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Context. Photometric surveys of transNeptunian objects (TNOs) and Centaurs have suggested possible correlations between some orbital parameters and surface colors of classical objects, scattered disk objects (SDOs), and Centaurs. However, larger sample sizes are needed in order to corroborate or rule out the possible correlations and find some possible new ones. The implications from these correlations for the formation and evolution of classical TNOs, SDOs, and Centaurs are important to understand possible physico-chemical coloring mechanisms and their influence on the surfaces of the TNOs and Centaurs, as well as their evolutionary history.

Context. One of the main scientific objectives of NASA's New Horizons mission is to map the icy surface compositions of Pluto and its moon Charon. The encounter will be in 2015. Meanwhile remote observations from earth and space are the most suitable means to enhance further our knowledge of the Pluto/Charon system.

Aims. The Jupiter family comet 67P/Churyumov-Gerasimenko (67P/C-G) is the target comet of ESA's ROSETTA mission. A detailed portrait of this comet has been drawn from observations around perihelion, but what needs to be enhanced is the description of the comet's behaviour at large heliocentric distance. It is not only important for planning the rendezvous of the ROSETTA spacecraft with the comet, but will also provide valuable information on basic physical properties of the nucleus and the possible distant activity of JFCs.

On 4 July 2005 at 05:52 UT, the impactor of NASA's Deep Impact (DI) mission crashed into comet 9P/Tempel 1 with a velocity of about 10 ${\rm km\,s}^{-1}$. The material ejected by the impact expanded into the normal coma, produced by ordinary cometary activity. The La Silla and Paranal sites of the European Southern Observatory (ESO) in Chile participated in the world-wide campaign to observe this event.

Context.On 4 July 2005 at 05:52UT the impactor of NASA's DeepImpact (DI) mission exploded at comet 9P/Tempel 1. The ejecta material of the impact expanded into the coma that is produced by normal cometary activity. The La Silla and Paranal sites of the European Southern Observatory ESO in Chile participated in the world-wide campaign to observe this event.

Context.Comet 9P/Tempel 1, the target of the Deep Impact (DI) Mission, has been monitored from June 18 until July 12, 2005, as a follow up to the long-term campaign started in Jan. 2005.

Comet 9P/Tempel 1, the target of the Deep Impact Mission, has been monitored for 6 months aiming at its characterization before the impact experiment. During this period of time, the coma has gone through a slow morphological evolution from a wide structure in the south-western quadrant in mid-February to a porcupine pattern in mid-April and up to seven features identified in June. In addition to this evolution, an arclet in the western coma hemisphere was detected on June 14, related to an outburst event. Interpretation of these features and their evolution seems to indicate the presence of at least 3 or 4 very active regions on the nucleus, consistent with the rotation axis being oriented close to previous solutions found by Belton et al. (2005), or close to the angular momentum vector of the orbital motion of the comet. The value of $Af{\rho}$varies with heliocentric distance as $r_{\rm h}^{-6.71}$; slightly enhanced $Af{\rho}$(above the $r_{\rm h}^{-6.71}$curve) was observed from mid-February until the end of March, when fan-shaped structures appeared in the coma for the first time. Somewhere between mid-April to mid-May (i.e. 80 to 60 days before perihelion), the comet peaked in activity. In terms of gas production rates, CN, C2and C3have been obtained at $r_{\rm h} \sim 1.7$, 1.60 and 1.51 AU, being slightly below those derived from previous passages. Abundance ratios of these species indicate that comet 9P/Tempel 1 is classified as typical in terms of C2abundance. The surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in north–south direction from the long-slit spectra can be well fit with $-1.9 \le m \le -1.14$in $\log B - \log \rho$representation. Steeper slopes are obtained at larger rhwhich might be related to variable dust size distribution with distance from the nucleus due to the radiation pressure dynamics and/or physical processing of the dust grains (sublimation, fragmentation). Normalized color of the dust inside the coma in the north–south direction is measured to be $\rm {\sim}20{-}30\% / 100~nm$.

Comet C/2000 WM1(LINEAR) was observed at ESO at the time of its closest approach to the Earth at the beginning of December 2001 (geocentric distance: 0.32 AU). The aim of the observations was characterization of the solid component of the coma. Observations have been acquired over a wide spectral range, from the visible to the thermal infrared using three telescopes simultaneously. In this paper we discuss the possible discovery of two sublimating components in the coma, one of them with scattering properties very different from those of the “common” cometary dust. We provide evidence that these two components possibly originate from different kinds of organic material.

