Properties of small, heliocentric bodies in the solar system share many attributes because of their small sizes, yet vary in other ways because of their different locations of formation and the diverse subsequent evolutionary processes that have affected them. Our insights concerning their properties range from highly detailed knowledge of a few specific bodies (like Eros), to rich knowledge about unspecific bodies (meteorite parent bodies), to no knowledge at all (other than existence and rough limits on size) concerning much smaller and/or more distant bodies. Today's state of learning about physical properties of TNOs is analogous to that for main-belt asteroids 35 years ago. This invited review attempts to elucidate linkages and differences concerning these populations from the highly heterogeneous data sets, emphasizing basic properties (size, shape, spin, density, metal/rock/ice, major mineralogy, presence of satellites) rather than the highly detailed knowledge we have of a few bodies or their dynamical properties. The conclusion is that there are vital interrelationships among these bodies that reinforce the precept that guided the original ACM meetings, namely that we should all think about small bodies in an integrated way, not just about subsets of them, whether divided by size, composition, or location.

We review processes leading to the orbital migration of bodies ranging from dust particles up to protoplanets in the earth mass range in protoplanetary disks. The objects most at risk of being lost from the disk have dimensions of the order of metres. To retain these it may be necessary to invoke either strong turbulence or trapping due to special flow features such as vortices. Migration also becomes important for protoplanets with mass exceeding $0.1M_{\oplus}$. In this case it can also lead to the attainment of commensurabilities for pairs of protoplanets. Such pairs could be left behind after disk dispersal. We review some recent work on the attainment of commensurabilities for protoplanets in the earth mass range.

The Deep Impact mission delivered 19 gigajoules of kinetic energy to the nucleus of comet 9P/Tempel 1 on 4 July 2005. Intensive observations, both from the two spacecraft and from Earth and Earth-orbit, while approaching the comet led to numerous new findings about comets. Observations of the impact event, from the flyby spacecraft have led us to a determination of numerous physical properties of the nucleus. Analysis of the near-IR spectra is still very preliminary.

Dust particles in the solar system are produced from the small bodies: asteroids, comets, meteoroids and Kuiper belt objects. A further source of dust is provided by the warm interstellar medium that the Sun is currently embedded in and that streams into the solar system. We review the physical properties of solar system dust and trace back its interrelation with the small solar system bodies. Comets contain relatively pristine material that they transport to the inner solar system. The alteration of dust in the vicinity of comets is complex and connected to the gas evolution, but a significant part of the organic dust material survives these alterations. The optical properties of cometary dust are best described with a mixture of silicate and carbon bearing materials. As far as the darkness of the cometary material is concerned, according to recent models, this is not a result of the porosity, but rather of the darkness of the carbon bearing component. This does not contradict the observation of silicate features in the thermal emission brightness of cometary dust, since porous mixtures of silicate and carbon bearing dust can produce the observed polarization and albedo characteristics, as well as the silicate features. The carbon-bearing component is most likely an organic refactory component. The relative contributions of different sources change within the solar system dust cloud and depend as well on the measurement technique considered. In particular, the dust from asteroids, which provides a large component of the dust near Earth orbit, is also preferably seen with most of the detection methods. The majority of dust inward from 1 AU is produced from cometary dust and meteoroids. Dust material evaporation induced by collisions inward from 1 AU produces a minor heavy ion component in the solar wind plasma known as inner source pick-up ions.

We review the recent progress in the exploration of the interrelations between primitive small bodies of the solar system which are preserved the pristine material in their interior: cometary nuclei, Transneptunian Objects, Centaurs, and primitive asteroids, and they are considered as primordial objects. In addition, we discuss the properties of the asteroid-comet transition objects which have really enigmatic behavior. The comets have most primitive, accessible material in the solar system but we do not know what is hidden below the evolved surface layers. Comets must become dormant but we do not know whether the ice is exhausted or sublimation is inhibited (blocked by quenching mechanisms). There must be many dormant comets masquerading as asteroids but we do not know to identify these bodies unless via serendipitous discovery observations. Indeed, there are some asteroids which temporarily show comet-like activity. These are among the Damocloids (C/2001 OG108 (LONEOS)), main belt asteroids (7968 Elst-Pizarro = 133P/E-P) and Near-Earth objects (4015 Wilson-Harrington = 107P/W-H). The important questions are: where is the pristine material in the cometary nuclei and in the asteroid-comet transition objects, do comets lose their ice or seal it in? Both the large survey projects and in-situ space missions will help to answer these questions in the near future.

Dawn is the first mission to attempt to orbit two distant planetary bodies. The objects chosen, 4 Vesta followed by 1 Ceres, are the two most massive members of the asteroid belt that appear to have been formed on either side of the dew line in the early solar nebula. This paper describes the present status of the mission development and the plans for operation at Vesta and Ceres.

