The perihelion activity of comet 67P/Churyumov-Gerasimenko as seen by robotic telescopes

Around the time of its perihelion passage the observability of 67P/Churyumov-Gerasimenko from Earth was limited to very short windows each morning from any given site, due to the low solar elongation of the comet. The peak in the comet's activity was therefore difficult to observe with conventionally scheduled telescopes, but was possible where service/queue scheduled mode was possible, and with robotic telescopes. We describe the robotic observations that allowed us to measure the total activity of the comet around perihelion, via photometry (dust) and spectroscopy (gas), and compare these results with the measurements at this time by Rosetta's instruments. The peak of activity occurred approximately two weeks after perihelion. The total brightness (dust) largely followed the predictions from Snodgrass et al. (2013), with no significant change in total activity levels from previous apparitions. The CN gas production rate matched previous orbits near perihelion, but appeared to be relatively low later in the year

Space missions to small bodies: asteroids and cometary nuclei

International audienceThe knowledge of the physical and dynamical properties, distribution, formation, and evolution of small bodies is fundamental to understand how planet formation occurred and, even more importantly, if and how these objects have played a role in the apparition of life on Earth. In the last century, asteroids began to no longer appear as starlike points of light in our telescopes, but to be resolved worlds with distinctly measurable sizes, shapes, and surface morphologies. Only in the last 25 years, the exploration of small bodies by spacecraft has begun and revealed objects widely diverse in formation region, evolution and properties (e.g. shape, albedo density, gravity, regolith size distribution, and porosity). In this paper we will provide a chronological analysis of comet nuclei and asteroids as revealed by space missions. The real breakthrough began with the ESA Giotto mission in 1986 to the comet Halley, while the latest JAXA Hayabusa mission was devoted to hover above the small asteroid Itokawa with a touch-and-go for a sample return of asteroidal regolith. Comet and asteroid science stands at the threshold of a new exceptional era, with many new missions to be devoted to these widely diverse and still poorly known small bodies

Polarisation de la lumière diffusée par des particules agrégées fractales

International audienceSome of the dust particles present in the solar system are likely to be aggregates. The study of polarization of the light scattered by such aggregated particles is performed for the six main formation processes. The analysis of the results shows that an irregular and fluffy structure of the particles allows a better understanding of the light scattering observations

Is 67P/Churyumov-Gerasimenko a classical JFC comet? Clues from recent polarimetric observations

Remote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance to determine the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. While dust observations have been made during different apparitions, polarization measurements were only obtained during the 1982 apparition by spectropolarimetry [1-2]. Recent imaging polarimetric observations were conducted at Haute-Provence observatory (France) on 2009 March 17-19 at 35 deg. phase angle and at IUCAA Girawali observatory (India) on 2008 December 25-27 at 36 deg. phase angle and on 2009 April 30-May 1 at 29 deg. phase angle. The imaging technique allows us to follow the intensity and polarization variations through the coma and their evolution. The decrease in intensity as a function of the distance to nucleus in log-log scale is close to -1 on average but important variations with values down to -1.5 are observed in agreement with previous observations in 1982-83 and 1995-96 [3]. Aperture polarization values are nominal before perihelion. Nevertheless, after perihelion, the increase in polarization suggests that an outburst occurred. Finally, comet 67P/C-G results will be compared to those obtained for other comets, including Jupiter Family Comets [4,5]. Polarization and intensity variations in the coma are reminiscent of those noticed for 9P/Tempel 1 (before Deep Impact) and comet C/2000 WM1 [5]. The presence of rather large particles can thus be suggested before and after perihelion. The properties of the particles ejected during post-perihelion will be discussed

Polarization observations of the dust ejected by comet 103P/Hartley 2 from OHP (France) and IUCAA (India)

EPSC-DPS2011-876Comet 103P/Hartley 2 was the target of the EPOXI mission. Remote observations from France and India, of the scattered light properties (and mainly its linear polarization) allowed us to determine an evolution of the physical properties of the dust particles in the coma. The properties displayed seem similar to those of other Jupiter Family comets (JFC)

