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We present new observations and models of the shapes and rotational states of the eight near-Earth Asteroids (1580) Betulia, (1627) Ivar, (1980) Tezcatlipoca, (2100) Ra-Shalom, (3199) Nefertiti, (3908) Nyx, (4957) Brucemurray, and (5587) 1990 SB. We also outline some of their solar phase curves, corrected to common reference geometry with the models. Some of the targets may feature sizable global nonconvexities, but the observable solar phase angles were not sufficiently high for confirming these. None is likely to have a very densely cratered surface. We discuss the role of the intermediate topographic scale range in photometry, and surmise that this scale range is less important than large or small scale lengths.

We describe the present state of an advanced physical model aimed at the simulation of the environment of active cometary nuclei. The model can handle the complicated shapes of real cometary nuclei, and is ready to handle alternative assumptions concerning the nucleus composition and structure; its present version is based on the simple, but hitherto highly successful, Whipple (1950) paradigm: at small heliocentric distances, strong sublimation theory is used to compute the surface gas production, taking into account the time-dependent heat flow in the nucleus interior; at large distances, different types of gas molecules may be assumed to dominate the gas production. Any size and shape distribution of dust can be assumed. The nucleus spin motion is modelled with allowance for the outgassing and solar tidal torques. The gas outflow is computed by solving quasistationary flow equations (Euler, or Navier–Stokes), hence the extent of the coma which can be modelled is limited either by the breakdown of the fluid approximation, or by that of the steady-state approximation. The dust outflow is computed by solving quasi-stationary “zero-temperature” multifluid Eulerian equations in the gas–dust interaction region, and from a “Keplerian fountain model” beyond it: the extent of the dust distribution which can be modelled is only limited by computer resources limitations. In addition to the detailed gas and dust coma structure, the resulting net nucleus mass loss, net sublimation recoil force, net sublimation torque, and net thermal emission are computed. We mention the past applications of the model to comets P/Halley and C/Hyakutake, and indicate some of the future steps of development of the model.

Asteroids have been used extensively as calibration sources for the Infrared Space Observatory (ISO) and are planned to be used by future groundbased, airborne and space-based projects in the thermal infrared (IR) and in the sub-millimetre. We summarize the general IR observational parameters with a focus on space observatories and discuss brightness variations, apparent velocities and background influences. During the ISO mission ten well- studied asteroids were used for the photometric calibration of ISOPHOT, but additionally the bright asteroids turned out to be of great interest for many technical tests and calibration aspects. We evaluated the dierent applications, like testing the photometry of the spectrometers, validation of relative spectral response functions, determination of beam proles or colour correction tests. The description of the asteroids' thermal emission has been obtained by a recent thermophysical model (TPM). The important model aspects are size, albedo, shape together with the spin vector, a beaming model, thermal inertia and a wavelength-dependent emissivity. With a large sample of observational data provided by three dierent ISO instruments we had for the rst time the possibility to study the thermal emission of several asteroids in detail. The intercomparison between results from dierent instruments allowed us to distinguish between observational errors and model shortcomings. It turned out that the accuracy of TPM predictions is in many cases strongly related to the limited knowledge of the asteroid shapes. The concepts of beaming, thermal inertia and wavelength dependent emissivities were nicely conrmed for a wide range of observing and illumination geometries under many aspect angles for dierent asteroids. The TPM predictions for Ceres, Pallas and Vesta are accurate within 5% over the full wavelength range from 5 to 200m, for Hygiea and a few other asteroids the predictions and observations agree within 10 to 15%. We found similar emissivity behaviour for the four large asteroids over the full ISO wavelength range. Up to now, no clear spectral features have been seen in the asteroid far-IR spectra.

We revisit the rotation of P/Halley taking into account the most essential observational constraints, as well as the external torques aecting the nucleus, in particular the outgassing torque. We solve the dirty ice sublimation equations at each point of the sunlit surface, using, for the rst time, the surface shape derived from the 1986 flybys imaging data. We assume that the nuclear surface is homogeneous in composition, thus reducing the number of model free parameters to one only: the dust-to-ice ratio on the surface. Our derived rotation model is a short-axis mode; it is consistent both with the 1986 nucleus imaging data, with the estimated non gravitational force, and with the observed time variations of the nucleus production rates. The outgassing torque results in a signicant variation of the angular momentum vector { for the assumed nucleus density of 0.5 g/cm 3 .

The subsurface thermal IR emission of asteroids becomes linearly polarized when scattered across the surface. The disk integrated mid-IR Stokes parameters are computed by extending the new thermophysical model by Lagerros (1996, Astron. Astrophys. 315 , 625–632; 1998, Astron. Astrophys. 332 , 1123–1132). The small scale surface roughness is approximated by hemispherical segment craters covering a smooth surface. Analytical solutions are used for the multiple scattered solar and thermally emitted radiation inside the craters. The surface roughness enhances the emission toward the solar direction, but randomizes the scattering planes which lowers the polarization. In general the predicted degree of linear polarization increases with higher refractive index, higher absorption coefficient, and a more elongated shape of the asteroid. The asteroids (6) Hebe and (9) Metis were observed at λ=25μm with ISOPHOT onboard the Infrared Space Observatory (ISO). The model absolute fluxes are in good agreement with the photometric results. Although no linear polarization was detected, the upper limits together with the extended model enable us to put useful constraints on the regolith properties of the target asteroids. The derived detection limits are compared to model polarization, by spanning a range in surface roughness, refractive index, and thermal inertia parameter space. The Metis observations favor a low refractive index and high surface roughness, but the Hebe observations are inconclusive since they coincided with a minimum in the polarization curve.