The Spitzer Space Telescope (SST), aloft for over two years at time of writing, has so far devoted almost 600 hours of observing time to Solar System science, and small bodies make up a significant fraction of the objects that have been observed. For the first time we now have high accuracy mid-infrared data to study the fundamental mineralogical and physical properties of a large number of objects of different types. In this paper we review some of the exciting recent results derived from SST photometry (in six bands from 3.6 to 70 $\mu$m) and spectroscopy (from 5 to 40 $\mu$m) of asteroids and comets. The observations reveal their spectral energy distributions (SEDs), and we discuss three important science goals that can be addressed with these data: (1) finding compositional diagnostics of these objects, (2) determining their bulk thermal properties, and (3) understanding the surface evolution of primitive and icy bodies. We focus primarily on comet-asteroid transition objects, low-albedo asteroids, cometary nuclei, Trojans, Centaurs, and trans-Neptunian objects. In particular, we will show: emissivity features in the SEDs and identification of the compositional sources, the constraints on thermal inertia and infrared beaming through the samples of the thermal continuum, and an intercomparison of albedos across dynamically-related bodies.

The present status of our knowledge of the composition of cometary nuclei is reviewed and compared with what we know on the composition of other Solar System minor bodies – interplanetary dust, meteorites, asteroids, trans-Neptunian objects. The current methods of investigations – by both in situ analysis and remote sensing – are described. Comets are active objects pouring their internal material to form a dusty atmosphere which can be investigated by remote sensing. This is not the case for minor planets and trans-Neptunian objects for which only the outer surface is accessible. Collected interplanetary dust particles and meteorites can be analysed at leisure in terrestrial laboratories, but we do not know for certain which are their parent bodies.

Considerable progresses have been made from spectroscopic observations of active comets, mainly at infrared and radio wavelengths. We probably know now most of the main components of cometary ices, but we still have a very partial view of the minor ones. The elemental composition of cometary dust particles is known from in situ investigations, but their chemical nature is only known for species like silicates which have observable spectral features. A crucial component, still ill-characterized, is the (semi-)refractory organic material of high molecular mass present in grains. This component is possibly responsible for distributed sources of molecules in the coma. A large diversity of composition from comet to comet is observed, so that no “typical comet” can be defined. No clear correlation between the composition and the region of formation of the comets and their subsequent dynamical history can yet be established.

There are three types of energy sources that affect comet nuclei and may render them active: thermal – solar radiation, nuclear – radioactive decay, and gravitational – through collisions and tidal forces. These sources give rise to processes that, in turn, may release, absorb or transport energy: sublimation or recondensation of volatiles, crystallization of amorphous ice, heat diffusion and advection, gas flow through the porous nucleus. Each of these sources and processes has its own characteristic time scale (or rate) and these may differ by many orders of magnitude. It is the competition between various processes and the interaction between them – as one triggers the other, or else impedes it – that determine the activity pattern and the internal structure of a comet nucleus. Examples of such interactions and their outcome are presented, such as apparently sporadic activity at large heliocentric distances, obtained from numerical simulations of the behavior and evolution of comet nuclei. Confrontation of modeling results with observations provides feedback and constraints for the assumptions and parameters on which models are based. Adjusting the latter to match observations reveals properties of the nucleus that are otherwise inaccessible (except for in situ measurements by space missions). However, the interpretation of observations, such as production rates in relation to nucleus abundances, may be misleading. It is shown that monitoring production rates over the active part of a periodic comet's orbit may lead to conclusions regarding the composition and structure of the nucleus.

Recent observations in visible photometry have provided B, V, R and I high quality colors for more than 130 objects. Color diversity is now a reality in the TNOs population. Relevant statistical analyses have been performed and all possible correlations between optical colors and orbital parameters have been analyzed. A taxonomy scheme based on multivariate statistical analysis of a subsample of 51 objects described by the 4 color indices (B-V, V-R, V-I and V-J) has been obtained. A tentative interpretation of the obtained groups in terms of surface characteristics is given. Moreover, an extension of this taxonomy to the other 84 objects for which only three colors indices (B-V, V-R, and V-I) are available, is also presented.

The faintness of these objects limits the spectroscopic observations. Despite this, our group provided visible and infrared spectra for 18 objects using the Very Large Telescope (ESO, Paranal, Chile). The wavelength region ranging 0.4–2.3 microns encompasses diagnostic spectral features to investigate organic compounds, minerals and ices present on the surface of the TNOs. The investigation of the surface variation can be an identifier of possible composition diversity and/or different evolution with different physical processes affecting the surface.

The current knowledge of the surface properties and composition of the population will be presented, analyzed and interpreted.