We have investigated the dust and gas coma of the comet C/2000 WM1 (LINEAR) during Dec. 2-4, 2001, its closest approach to Earth. The gaseous coma is slightly asymmetric in CN and C2, indicating the presence of some structures which are detected either by applying enhancement techniques or studying the azimuthal distribution of material. This asymmetry is due to the existence of a double jet in a direction almost perpendicular to the Sun–comet direction. These structures are not clearly detected in C3. The morphological analysis of the dust coma shows a faint, short and narrow sunward structure, besides the dust tail in a position angle of ~60°.
CN is produced at a rate of $2.43 \times 10^{26}$$\rm s^{-1}$with lifetimes that can be considered as representative of HCN being the parent species. In the case of C2, the use of usual lifetimes ($\tau_{\rm p},\tau_{\rm d}$) requires a modified two-steps photolytic model to account for the pronounced flatness of the column density profile in the inner coma. Usual steady-state model gives rise to a $\rm Q_{C_2} = 3.11 \times 10^{26}$$\rm s^{-1}$. Other gas species as C3and NH2have been also detected from the spectrum, although with a lower S/N. The computed production rates are $3.6 \times 10^{24}$$\rm s^{-1}$and $2.15 \times 10^{26}$$\rm s^{-1}$, respectively. Determination of the CN and C2production rates by means of the Haser modeling (Haser [CITE]) with customary scalelengths indicates that C/2000 WM1 is a C2-enriched comet at 1.178 AU heliocentric distance. The dust production rate, parameterised by $ A(\theta) f \rho$, is ~$300{-}400$cm when measured in two continuum regions, centered at 4845 and 6840 Å, whilst this value is slightly lower in Bessel Rand Gunn i(~230 cm). Thus, the gas-to-dust mass ratio is in the order of 6.3, a relatively gassy comet.
The surface brightness profiles of the continuum, either azimuthally averaged profiles from the broadband images or in east-west direction from the long-slit spectrum, can be well fitted with $m \approx 1$in $\log~B - \log~\rho$representation. On the other hand, the study of the dust coma as imaged with the broadband and narrowband filters reveals that variations in the size and/or composition of the grains might be occurring while traveling outward. This fact is manifested as a sharp decrease of $\sum A f$vs. ρ at $700 \le \rho \le 15\,000$km. The fit of these $\sum A f$profiles vs. ρ points to the existence of two different grain populations with their own scattering properties and scalelengths (i.e. lifetimes). These pronounced variations of $\sum A f$vs. ρ are accompanied by clear dust color gradients in the 2D maps, unlike the dust color vs. ρ as derived from the spectrum. Since spectroscopic measurements are only obtained in east-west direction and with a very long exposure time, the effect of color variations inside the coma due to disintegrating grains is difficult to detect.

The Large Program on physical studies of TNOs and Centaurs, started at ESO Cerro Paranal on April 2001, has recently been concluded. This project was devoted to the investigation of the surface properties of these icy bodies through photometric and spectroscopic observations. In this paper we present the latest results on these pristine bodies obtained from the spectrophotometric investigation in the visible range. The newly obtained spectrophotometric data on 3 Centaurs and 5 TNOs, coming from 2 observing runs at the Very Large Telescope (VLT), show a large variety of spectral characteristics, comprising both gray and red objects in the two different populations. A very broad and weak absorption feature, centered around 7000 Å , has been revealed in the spectrum of the gray TNO 2003 AZ84. This absorption is very similar to a feature observed on low albedo main belt asteroids and attributed to the action of the aqueous alteration process on minerals. This process was previously also claimed as the most plausible explanation for some peculiar visible absorption bands observed on 2000 EB173 and 2000 GN171 in the framework of the Large Program (Lazzarin et al. [CITE]; de Bergh et al. [CITE]). This detection seems to reinforce the hypothesis that aqueous alteration might have taken place also at such large heliocentric distances.
We also report the results of a spectroscopic investigation performed outside the Large Program on the very interesting TNO 2000 GN171 during part of its rotational period. This object, previously observed twice in the framework of the Large Program, had shown during the early observations a very peculiar absorption band tentatively attributed to aqueous alteration processes. As this feature was not confirmed in a successive spectrum, we recently repeated the investigations of 2000 GN171, finding out that it has an heterogeneous composition. Finally an analysis of the visible spectral slopes is reported for all the data coming from the Large Program and those available in literature.

Comet Lee (C/1999 H1) was observed on June 6, 1999 when it was at rh= 0.98 AU and $\Delta = 1.195$AU. The spectrophotometric observations, between 0.6 and 1 μm, were aimed at the detection of the CI(1D) doublet $\lambda \lambda$9823/9850 Å. The non-detection of these lines, with a $3 \sigma$flux upper limit of the order of $4.6 \times 10^{-17}$$\rm erg\,cm^{-2}\,s^{-1}$, confirms the CO depletion already noted by other authors. Several CN and NH2emission bands lie in that spectral range, making it possible to derive production rates for both species as ~$ 3.1 \times 10^{26}$$\rm s^{-1}$and $1.2 \times 10^{27}$$\rm s^{-1}$, respectively. The oxygen forbidden line at 6300 Å was used to obtain $Q_{\rm H_2O} = (1.22 \pm 0.7) \times 10^{29}$$\rm s^{-1}$. Assuming that CN and NH2are directly produced by HCN and NH3, Comet Lee has a $\rm HCN/H_2O \approx 0.25\%$and $\rm NH_3/H_2O \approx 1\%$at rh= 0.98 AU, in reasonable agreement with what has been found in other long-period comets. The structural analysis carried out on cometary images acquired with broad band RBessel filter clearly displays two pairs of ion rays likely produced by the H2O+doublet at 6198 and 6200 Å, wavelengths covered by the bandpass filter. Identical features are found in the images acquired with the Gunn ifilter.
The dust brightness profiles in the east-west direction do not deviate from a $\rho^{-m}$law (with $0.7 < m \le 1.2$) as expected for a steady state model coma with a constant dust production rate and expanding at constant velocity. The dust production rate, as obtained from the $A f \rho$parameter, is ~500 cm, which compared with the gas production rate classifies this comet as a dust poor one with relatively high (6.5–11.7) gas-to-dust mass ratio. Analysis of the normalized reflectivity gradient (i.e. continuum color) as a function of ρ indicates a slight reddening of the solid component in the coma at large cometocentric distances, whereas the average dust color within an aperture of 20 000 km, centered at the nucleus, is ~$9\%$per 1000 Å. Mie scattering computations applied to an ensemble of particles indicate that the dust coma is characterised by a relative broad size distribution with a typical mean size of 1 μm. These grains might be composed of a mixture of silicates and icy material.

