and applications" (20-22 January 2003, University of Maryland, College Park
A revised asteroid polarization-albedo relationship using WISE/NEOWISE data
We present a reanalysis of the relationship between asteroid albedo and
polarization properties using the albedos derived from the Wide-field Infrared
Survey Explorer. We find that the function that best describes this relation is
a three-dimensional linear fit in the space of log(albedo)-log(polarization
slope)-log(minimum polarization). When projected to two dimensions the
parameters of the fit are consistent with those found in previous work. We also
define p* as the quantity of maximal polarization variation when compared with
albedo and present the best fitting albedo-p* relation. Some asteroid taxonomic
types stand out in this three-dimensional space, notably the E, B, and M Tholen
types, while others cluster in clumps coincident with the S- and C-complex
bodies. We note that both low albedo and small (D<30 km) asteroids are
under-represented in the polarimetric sample, and we encourage future
polarimetric surveys to focus on these bodies.Comment: 16 pages, Accepted to Ap
Polarimetry and photometry of the peculiar main-belt object 7968 = 133P/Elst-Pizarro
133P/Elst-Pizarro is an object that has been described as either an active
asteroid or a cometary object in the main asteroid belt. Here we present a
photometric and polarimetric study of this object in an attempt to infer
additional information about its origin.
With the FORS1 instrument of the ESO VLT, we have performed during the 2007
apparition of 133P/Elst-Pizarro quasi-simultaneous photometry and polarimetry
of its nucleus at nine epochs in the phase angle range 0 - 20 deg. For each
observing epoch, we also combined all available frames to obtain a deep image
of the object, to seek signatures of weak cometary activity. Polarimetric data
were analysed by means of a novel physical interference modelling.
The object brightness was found to be highly variable over timescales <1h, a
result fully consistent with previous studies. Using the albedo-polarization
relationships for asteroids and our photometric results, we found for our
target an albedo of about 0.06-0.07 and a mean radius of about 1.6 km.
Throughout the observing epochs, our deep imaging of the comet detects a tail
and an anti-tail. Their temporal variations are consistent with an activity
profile starting around mid May 2007 of minimum duration of four months. Our
images show marginal evidence of a coma around the nucleus. The overall light
scattering behaviour (photometry and polarimetry) resembles most closely that
of F-type asteroids.Comment: Accepted by Astronomy and Astrophysic
Origin of volatiles in the Main Belt
We propose a scenario for the formation of the Main Belt in which asteroids
incorporated icy particles formed in the outer Solar Nebula. We calculate the
composition of icy planetesimals formed beyond a heliocentric distance of 5 AU
in the nebula by assuming that the abundances of all elements, in particular
that of oxygen, are solar. As a result, we show that ices formed in the outer
Solar Nebula are composed of a mix of clathrate hydrates, hydrates formed above
50 K and pure condensates produced at lower temperatures. We then consider the
inward migration of solids initially produced in the outer Solar Nebula and
show that a significant fraction may have drifted to the current position of
the Main Belt without encountering temperature and pressure conditions high
enough to vaporize the ices they contain. We propose that, through the
detection and identification of initially buried ices revealed by recent
impacts on the surfaces of asteroids, it could be possible to infer the
thermodynamic conditions that were present within the Solar Nebula during the
accretion of these bodies, and during the inward migration of icy
planetesimals. We also investigate the potential influence that the
incorporation of ices in asteroids may have on their porosities and densities.
In particular, we show how the presence of ices reduces the value of the bulk
density of a given body, and consequently modifies its macro-porosity from that
which would be expected from a given taxonomic type.Comment: Accepted for publication in MNRA
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