39 research outputs found
Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by
A wide observational campaign was carried out in 2004-2009 aimed to complete
the ground-based investigation of Lutetia prior to the Rosetta fly-by in July
2010. We have obtained BVRI photometric and V-band polarimetric measurements
over a wide range of phase angles, and visible and infrared spectra in the
0.4-2.4 micron range. We analyzed them together with previously published data
to retrieve information on Lutetia's surface properties. Values of lightcurve
amplitudes, absolute magnitude, opposition effect, phase coefficient and BVRI
colors of Lutetia surface seen at near pole-on aspect have been determined. We
defined more precisely parameters of polarization phase curve and showed their
distinct deviation from any other moderate-albedo asteroid. An indication of
possible variations both in polarization and spectral data across the asteroid
surface was found. To explain features found by different techniques we propose
that (i) Lutetia has a non-convex shape, probably due to the presence of a
large crater, and heterogeneous surface properties probably related to surface
morphology; (ii) at least part of the surface is covered by a fine-grained
regolith with particle size less than 20 microns; (iii) the closest meteorite
analogues of Lutetia's surface composition are particular types of carbonaceous
chondrites or Lutetia has specific surface composition not representative among
studied meteorites
YORP and Yarkovsky effects in asteroids (1685) Toro, (2100) Ra-Shalom, (3103) Eger, and (161989) Cacus
The rotation states of small asteroids are affected by a net torque arising
from an anisotropic sunlight reflection and thermal radiation from the
asteroids' surfaces. On long timescales, this so-called YORP effect can change
asteroid spin directions and their rotation periods. We analyzed lightcurves of
four selected near-Earth asteroids with the aim of detecting secular changes in
their rotation rates that are caused by YORP. We use the lightcurve inversion
method to model the observed lightcurves and include the change in the rotation
rate as a free parameter of optimization. We
collected more than 70 new lightcurves. For asteroids Toro and Cacus, we used
thermal infrared data from the WISE spacecraft and estimated their size and
thermal inertia. We also used the currently available optical and radar
astrometry of Toro, Ra-Shalom, and Cacus to infer the Yarkovsky effect. We
detected a YORP acceleration of for asteroid Cacus. For
Toro, we have a tentative () detection of YORP from a significant
improvement of the lightcurve fit for a nonzero value of . For asteroid
Eger, we confirmed the previously published YORP detection with more data and
updated the YORP value to . We also updated the shape model of
asteroid Ra-Shalom and put an upper limit for the change of the rotation rate
to . Ra-Shalom has a greater than
Yarkovsky detection with a theoretical value consistent with observations
assuming its size and/or density is slightly larger than the nominally expected
values
Analysis of the rotation period of asteroids (1865) Cerberus, (2100) Ra-Shalom, and (3103) Eger - search for the YORP effect
The spin state of small asteroids can change on a long timescale by the
Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, the net torque that arises
from anisotropically scattered sunlight and proper thermal radiation from an
irregularly-shaped asteroid. The secular change in the rotation period caused
by the YORP effect can be detected by analysis of asteroid photometric
lightcurves. We analyzed photometric lightcurves of near-Earth asteroids (1865)
Cerberus, (2100) Ra-Shalom, and (3103) Eger with the aim to detect possible
deviations from the constant rotation caused by the YORP effect. We carried out
new photometric observations of the three asteroids, combined the new
lightcurves with archived data, and used the lightcurve inversion method to
model the asteroid shape, pole direction, and rotation rate. The YORP effect
was modeled as a linear change in the rotation rate in time d\omega /dt. Values
of d\omega/ dt derived from observations were compared with the values
predicted by theory. We derived physical models for all three asteroids. We had
to model Eger as a nonconvex body because the convex model failed to fit the
lightcurves observed at high phase angles. We probably detected the
acceleration of the rotation rate of Eger d\omega / dt = (1.4 +/- 0.6) x
10^{-8} rad/d (3\sigma error), which corresponds to a decrease in the rotation
period by 4.2 ms/yr. The photometry of Cerberus and Ra-Shalom was consistent
with a constant-period model, and no secular change in the spin rate was
detected. We could only constrain maximum values of |d\omega / dt| < 8 x
10^{-9} rad/d for Cerberus, and |d\omega / dt| < 3 x 10^{-8} rad/d for
Ra-Shalom
Photometry of asteroids: Lightcurves of 24 asteroids obtained in 1993â2005
The results of 1993â2005 photometric observations for 24 main-belt asteroids: 24 Themis, 51 Nemausa, 89 Julia, 205 Martha, 225 Henrietta, 387 Aquitania, 423 Diotima, 505 Cava, 522 Helga, 543 Charlotte, 663 Gerlinde, 670 Ottegebe, 693 Zerbinetta, 694 Ekard, 713 Luscinia, 800 Kressmania, 1251 Hedera, 1369 Ostanina, 1427 Ruvuma, 1796 Riga, 2771 Polzunov, 4908 Ward, 6587 Brassens and 16541 1991 PW18 are presented. The rotation periods of nine of these asteroids have been determined for the first time and others have been improved
Opposition polarimetry and photometry of the low albedo asteroid 419 Aurelia
International audienc