We have performed a Near Earth Asteroid search and follow-up test beyond 22nd magnitude with the 2.2-m MPG/ESO and the New Technology Telescope (NTT) facilities at La Silla. The experiment comprised a total number of 4 nights at the 2.2-m telescope and 3 nights at the NTT on two separate runs. In addition to the discovery of two NEAs and the recovery of many more, this pilot program has shown the advantages as well as the problems of a dedicated program using much larger facilities than the ones currently used worldwide. We confirm the results of Jedicke et al. ([CITE]), that by observing at fainter magnitudes and finding objects at larger distances, such a system will discover km-sized NEAs with higher orbital eand ias well as a larger proportion of the smaller NEAs; moreover, it will shorten the time needed to reach 90% completeness for km-sized objects. The pilot program also evidenced the need for follow-up facilities compatible with the discovery telescopes.

We present simultaneous visible-near-IR (BVRIJHK)photometry of 8 Kuiper belt objects (KBOs) and 4 Centaurs. The observations were carried out using two 8m-units of the ESO/Very Large Telescope operated simultaneously on November 29 and 30, 2000. The KBOs displayed linear reflectivity spectra in the optical with a wide range of slopes from neutral (solar) to very red. In most cases, the reflectivity spectra are linear from Bto Jbands. We notice a change of regime in the spectra which flatten toward the Hand Kbandpasses. (33128) 1998 BU48even has a significantly negative $H-K$value. Only (54598) 2000 QC243does not flatten in the infra-red. (24835) 1995 SM55has a noticeably blue spectrum in the near-IR which might be indicative of water ice absorption features. We also present an empirical model for KBO surface evolution, based on models from [CITE] and [CITE], in which surface colors evolve because of collisions. In the model presented here we propose that cometary activity can refresh the surface. This model predicts that (1) small (e.g. ≪80 km in radius) KBOs should have color variations with rotation, (2) the larger objects should display uniform colors, (3) there should be no color dependence with inclination for objects >100 km in radius, and (4) very distant KBOs are fully irradiated.

In April 2001 we obtained visible and near-infrared spectra, as well as photometric data, for two Trans-Neptunian Objects (TNOs), 2000 GN171(now numbered 47932) and 2000 EB173(now designated 38628 Huya), which belong to the dynamical class of Plutinos. These observations were made with the FORS1 and ISAAC instruments at the European Southern Observatory-Very Large Telescope (ESO-VLT). The detection of weak absorption features in the visible spectra of these two Trans-Neptunians has been reported elsewhere (Lazzarin et al. [CITE]). In this paper, we discuss the interpretation of the features, which are different for the two objects, and we present some complementary observations in the near-infrared, as well as more recent (May 2002) visible spectra in which the features are absent. Although the visible colors of the two objects are comparable, the near-infrared spectra are different: (47932) 2000 GN171shows a nearly flat spectrum, except in the Hband where a broad absorption appears, whereas the spectrum of (38628) Huya (2000 EB173) has a red slope in the Jband and some absorption beyond 2 micron. The features detected in the visible spectra of the two objects are tentatively attributed to the presence of iron oxides or phyllosilicates at the surfaces of the two objects. The differences between the April 2001 and May 2002 visible spectra are attributed to spatial variations at the surfaces of the objects. We briefly discuss possibilities for aqueous alteration in TNOs, after reviewing what we know about the presence of aqueously altered minerals in other small bodies of the solar system. Further studies monitoring the rotation of these two objects are highly desirable.

The Plutino 28978 Ixion, one of the largest objects in the Kuiper Belt, is measured by Rfilter photometry and linear polarimetry over the phase angle range as well as by BVRIcolours and visible spectroscopy. Ixion is a medium red object with spectral slope of 17.7%/100 nm (400-900 nm). While its opposition phase curve surge (0.2 mag/deg) is within the typical range found for other solar system objects, it displays unusually high negative polarization (min. -1.3%). Comparison with model spectra and numerical modeling of the opposition brightening and polarization effects suggest an areal mixture of absorbing (dark) and icy (bright) compounds on its surface: the mixing ratio is about 6:1 for dark and bright material of 0.21 and 0.79 single-scattering albedo and of 250 and 33 dimensionless mean free path (length multiplied by wave number), respectively.

We revisit the Trans–Neptunian Objects (TNOs) color controversy allegedly solved by Tegler & Romanishin (2003). We debate the statistical approach of the quoted work and discuss why it can not draw the claimed conclusions, and reanalyze their data sample with a more adequate statistical test. We find evidence for the existence of two color groups among the Centaurs. Therefore, mixing both centaurs and TNOs populations lead to the erroneous conclusion of a global bimodality, while there is no evidence for two color groups in the TNOs population alone. We use quasi–simultaneous visible color measurements published for 20 centaurs (corresponding to about half of the identified objects of this class), and conclude on the existence of two groups. With the surface evolution model of Delsanti et al. (2003). we discuss how the existence of two groups of Centaurs may be compatible with a continuous TNOs color distribution.

