Asteroid lightcurve phase shift from rough‐surface shadowing

We have simulated asteroid lightcurves for simple shape models using a realistic surface scattering law. The scattering law includes a shadowing function computed with numerical ray‐tracing. We computed lightcurves in a variety of illumination geometries for both the traditional Lommel–Seeliger law and our seminumerical law. We observe a shift in the rotational phase of the lightcurves, which depends on the parameters of the scattering law as well as the illumination geometry and the direction of the spin axis of the asteroid. This phase shift is always zero at opposition, and can be as large as 10° for illumination geometries typical for Main Belt asteroids. The phase shift has implications on the accuracy of other results which are based on asteroid lightcurve analysis, such as spin‐state or shape determination.     

Collision Probability for Earth-Crossing Asteroids Using Orbital Ranging

We introduce new techniques for the computation of the collision probability for Earth-crossing asteroids in the case of short observational arcs and/or small numbers of observations. The techniques rely on the orbital element probability density computed using statistical orbital ranging. We apply the techniques to the Earth-crossing asteroid 1998 OX₄with non-vanishing collision probability in numerous close approaches after the year 2012 (inclusive). We study the invariance of the collision probability in transformations between different orbital element sets, and develop a Spearman rank correlation measure for the validity of the linear approximation. We introduce an optimized, fast version of the statistical ranging method.     

Asteroid photometric and polarimetric phase curves: Joint linear‐exponential modeling

Abstract— We present Markov‐Chain Monte‐Carlo methods (MCMC) for the derivation of empirical model parameters for photometric and polarimetric phase curves of asteroids. Here we model the two phase curves jointly at phase angles ≤25° using a linear‐exponential model, accounting for the opposition effect in disk‐integrated brightness and the negative branch in the degree of linear polarization. We apply the MCMC methods to V‐band phase curves of asteroids 419 Aurelia (taxonomic class F), 24 Themis (C), 1 Ceres (G), 20 Massalia (S), 55 Pandora (M), and 64 Angelina (E). We show that the photometric and polarimetric phase curves can be described using a common nonlinear parameter for the angular widths of the opposition effect and negative‐polarization branch, thus supporting the hypothesis of common physical mechanisms being responsible for the phenomena. Furthermore, incorporating polarimetric observations removes the indeterminacy of the opposition effect for 1 Ceres. We unveil a trend in the interrelation between the enhancement factor of the opposition effect and the angular width: the enhancement factor decreases with decreasing angular width. The minimum polarization and the polarimetric slope at the inversion angle show systematic trends when plotted against the angular width and the normalized photometric slope parameter. Our new approach allows improved analyses of possible similarities and differences among asteroidal surfaces.     

Polarimetric and Photometric Phase Effects Observed on Transneptunian Object (29981) 1999 TD10

The Kuiper-Belt Object (29981) 1999 TD₁₀, classified as a Scattered-Disk Object, has been observed at three different phase angles with the ESO 8.2-m VLT and FORS 1 instrument in polarimetric mode in November and December 2003. These observations have been used to compute the Stokes parameter q, which represents the linear polarization degree. We have also used the previously published photometric observations to improve the R-band phase function. The main conclusions are as follows: (i) a negative linear polarization degree decreasing with phase angle α up to, at least, α=3°, (ii) for α=3°, (iii) a possible color effect between the R and V band, the polarization degree being more negative in R. The R-band polarimetric observations can be explained by the coherent-backscattering mechanism and fitted by a two-component Rayleigh-scatterer model for a spherical small body. The rotation period of 15.382±0.001 h published by Mueller et al. (2004, Icarus 171, 506–515) and Choi et al. (2003, Icarus 165, 101–111) is confirmed. The R-band phase curve provides H=8.35±0.02 and G=−0.25±0.022 parameters with the IAU H–G formalism.Based on observations obtained at the Cerro Paranal observatory of the European Southern Observatory (ESO) in Chile.     