Before the discovery of the first member of the Kuiper belt in 1992, the trans-Neptunian population was supposed to lie on a flat disk and each member would follow a barely eccentric orbit. While less conventional orbits for the trans-Neptunian objects were being discovered, our understanding of its orbital structure and origin was continually changed. A basic classification of the trans-Neptunian population as to their orbits identifies a classical low inclination Kuiper belt population, a resonant population, a high inclination Kuiper belt population, a scattered population and an extended population. Several mechanisms have been proposed to explain the orbital architecture of the Kuiper belt population. Presently, the most plausible scenarios are unequivocally related with the primordial planetary migration induced by a planetesimal disk. Low inclination orbits in the Kuiper belt may have been moderately pushed out from a dynamically cold primordial disk by the resonance sweeping mechanism. The origin of high inclination objects in the classical Kuiper belt is however to be found in a primordial Neptune scattered population, through a perihelion increasing mechanism based on secular resonances. Another push-out mechanism based on the sweeping of the 1:2 resonance with Neptune has also been invoked to explain the low inclination orbits in the classical Kuiper belt. Assuming these last two mechanisms, Kuiper belt objects do not need to have been formed in situ. This kind of formation process would demand a quite large original mass in the Kuiper belt region, which would have brought Neptune beyond its present position at 30 AU. Thus with the exception of the low inclination classical Kuiper belt objects and a few resonant ones, all other trans-Neptunian objects are present or past scattered objects. This notion also includes the case for Sedna, so far the only certain member of the extended population. In its most plausible formation scenario, it was a primordial scattered object by Neptune whose perihelion was increased by the close passage of a star.

Within the near-Earth object population, one finds asteroids, comets, and meteorites thereby placing the NEO population at the center of the ACM conference. The longstanding gulf between the spectral properties of S-type asteroids and ordinary chondrite meteorites appears to be bridged, where the observational data are consistent with a space weathering type process. As much as 30% of the entire NEO population may reside in orbits having a Jovian Tisserand parameter <30, and among these roughly half are observed to have comet-like physical properties in terms of their albedos and spectra (taxonomy). Thus 15±5% of the entire NEO population may be comprised by extinct or dormant comets.

Orbit determination for Near-Earth Asteroids presents unique technical challenges due to the imperative of early detection and careful assessment of the risk posed by specific Earth close approaches. This article presents a case study of asteroid 99942 Apophis, a 300-400 meter object that, for a short period in December 2004, held an impact probability of more than 2% in 2029. Now, with an orbit based on radar ranging and more than a year of optical observations, we can confidently say that it will pass safely by the Earth in 2029, although at a distance of only about six Earth radii from the geocenter. However, the extremely close nature of this encounter acts to obscure the trajectory in subsequent years, when resonant returns to the vicinity of the Earth are possible. In particular, an impact possibility in the year 2036 has a roughly 5% probability of persisting through the very favorable 2013 radar and optical observing apparition. In the event that the 2036 potential impact has not been eliminated by 2013, a precise characterization of the Yarkovsky accelerations acting on the asteroid may become an important part of the orbit estimation and impact prediction problem. Even so, the sixteen years available to effect a deflection from 2013 until 2029, after which the problem would become intractable, are sufficient to respond to the threat should a deflection effort become warranted.

Mathematical modelling of meteoroid streams formation and evolution is a very fruitfull method for obtaining not only cosmogonical knowledges, but information about the stream up-to-date structure as well. In this review we consider advances of the method, and applications to meteoroid streams of different schemes of formation. We also discuss the part played by observations, and feedback between models and observations. Attention is also drawn to some unresolved problems and promising areas of application.

A review of the current knowledge of physical properties and chemical composition of meteoroids entering the Earth's atmosphere is presented. Meteoroid penetration ability, ablation coefficients, beginning heights, light curves, fragmentation, and spectra are considered. The inferred bulk densities, mechanical strengths, rotation, and atomic elemental abundances are discussed. Cometary meteoroids are effectively grain aggregates with low bulk density (100–1000 kg m$^{-3}$), high porosity and low cohesivity. A volatile matrix holding the grains together may be present. Presence of large amounts of organic material is not firmly established. Small chunks ($\sim $mm) of denser material are sometimes contained in cometary meteoroids. Chemically, cometary grains are similar to CI chondrites but there is a hint of enhancement of Na, Si, and Mg and depletion of Fe, Cr, and Mg. Larger chemical diversity is observed among small meteoroids on cometary orbits not belonging to meteoroid streams. The relatively frequent Na-free meteoroids are probably fragments of cometary irradiation crust. Asteroidal meteoroids exhibit much lower mechanical strengths than stony meteorites, clearly due to the presence of large scale cracks. Iron meteoroids dominate among asteroidal meteoroids smaller than 1 cm.