Comet C/2001 Q4 (NEAT) was observed inbound at an heliocentric distance of 8.6 AU. Broadband filter images in the visible and long slit spectroscopy in the 3500–7000 Å range was recorded. No sign of CN emission was detected and an upper limit for CN production rate of $1.4\times10^{25}$molecules s-1is estimated. The color of the dust in the inner coma is very red with a spectral index of 29%/1000 Å. The radial profile of the dust coma displays steep gradients of 1.7 (inner part) to 2.4 (outer part). The afρ value, a proxy for the dust production, is measured to be 1500 cm. The coma shows a weak and short conic structure in the northern hemisphere that is interpreted as dust fan produced by an active region on the nuclear surface.

We present thermal observations of MUSES-C mission target, (25143) 1998 SF36. The mid-IR radiation in the N-band (11.9 μm) was measured using the ESO 3.6 m telescope with TIMMI2. Assuming the Standard Thermal Model and combining the thermal flux in the N-band with the time-resolved absolute magnitude in optical, the derived diameter and the geometric albedo are 0.35 (±0.03) km and 0.23 (+0.07, -0.05), respectively. While these values are model-dependent, we conclude that this Apollo class asteroid is of small size and its albedo is consistent with spectroscopic classification as an S-type asteroid.

We observed comet C/1995 O1 (Hale-Bopp) at 4.6–2.9 AU pre-perihelion and 2.8–12.8 AU post-perihelion with optical long-slit spectroscopy. Emission bands of CN, C3, C2and NH2have been covered. Emission of C3was detected up to 7.0 AU, and CN could be followed up to 9.8 AU post-perihelion. Spatial column density profiles of the radicals have been used to derive effective parent Haser scale lengths for heliocentric distances beyond 3 AU. Production rates were derived based on these Haser scale lengths. The observations of CN are in agreement with HCN as the major parent molecule of this radical at large distances from the Sun (i.e. beyond ~3 AU). We compare the measured CN production rate to sublimation rates of HCN from a simple nucleus sublimation model. The variation of CN production rates with changing heliocentric distance gives no indication for sublimation from the interior and is consistent with very little thermal lag of the nucleus.

We present the results of our monitoring of comet 81P/Wild 2, the target of the Stardustmission. Broad-band filter images and medium resolution spectra of this comet obtained along its pre- and postperihelion orbit between 1996 and 1998 were analysed in terms of coma morphology and activity. The morphological coma investigation revealed the presence of long-lasting fan structures, which remained essentially unchanged for at least three months. The ($B-V$) and ($V-R$) colour indices were determined at three different positions along the orbit. The two-dimensional analysis showed a uniform colour over the entire coma. Hence, the material composing the structures is not different from that of the underlying coma. The distribution of coma brightness as a function of projected distance from the nucleus shows no peculiarities. The evolution of the comet's activity during the pre- and postperihelion phase appears to be asymmetric in that activity increases faster preperihelion than it decreases postperihelion. The production rates (or their upper limits in the case on a non-detection) of CN, C2, C3, and NH2were determined from the spectrophotometric observations and compared to other values published in the literature.

We present the first results of BVRIphotometry of Transneptunian Objects (TNOs) and Centaurs obtained through the ESO Large Program on physical studies of these icy bodies in the outer solar system. In total 28 objects were observed of which 18 are new measurements. Combining our new BVRIphotometry with the data summary published by Hainaut & Delsanti ([CITE]) results in a database of 94 objects: 45 Cubewanos, 22 Plutinos, 13 scattered disk objects, 14 Centaurs. The reddening range seems to be similar among the four dynamical classes (-5 to 55%/l00 nm) and only one outlier (1994 ES2) exists. The spectral gradient distribution of the Cubewanos peaks between 25 to 35%/l00 nm, while for the three other types the maximum seems to fall below 20%/l00 nm. A clustering of red Cubewanos with perihelia beyond ~41 AU in low eccentricity and low inclination orbit suggests that these objects are less affected by the physical processes that potentially produce neutral colors, i.e. resurfacing by collision and by intrinsic activity. For Cubewanos and scattered disk objects, the range of reddening increases with decreasing perihelion distance and with increasing orbital excitation. A correlation of the spectral slope with inclination is present for Cubewanos and scattered disk objects, and is non-existent for the other dynamical types. It is unclear whether these trends (or their absence) are discriminative for the correctness of the resurfacing scenarios. If intrinsic activity is responsible for resurfacing, the start of the effect inside ~41 AU from the Sun may be indicative for the driving agent, while in the collision scenario the survival of the red Cubewano cluster in the central region of the Kuiper-Belt argues for the existence of a population of bodies the surface of which is heavily radiation processed without impact resurfacing.