Slope variations on the surface of Phobos

It has been suggested that slope fluctuations on the scale of pixel dimensions could be determined by statistical photoclinometry. A closer study of the surface of Phobos reveals variations in the scattering properties of single particles and micro-structures formed by the particles. In the present context, the photoclinometric method of brightness moments is extended to account for these variations by allowing statistical fluctuations in the phase function of the assumed Lommel-Seeliger scattering law. The mean slope on the investigated regions of Phobos has been found to vary from approximately 12 degrees on a 61m scale to approximately 7 degrees on a 216-272m scale. On the same scales, a value of the order of 2% has been obtained for the standard deviation of the scattering phase function. Hints of a fractal-like scale-invariance have been noticed in the covariance function of brightness.     

Polarimetry in Planetary Science—A Step Forward with the VLT and a Need for the ELTs

We present a brief review of polarimetric measurements of solar system objects, both linear and circular, obtained with the FORS1 instrument at the Very Large Telescope VLT over the past years. A number of first and new results have been obtained by using this unique observing mode at an 8 m class telescope, among them polarimetry of faint planetary bodies like near-Earth asteroids, Kuiper Belt objects and cometary nuclei, spectropolarimetry of cometary coma material and of the Earthshine of the Moon (in order to verify that life exists on Earth!). We outline the science cases for planetary polarimetry at a future Extremely Large Telescope ELT and provide high level requirements for polarimetric equipment to be used at the ELTs for the study of the science cases described.     

Asteroid orbits using phase-space volumes of variation

We present a statistical orbit computation technique for asteroids with transitional observational data, that is, a moderate number of data points spanning a moderate observational time interval. With the help of local least-squares solutions in the phase space of the orbital elements, we map the volume of variation as a function of one or more of the elements. We sample the resulting volume using a Monte Carlo technique and, with proper weights for the sample orbital elements, characterize the six-dimensional orbital-element probability density function. The volume-of-variation (VOV) technique complements the statistical ranging technique for asteroids with exiguous observational data (short time intervals and/or small numbers of observations) and the least-squares technique for extensive observational data. We show that, asymptotically, results using the new technique agree closely with those from ranging and least squares. We apply the technique to the near-Earth object 2004 HA₃₉, the main-belt object 2004 QR and the transneptunian object 2002 CX₂₂₄ recently observed at the Nordic Optical Telescope on La Palma, illustrating the potential of the technique in ephemeris prediction. The VOV technique helps us assess the phase transition in orbital-element probability densities, that is, the non-linear collapse of wide orbital-element distributions to narrow localized ones. For the three objects above, the transition takes place for observational time intervals of the order of 10 h, 5 d and 10 months, respectively, emphasizing the significance of the orbital-arc fraction covered by the observations.     

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESA's SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.     

Transneptunian Object Ephemeris Service (TNOEPH)

We present a web service called TNOEPH ( http://asteroid.lowell.edu/)for ephemeris uncertainty prediction and dynamical classification ofshort-arc trans\-neptunian objects (TNOs). User-supplied observations are transformed to a rigorous sky-plane uncertainty map using the technique of statistical orbital ranging. We show examples of thegrowth of ephemeris uncertainty with time, and give the probabilities ofdifferent dynamical classifications for a few short-arc TNOs.     

Asteroid photometric and polarimetric phase curves: empirical interpretation

A method for interpretation of asteroid phase curves, based on empirical modeling and laboratory measurements, is outlined and preliminary results are presented. A linear-exponential function is used to describe the opposition peaks and negative polarization surges of various asteroids and laboratory samples and a statistical algorithm is used in parameter estimation. The linear-exponential function describes well the phase curves, but dense phase angle coverage, particularly at small phase angles must be obtained to improve the results. Major emphasis should also be put on laboratory study: with an extensive library of laboratory measurements, a stronger connection between the phase curve properties and surface characteristics is possible.