Dust properties in comet 67P/Churyumov- Gerasimenko's coma during its 2008-2009 apparition from polarization observations

Comet 67P/Churyumov-Gerasimenko is the main target of the Rosetta mission. Remote observations from India and France, of the scattered light (and mainly its linear polarization) by the dust allowed us to conclude on different physical properties pre- and post-perihelion. Large dust particles are likely ejected from the surface pre-perihelion and small grains eventually in fluffy aggregates post-perihelion possibly originating from the subsurface

Polarimetric observations of comet 67P/Churyumov-Gerasimenko during its 2008-2009 apparition

International audienceRemote observations of the light scattered by comet 67P/Churyumov-Gerasimenko dust coma are of major importance for determining the physical properties of the particles and prepare the rendezvous with the ESA/Rosetta spacecraft in 2014. Aims. Light scattering and especially linear polarization observations allow comparison between different coma regions and different comets, including comets that have been studied by space probes. Our aim is to retrieve physical properties of the dust particles and to characterize their evolution around perihelion passage. Methods. Recent imaging polarimetric observations were conducted at the Haute-Provence observatory (France) on 2009 March 17-19 at 35° phase angle and at IUCAA Girawali observatory (India) on 2008 December 25-27 at 36° phase angle and on 2009 April 30-May 1 at 29° phase angle. With the imaging technique, the intensity and linear polarization variations are studied through the various coma regions. These observations are compared to other cometary data (e.g. Jupiter family comets) and to numerical and experimental simulations. Results. The decrease in intensity as a function of the distance to nucleus in log-log scale is on average close to -1, although important variations with values down to -1.5 are noticed, in agreement with previous observations in 1982-83 and 1995-96. The intensity along the tailward direction decreases with a slope between -1.2 two months before perihelion (2009 February 28) to -1.0 two months after perihelion, and the decrease is more pronounced in the sunward direction. Before perihelion, aperture polarization values are comparable to polarization values measured on other comets at similar phase angles. The sharp decrease in intensity and the feature in the tailward direction, without any difference in polarization in the coma before perihelion, could suggest the presence of large dark particles. The post-perihelion increase in intensity and in polarization suggests that an outburst has occurred. The freshly ejected dust polarizes more the scattered light and is more sensitive to the solar radiation pressure, suggesting small micron- or submicron-sized grains. Conclusions. Polarization and intensity variations in the coma of 67P/Churyumov-Gerasimenko are reminiscent of those noticed for some comets such as comet 81P/Wild 2 and comet 9P/Tempel 1. The presence of rather large particles can thus be suggested before and just after perihelion and the ejection of post-perihelion smaller grains, eventually in fluffy aggregates. A strong seasonal effect related to the obliquity of the comet suggests that the different grains originate in different hemispheres of the nucleus

Observations and laboratory simulations of asteroids by polarization measurements

International audienceWe present polarization observations of C-type asteroids and of (21) Lutetia, one of the targets of the Rosetta mission. The (21) Lutetia phase curves are compared to C-type and M-type asteroid phase curves. In a second part of the work, we use powdered meteorites to compare their polarization at two wavelengths to those observed for different asteroids, e.g., (21) Lutetia and (2867) Steins (the second asteroid observed by Rosetta)

Polarimetric observations and laboratory simulations of asteroidal surfaces:the case of 21 Lutetia

International audienceThe Rosetta spacecraft flew by 21 Lutetia on July 2010. This event provides a unique opportunity to enhance our knowledge of solar system small bodies, by comparing the surface properties measured in situ and the properties deduced from the linear polarization of scattered light, and prepare future observations. The linear polarization is studied as a function of the phase angle at different wavelengths and compared to phase curves of M-type and C-type asteroids. In a second part of the work, 21 Lutetia's polarization phase curves are compared to phase curves measured in the laboratory for powdered carbonaceous chondrites suggested as eventual analogs by spectroscopic studies. The importance of the variation of the linear polarization as a function of the wavelength is emphasized. CV3-class meteorite is found to be the best polarimetric laboratory analog with an average size of regolith grain lower than 50 μm