We present photometric and spectroscopic data obtained through visible and near-infrared observations of the Centaur 32532 (2001 PT13) performed with FORS1 and ISAAC at the Very Large Telescope (VLT-ESO) within the framework of an ESO large program on the Trans-Neptunian objects (TNOs) and Centaurs. The results show evidence for a difference in the near-infrared spectral behaviour obtained during two observations carried out one month apart. In one spectrum there is the possible presence of signatures of water ice in small amounts. Two models have been proposed to interpret the surface composition of this Centaur.

Comet 46P/Wirtanen, prime target of ESA's ROSETTA mission, was successfully observed at the Very Large Telescope Observatory in Chile: on 17 May 1999 with the Test Camera at the Cassegrain focus of the 8.2-m VLT Kueyen telescope and on 8 December 2001 with FORS1 at Unit Telescope 4 Yepun. May 1999: no coma was detected at heliocentric distance $r = 4.98$AU. From the measured brightness in the Bessell R-filter, a mean nucleus radius of $555\pm40$m is derived (for a geometric albedo of 0.04 and a phase darkening of 0.04 mag/deg). The nucleus signal varies during the 2.7 h observing interval and a peak-to-peak amplitude of ~0.38 mag is determined. The measured lightcurve is in agreement with a rotation period of 6–7.5 hours and a ratio of the main nucleus axes of at least 1.4. The non-detection of a coma allows one to put an approximate upper limit for Afρ of <0.45 cm (suggesting a dust production rate of 0.05 kg/s). December 2001: a weak and condensed coma seems to be present in the seeing disk of the comet at 2.9 AU inbound, causing a higher brightness than expected from the previous size estimates of the nucleus. The colour of the comet appears very red ($V-R$spectral gradient ~47%/100 nm). The Afρ value of the comet was $6.5\pm2$cm (equivalent to a dust production rate of about 1 kg/s).

We performed high resolution imaging of 1996 TO66in Vand Rband to verify and monitor the lightcurve change, to address a colour change over a rotation and to search for a cometary activity. No activity was detected at the 29 mag/sq.arcsec level with 5400 s integration time with the ESO-VLT. Combining the data in Nov. and Dec. 1999, we obtained a complete rotation period coverage. The lightcurve was a single-peaked, with an amplitude of 0.21 mag in R. The ($V - R$) colour displays the inhomogeneity of TNO surface. TNO's patchy surface may be cause the intensity change of water absorption feature in near-IR spectra. These observations can be the starting points for a challenging work of surface mapping of TNOs.

We present visible (BVRI) photometric measurements of 27 Trans-Neptunian objects, obtained with the FORS2 instrument on the ESO/8 m Very Large Telescope (Unit 2) from runs during September and November 2000. The objects display a broad and continuous range of colors from neutral-bluish to very red. Most of the objects also have a linear reflectivity spectrum over the VRIrange. There is no evidence of a bimodal color distribution as has been previously reported by other groups. Several objects (1994 TB, 1995 SM55, 1998 UR43, 1999CF119and 2000 OK67) show evidence of changing color, and should be investigated further. The object 1995 SM55deserves a more complete study since its neutral-blue colors suggest it could be a good candidate for hosting cometary activity.

We present visible (BVRI) and near-IR (JHKs) broadband photometry and visible low-dispersion spectroscopy of Transneptunian Objects (TNOs) and Centaurs. In total, 16 TNOs and 1 Centaur were observed over the past two years at ESO telescopes in La Silla and Paranal in Chile as well as at the Calar Alto Observatory in Spain. The sample consists of objects measured for the first time and those for which comparison data is available from literature. The targets were: 1992QB1, 1993RO, 1994EV3, 1995HM5, 1995SM55, 1996RQ20, 1996TL66, 1996TO66, 1996TP66, 1997CQ29, 1997CS29, 1998HK151, 1998TF35, 1998VG44, 1998WH24, 1998XY95, 1999TC36. The spectra of 5 TNOs (1995SM55, 1996TO66, 1997CQ29, 1997CS29, 1998HK151) show almost constant gradients over the visible wavelength range with only marginal indication for a flatter slope beyond 750-800 nm. The photometric colour gradients obtained quasi-simultaneously are in good agreement with the spectral data. This suggests that in general photometric colour gradients are a valuable diagnostic tool for spectral type classification of TNOs. The photometric study revealed a number of new objects with neutral and red colours. For re-measured objects the published broadband colours were -in general -confirmed, although a few remarkable exceptions exist. Two TNOs appear to be outlyers according to the available broadband colours: 1993EV3 and 1995HM5. 1995SM55 is the bluest TNO measured so far. No clear global correlation between $V-I$colour and absolute Rfilter brightness of our TNO targets is found. However, the data for the 5 brightest TNOs (brighter than 5 mag absolute magnitude) could also be interpreted with a linear increase of $V-I$colour by about 0.75 mag per brightness magnitude. The colour-colour diagrams show continuous reddening of the TNOs in $V-R$vs. $B-V$, $R-I$vs. $B-V$and $R-I$vs. $V-R$. The bimodality suggested from earlier measurements of Tegler & Romanishin ([CITE]) is not confirmed. According to our colour gradient statistics (number of objects per gradient interval) most of the TNOs have surface reddening between 0 and 40% /100 nm. For the Cubewanos the major population falls between 20-40% /100 nm. The Plutinos and Centaurs show a bifold grouping, i.e. a neutral/slightly reddish group (reddening <20% /100 nm) and a red group (reddening 30-40% /100 nm). The statistical significance of the various populations found is suffering -for the Centaurs and scattered disk objects very severely - from the small number of objects measured. However, the diversity of the reddening distribution of Centaurs/Plutinos and Cubewanos, if confirmed by new observations, may indicate a different balancing of resurfacing processes for these object types: for instance, for Centaurs a possibility is that re-condensed frost from coma activity may be dominant over impact re-surfacing and high energy surface processing.

Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on board ESA's Infrared Space Observatory (ISO) between 4.6 and 2.8 AU. Each time, broadband photometry was performed using 4 different detectors, 5 apertures and 10 filters covering the range between 3.6 and 170 μm. Background observations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the data reduction steps are described in some detail, including the techniques to correct for variable detector responsivity. The resulting inband power values of the Hale-Bopp observations and their uncertainties are given. The mean uncertainty is 25% . The final fluxes were computed, taking into account the zodiacal background, possible offset of the comet's position from the center of the aperture, the brightness distribution within the coma, and the spectral energy distribution of the comet's emission. Strong thermal emission from a broad size distribution of dust particles was detected in all of the data sets, even at $r=4.6$-4.9 AU pre-perihelion and 3.9 AU post-perihelion; the total thermal energy varied as r-3. The 7.3-12.8 μm color temperature was ~1.5 times the blackbody temperature, higher than that observed in any other comet. Silicate features at 10 and 25 μm were prominent in all 5 data sets, the largest heliocentric distances that silicate emission has been detected in a comet. The presence of crystalline water ice grains is suggested from the 60 μm excess emission at 4.6-4.9 AU, consistent with the observed $Q_{\mathrm{OH}}$if the icy grains were slightly warmer than an equilibrium blackbody. The average albedo of the dust is higher than that of comet P/Halley, but lower than other albedo measurements for Hale-Bopp nearer perihelion. There is no evidence for a component of cold, bright icy grains enhancing the scattered light at 4.6 AU. Simple models for a mixture of silicate and absorbing grains were fit to the ISO spectra and photometry at 2.8 AU. The observed flux at $\lambda > 100 \mu$m requires a size distribution in which most of the mass is concentrated in large particles. Dust production rates of order $1.5\times 10^5$kg s-1at 2.8 AU and $3\times 10^4$kg s-1at 4.6 AU have been found. They correspond to dust to gas mass ratios of 6 to 10.

One of the prime objectives of the Rosetta Radio Science Investigations (RSI) experiment is the determination of the mass, the bulk density and the low degree and order gravity of the nucleus of comet P/Wirtanen, the target object of the international Rosetta mission. The RSI experiment will use the spacecraft's radio carrier frequencies at X-band (8.4 GHz) and S-band (2.3 GHz) in order to measure slight changes of the orbit velocity via the classical Doppler effect induced by the gravity attraction of the comet nucleus. Based on an estimate of the background Doppler noise, it is expected that a mass determination (assuming a representative radius of 700 m and a bulk density of 500 kg/m3) at an accuracy of 0.1% can be achieved if the spacecraft's orbit is iteratively reduced below 7 km altitude. The gravity field of degree and order two can be detected for reasonable tracking times below 5 km altitude. The major competing forces acting on the spacecraft are the radiation pressure and the gas mass flux from cometary activity. While the radiation pressure may be predicted, it is recommended to begin a gravity mapping campaign well before the onset of outgassing activity (>3.25 AU heliocentric distance). Radial acceleration by water outgassing is larger by orders of magnitude than the accelerations from the low degree and order gravity field and will mask the contributions from the gravity field.

We report the results of the first statistical analysis of colours ($B-V$, $V-R$, $V-I$, and $V-J$) of the Trans-Neptunian and Centaur populations. Using the same statistical techniques applied to define the current asteroid taxonomy, we find a continuous spread of the objects between neutral colour to very red. Pushing further the analysis, the TNOs may be split into four groups. The differences in colour content are interpreted as a consequence of the TNOs evolution (i.e. collisional history, space weathering, ...)

Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ~2.1 days after the event and continued until 4 Apr. 2000. The flux decay constant of the optical afterglow in the R-band is -2.30 ±0.07 up to ~5 days, which is very likely due to the jet e ffect, and it is followed by a much steeper decay with constant -3.2 ±0.2, the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously with the event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ~2 days after the occurrence of the GRB (as expected from the jet effect) or a maximum, as observed in GRB 970508. The existence of a se cond break might indicate a steepening in the electron spectrum or the superposition of two events, resembling GRB 000301C. Once the afterglow emission vanished, contribution of a bright underlying supernova was found on the basis of the late-time R-band measurements, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our redshift determination of $z = 0.706$indicates that GRB 991208 is at 3.7 Gpc (for $H_{0}= 60$km s-1Mpc-1, $\Omega_{0}= 1$and $\Lambda_{0}= 0$), implying an isotropic energy release of 1.15 1053erg which may be relaxed by beaming by a factor >102. Precise astrometry indicates that the GRB coincides within 0.2″ with the host galaxy, thus supporting a massive star origin. The absolute magnitude of the galaxy is MB= -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of ($11.5 \pm 7.1$) $M_{\odot}$yr-1, which is much larger than the present-day rate in our Galaxy. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow was determined ~3.5 day after the burst (Dec. 12.0) implying a cooling frequency $\nu_{\rm c}$below the optical band, i.e. supporting a jet model with $p = -$2.30 as the index of the power-law electron distribution.