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST) on La Palma, Spain, during the period April 21–25, 1997. CCD observations were made with and filters, with band-passes excluding major cometary gaseous emissions. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalisation, rotational and temporal derivative and unsharp masking techniques. The rotation period of the nucleus was determined to and the mean projected dust outflow velocity to measured over a radius range of . No difference in quasi-simultaneous dust morphology between the filter wavelengths was seen. Up to five shell envelopes were visible in the sun-ward side of the coma out to a radius of , separated by a projected distance of ~ and spiralling towards smaller radii in the direction of local evening. Outer boundaries of shells were more sharply defined than inner, with shell intensity maxima occurring at ~35° past the sun-comet line in the direction of rotation. Small scale inhomogeneities of size were seen at radii . Two superposed shell systems with a relative lag angle of ~55° were evident during the period of observation. The sense of rotation of dust emission features indicates that the north pole of the nucleus was directed towards the Earth. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotation axis of the nucleus was not very large.

The results of a photometric survey of Hilda asteroids are presented. We have obtained 224 lightcurves of 47 Hilda asteroids. A wide span of rotation periods have been found, ranging from 2.851 to 31 h. However, indications of rotation periods longer than 100 h were found for two objects. The largest lightcurve amplitude found is 1.38 mag, and 13 additional objects have observed amplitudes ≥0.30 mag. Improved absolute magnitudes have been obtained for 20 objects as well as new V–R color indices for 17 of these. Improved albedos, derived from the new absolute magnitudes, have been obtained for 10 Hilda asteroids. The V–R color indices range between 0.38 and 0.49 mag, indicating that only P- and D-type asteroids are present among these asteroids. A probable taxonomy was assigned to four Hilda asteroids (one P-type and three D-types) which previously were unclassified. The Hilda Asteroid 3694 Sharon was observed at low solar phase angles, and the obtained lightcurves indicate a reduced to nonexisting opposition effect for this asteroid. One Hilda asteroid, 2067 Aksnes, may be in a state of precession as no satisfactory two-maxima and two-minima lightcurve could be fitted to the data. However, the short damping time scale makes this very unlikely. This favours a four-maxima and four-minima lightcurve of 2067 Aksnes, which is a better fit to the data than a two-maxima and two-minima lightcurve. The Hilda Asteroid 3415 Danby has a very short rotation period ( P = 2.851 ± 0.003 h), making Danby the fastest spinning asteroid known within its size range ( D ∼ 50 km). Due to its fast spin some constraints can be set on 3415 Danby's internal structure and mean density. The assumption that 3415 Danby is a rubble-pile, with its shape governed by hydrostatic equilibrium, leads to an unrealistically high mean density. On the other hand, assuming that 3415 Danby has internal strength results in a mean density larger than 1.4 g cm −3 .

More than 7000 positions of moving objects were detected on 33 films obtained with the Schmidt telescope at the European Southern Observatory in Chile during August and September 1995. The majority of these asteroids are previously unobserved. This paper presents the results for 62 unnumbered asteroids with provisional designations found in the fields. References to the derived positions for these asteroids are given and improved orbital elements have been calculated using all available positions.

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST)on La Palma in April, 1997. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalization, rotational derivative, temporal derivative and unsharp masking processing.

The NTT at La Silla, Chile and NOT on La Palma were used for VRI photometry of 9 Edgeworth-Kuiper objects and 3 Centaurs. For the NOT observations growth-curve fitting was applied in the photometric reductions. The spread in colour of the observed Centaurs and Edgeworth-Kuiper objects is large.

In this paper we have studied how the number density of asteroids varies as a function of the mean distance from the sun.

We investigate a broad system design for a space-based observatory operating at mid-infrared and visible wavelengths to perform physical characterization and discovery of near-Earth objects (NEOs) in the inner solar system. Our goals require measurements that are much more efficiently done from space. The mission objectives are to obtain accurate diameters, albedos and multiband reflectance properties for the known NEOs, and to conduct a search for objects spending most or all their orbital period inside Earth's orbit. The purpose is to observe a large fraction of the existing population during a mission operational lifetime of two years. A rather modest sized telescope (70 cm primary mirror and Ritchey-Chretien optical configuration) is found to be adequate to meet the objectives.

In this paper we present lightcurves and physical parameters for nine near-Earth objects (NEOs) (433 Eros, 1917 Cuyo, 2368 Beltrovata, 3753 Cruithne, 4769 Castalia, 4953 1990 MU, 6491 1991 OA, 1995 EK1, and 1995 FJ) and three Mars crossers (5738 Billpicke