Although a direct link between the HED meteorites and the asteroid 4 Vesta is generally acknowledged, several issues continue to be actively examined that tie Vesta to early processes in the solar system. Vesta is no longer the only basaltic asteroid in the Main belt. In addition to the Vestoids of the Vesta family, the small asteroid Magnya is basaltic but appears to be unrelated to Vesta. Similarly, diversity now identified in the collection of basaltic meteorites requires more than one basaltic parent body, consistent with the abundance of differentiated parent bodies implied by iron meteorites. The timing of the formation of the Vestoids (and presumably the large crater at the south pole of Vesta) is unresolved. Peaks in Ar-Ar dates of eucrites suggest this impact event could be related to a possible late heavy bombardment at least 3.5 Gyr ago. On the other hand, the optically fresh appearance of both Vesta and the Vestoids requires either a relatively recent resurfacing event or that their surfaces do not weather in the same manner thought to occur on other asteroids such as the ordinary chondrite parent body. Diversity across the surface of Vesta has been observed with HST and there are hints of compositional variations (possibly involving minor olivine) in near-infrared spectra.

An asteroid family is a group of asteroids with similar orbits and spectra that was produced by a collisional breakup of a large parent body. To identify asteroid families, researchers look for clusters of asteroid positions in the space of proper orbital elements. These elements, being more constant over time than osculating orbital elements, provide a dynamical criterion of whether a group of bodies has a common ancestor. More than fifty asteroid families have been identified to date. Their analysis produced several important insights into the physics of large scale collisions, dynamical processes affecting small bodies in the Solar System, and surface and interior properties of asteroids.

The discovery of binaries in each of the major populations of minor bodies in the solar system is propelling a rapid growth of heretofore unattainable physical information. The availability of mass and density constraints for minor bodies opens the door to studies of internal structure, comparisons with meteorite samples, and correlations between bulk-physical and surface-spectral properties. The number of known binaries is now more than 70 and is growing rapidly. A smaller number have had the extensive followup observations needed to derive mass and albedo information, but this list is growing as well. It will soon be the case that we will know more about the physical parameters of objects in the Kuiper Belt than has been known about asteroids in the Main Belt for the last 200 years. Another important aspect of binaries is understanding the mechanisms that lead to their formation and survival. The relative sizes and separations of binaries in the different minor body populations point to more than one mechanism for forming bound pairs. Collisions appear to play a major role in the Main Belt. Rotational and/or tidal fission may be important in the Near Earth population. For the Kuiper Belt, capture in multi-body interactions may be the preferred formation mechanism. However, all of these conclusions remain tentative and limited by observational and theoretical incompleteness. Observational techniques for identifying binaries are equally varied. High angular resolution observations from space and from the ground are critical for detection of the relatively distant binaries in the Main Belt and the Kuiper Belt. Radar has been the most productive method for detection of Near Earth binaries. Lightcurve analysis is an independent technique that is capable of exploring phase space inaccessible to direct observations. Finally, spacecraft flybys have played a crucial paradigm-changing role with discoveries that unlocked this now-burgeoning field.

Outer satellites of the planets have distant, eccentric orbits that can be highly inclined or even retrograde relative to the equatorial planes of their planets. These irregular orbits cannot have formed by circumplanetary accretion and are likely products of early capture from heliocentric orbit. The irregular satellites may be the only small bodies remaining which are still relatively near their formation locations within the giant planet region. The study of the irregular satellites provides a unique window on processes operating in the young solar system and allows us to probe possible planet formation mechanisms and the composition of the solar nebula between the rocky objects in the main asteroid belt and the very volatile rich objects in the Kuiper Belt. The gas and ice giant planets all appear to have very similar irregular satellite systems irrespective of their mass or formation timescales and mechanisms. Water ice has been detected on some of the outer satellites of Saturn and Neptune whereas none has been observed on Jupiter's outer satellites.

Since Pietrowsky's first analytical study of collisional systems of asteroids (1953), through Dohnanyi's comprehensive theory (1969), to the analytical and numerical studies of the last two decades, the collisional evolution of populations of asteroids — and to a less extent, of Trojans and TNOs— has been investigated by many researchers.

The study of such systems is an intrinsically delicate mathematical problem, as their evolution in time is properly described in terms of systems of first–order, non–linear differential equations. Physically, the limited knowledge of some of the collisional properties, rotations and internal structure of bodies, and the complex interplay with dust, non–gravitational effects and dynamical interactions with planets, make the study of the collisional evolution a hard multi–parametric problem. Nevertheless, the task is worth the effort, in fact the understanding of evolutionary processes in the solar system's small body belts provides the main tools to discriminate between the many different theoretical scenarios proposed to explain the formation of the solar system itself.

This review tries to give an updated overall view of the research done in this field, and to show the connections between apparently independent phenomena that may affect the evolution of collisional systems of asteroids and TNOs.