Aims.The asteroid (132524) 2002 JF56was the distant fly-by target of the New Horizons spacecraft on June 13, 2006. To further enhance our knowledge of this asteroid and facilitate the cross-calibration of the spacecraft instruments, ground based observations of the asteroid were performed in May 2006 at ESO VLT.

The evolution of the activity and composition of the coma of Rosettatarget comet 67P/Churyumov-Gerasimenko was studied along its postperihelion orbit from 2.29 AU to 3.22 AU. The comet had a major drop of activity between 2.5 AU and 2.9 AU, which manifested most obviously in its light curve (the brightness decreased by 3 mag), but is also confirmed by the production rates derived from spectrophotometric measurements. The strong decrease of activity indicates a change in the outgassing behaviour of the nucleus and might be due to water sublimation becoming less efficient. The spectrophotometric measurements at 2.9 AU show that the coma is at least slightly depleted in C2with respect to CN.

Context. The split Jupiter family comet 73P/Schwassmann-Wachmann?3 was monitored between January?21 and May?25?2006, for 24?nights.Aims. The goals of the campaign were to characterize the two principal comet components?(C?and?B) and the smaller component?G during their approach to perihelion, and study differences and commonalities in their evolution to obtain insight into the nature of the nuclei (gas and dust). We aimed to assess the chemical homogeneity/heterogeneity of the different components, the presence of jets and other coma structures, the rotation axis, the long-term activity evolution, and the detection of new fragmentation events.Methods. Long-slit spectra and optical broadband images were acquired using the CAFOS?instrument at the 2.2-m telescope at Calar?Alto Observatory (CSIC-MPG). Data obtained in service mode consisted of spectra and Johnson R?filter images. By observing for four nights close to perigee, the comet could be imaged in the Johnson UBVRI?filters. When possible, we analyzed the radial profile of the dust brightness and we derived the dust and gas production rates, the dust reddening, and the N-S?profiles of the CN, C2, and C3?column densities. The analysis of the morphological evolution of coma structures and the determination of the rotation axis is performed in a separate paper.Results. We found that components C and B behave differently during most of our observations. While component?C did not show any sudden increase in dust productivity, as measured by Af?, component?B was characterized by a higher activity variation, exhibiting two outburst peaks and fragmentation events. Excluding outburst dates, component?B always had lower dust productivity than component?C. We also found differences in the behavior of the dust brightness radial profiles and in the dust colors. Differences in the dust colors are found also with respect to component?G. In the spectral analysis, we found that both C and B?components seem to be carbon-chain depleted, their compositions being almost the same. This indicates that the pre-split original intact nucleus probably had a homogenous composition. A two-dimensional color map of component?B on May?13 shows relative color variations in the inner coma that can be interpreted qualitatively in terms of Mie?theory as fragmentation of silicate dust particles emanating from the nucleus.

Context. Photometric surveys of transNeptunian objects (TNOs) and Centaurs have suggested possible correlations between some orbital parameters and surface colors of classical objects, scattered disk objects (SDOs), and Centaurs. However, larger sample sizes are needed in order to corroborate or rule out the possible correlations and find some possible new ones. The implications from these correlations for the formation and evolution of classical?TNOs, SDOs, and Centaurs are important to understand possible physico-chemical coloring mechanisms and their influence on the surfaces of the TNOs and Centaurs, as well as their evolutionary history.Aims. We aim to obtain a sufficiently large photometric dataset in order to carry out a more significant statistical analysis, with emphasis on improving the sample size of observed objects, in particular for classical?TNOs and SDOs.Methods. We use VLT-FORS images through BVRI filters of 32?Kuiper Belt Objects (KBOs) and obtain their colors after proper reduction and calibration. We study the possible correlations (Peixinho et?al. 2004, Icarus, 170, 153; Doressoundiram et?al. 2005, Icarus, 174, 90) merging these new measurements with the VLT published results from the ESO large program and with the latest published results of the Meudon Multicolor Survey via non-parametric statistical tests.Results. We obtain a large dataset of 116 objects (classical, SDOs and Centaurs) and, in addition to confirming most of the correlations and conclusions reached in the literature, some possible new correlations are found. The most interesting ones are some correlations of color?vs. orbital parameters for the different dynamical groups. We find that some correlations in the classical group, as well as the (dynamically) cold and hot subgroups depend on the size of the objects. As a by-product of our study, we were able to identify new candidates for light curve studies and found that ~55% of the objects showed variability above 0.15?mag. This is a higher value than what is found in other studies. Since our sample contains smaller objects than samples from other studies, this result might be an indication that the smaller TNOs are more elongated than the larger ones.

Context. Comet 2P/Encke, a short-period comet, is a favorable target for studies of light scattering by bare cometary nuclei. These studies enable assessment of the nucleus size, the material albedo, and the light-scattering properties of the cometary surface. Observations of 2P/Encke were completed between October to December?2006 when the comet approached the Sun between 2.7 and 2.1?AU.Aims. We characterize the activity of the cometary coma and the physical properties of the nucleus of 2P/Encke such as its size, albedo, colors, spectral slope, surface roughness, porosity, and single-particle properties. Methods. Broadband imaging photometry and broadband and narrowband linear polarimetry is measured for the nucleus of 2P/Encke over the phase-angle range 4-28?deg. Results. An analysis of the point spread function of the comet in polarimetric images reveal only weak coma activity in 2P/Encke, corresponding to dust production of the order of 0.05?kg/s. Over the measured phase-angle range the nucleus displays a color independent photometric phase function of almost linear slope (??= 0.050?? 0.004?mag/deg). The absolute R?filter magnitude at zero phase angle is 15.05?? 0.05?mag and corresponds to an equivalent radius for the nucleus of 2.43?? 0.06?km (adopted albedo of 0.047). The nucleus color?V-R was measured to be 0.47?? 0.07, suggesting a spectral slope?S' of 11?? 8%/100?nm. The phase function of linear polarimetry in the V and R?filter shows a widely color independent linear increase with phase angle (0.12?? 0.02%/deg). The test of the empirical albedo-polarization relationship for asteroids reveals unreasonably high albedo values for the nucleus. We find discrepancies in the photometric and polarimetric parameters between 2P/Encke and other minor bodies in the solar system, which may indicate significant differences in the surface material properties and light-scattering behavior of the bodies.Conclusions. The linear polarimetric phase function of 2P/Encke presented here is the first ever measured for a cometary nucleus. It provides encouragement to future studies of cometary nuclei in order to characterize the light-scattering behavior of comets on firm empirical grounds and provide suitable input to a comprehensive modeling of the light scattering by cometary surfaces.

Context.The Jupiter family comet 73P/Schwassmann-Wachmann 3 (SW3) split into several pieces in 1995. Some of the original fragments were observed during the next apparition of the comet in 1999-2001. The last return of the comet in 2005-2006 was accompanied by tremendous further splitting of some SW3?components?? in particular component?B?? into a large number of subfragments.Aims.We present observations of components?B and C during their closest approach to Earth in the first half of May?2006. These results aim at characterizing the properties of dust particles released from the fragments of comet?SW3 and at identifying dusty and gaseous structures in the comae and tail regions of components?B and?C, which could be useful for conclusions about the presence of active regions and break-up events of the components. Methods.We used narrowband imaging polarimetry and an analysis of the broadband surface brightness for the characterization of the dust comae of the SW3?components. Coma and tail structures in components?B and?C were enhanced by numerical methods.Results.The degree of linear polarization of components?B and?C follows the empirical phase-angle dependence as derived from other comets. Fragment?B was found in outburst on May?8. The brightness outburst was accompanied by changes in the shape of the coma. During the outburst, the spatial distribution of the linear polarization showed some peculiarities that had disappeared again on May?9. Arc-like signatures of a temporary nature, typical of fragmentation events, are seen in broadband filter images of component?B on 3, 8, and 9?May?2006. The secondary dust tail, seen in component?B on 3 and 4?May?2006, marks the synchrone of an earlier splitting event around 25?April?2006. Fan-like coma structures are found in the sunward coma hemisphere of components?B and?C that might be related to localized enhanced activity on the rotating nuclei.

Context.Polarization is a powerful remote-sensing method to investigate solar system bodies. It is an especially sensitive diagnostic tool to reveal physical properties of the bodies whose observational characteristics are governed by small scatterers (dust, regolith surfaces). For these objects, at small phase angles, a negative polarization is observed, i.e., the electric vector?E oscillates predominantly in the scattering plane, contrary to what is typical for rather smooth homogeneous surfaces. The behavior of negative polarization with phase angle depends on the size, composition and packing of the scatterers. These characteristics can be unveiled by modelling the light scattering by the dust or regolith in terms of the coherent backscattering mechanism. Aims.We investigate the surface properties of TNOs and Centaurs by means of polarimetric observations with FORS1 of the ESO VLT. Methods. We have obtained new broadband polarimetric measurements over a range of phase angles for a TNO, 50000?Quaoar (in the R Bessel filter), and a Centaur, 2060?Chiron (in the BVR Bessel filters). Simultaneously to the polarimetry, we have obtained R?broadband photometry for both objects. We have modelled these new observations of Quaoar and Chiron, and revised the modelling of previous observations of the TNO?28978?Ixion using an improved value of its geometric albedo. Results. TNOs Ixion and Quaoar, and Centaur Chiron show a negative polarization surge. The Centaur Chiron has the deepest polarization minimum (?1.5?1.4%). The two TNOs show differing polarization curves: for Ixion, the negative polarization increases rapidly with phase; for Quaoar, the polarization is relatively small (?-0.6%), and nearly constant at the observed phase angles. For all three objects, modelling results suggest that the surface contains an areal mixture of at least two components with different single-scatterer albedos and photon mean-free paths.

Context.Dynamically, 133P/Elst-Pizarro and 176P/LINEAR are main belt asteroids, likely members of the Themis collisional family, and unlikely of cometary origin. They have been observed with cometary-like tails, which may be produced by water-ice sublimation. They are part of a small group of objects called Main Belt Comets (MBCs, Hsieh & Jewitt 2006).

Context.Comet 67P/Churyumov-Gerasimenko is the main target of ESA’s Rosetta mission and will be encountered in May 2014. Because the spacecraft will orbit the comet nucleus before and after release of the lander Philae, it is necessary to know the conditions in the coma.