The ACM conference series occupies a unique position in planetary science. Unlike meetings devoted to a single mission or technique, it deliberately bridges communities that often work in parallel: dynamicists who model orbital evolution over millions of years, spectroscopists who infer composition from reflected sunlight, laboratory analysts who study meteorites in the clean room, and observers who track the fleeting trails of meteors burning in the upper atmosphere. The triennial cadence means each meeting functions as a census of the field, capturing which questions have matured into consensus and which remain open. The 2026 edition in Poznań continued that tradition, bringing together current and future generations of small body scientists, with a scientific programme assembled by an international Scientific Organizing Committee and local logistics managed by a committee based in the Wielkopolska region.

A recurring theme of the meeting was the flood of high-precision data now arriving from next-generation surveys and space telescopes. Work presented in the session literature includes studies of trans-Neptunian and Centaur populations informed by recent survey discoveries, analyses of the orbital structure of near-Earth asteroid populations, and characterizations of active asteroids and main-belt comets whose dust tails blur the classical boundary between asteroids and comets. The increasing sensitivity of ground-based facilities has pushed detections toward smaller and fainter objects, revealing populations that were statistically invisible a generation ago. For dynamicists, this means models of impact flux, delivery from the main belt and the Kuiper belt, and resonant transport can now be tested against observed size-frequency distributions rather than fitted to sparse catalogs.

Cometary science featured prominently, reflecting the maturation of remote sensing at unprecedented spatial resolution. Research discussed at the conference included observations of cometary activity captured with the James Webb Space Telescope, which has begun resolving the distributions of water, carbon dioxide and other volatiles within cometary comae, and studies of long-period comets making their first pass through the inner Solar System. These measurements bear directly on questions of where comets formed and how the primordial ice inventory of the outer disk was distributed during the giant planets’ migration. Compositional mapping of individual comets, combined with the in situ results from the Rosetta mission at comet 67P/Churyumov-Gerasimenko, is converging on a picture in which comets are heterogeneous aggregates assembled from material processed at different distances from the young Sun.

Planetary defence was another major strand of the programme, and one with particular urgency following the first deliberate alteration of an asteroid’s orbit. The Double Asteroid Redirection Test demonstrated that a kinetic impactor can measurably change the trajectory of a satellite asteroid, and follow-up work on the ejecta dynamics, momentum transfer efficiency and structural response of the target Dimorphos has reshaped how researchers model asteroid interiors. Presentations at ACM 2026 drew on this legacy as the European Space Agency’s Hera spacecraft travels toward the Didymos system to survey the impact outcome in detail. Related work on the physical properties of rubble-pile asteroids, their cohesion, porosity and rotational failure modes, informs both deflection planning and the broader understanding of how small bodies evolve through collisions and spin-up over Solar System history.

Meteor science, the third pillar of the conference’s remit, is undergoing its own quiet revolution driven by global camera networks and seismic monitoring. Modern fireball networks now triangulate meteoroid trajectories with sufficient precision to recover falls and compute pre-impact orbits, linking meteorites directly to their source regions in the main belt. Studies presented in the meeting literature include analyses of meteoroid fragmentation in the atmosphere, statistical treatments of fireball datasets, and the use of seismometers to detect and characterize bolide airbursts, a technique that extends monitoring to events too small or too remote for optical networks. Together these approaches constrain the flux of material reaching Earth and the physical structure of the parent bodies, connecting the meteorite record in laboratories to the asteroid populations observed telescopically.

Sample return missions have fundamentally changed the evidentiary basis of the field, and their results resonated through many sessions. Material returned from the carbonaceous asteroids Ryugu and Bennu has provided uncontaminated, space-weathered primitive material whose mineralogy, organic content and isotopic composition can be compared directly with meteorite classes. Work building on these samples, including studies of the volatile inventory and thermal history of primitive bodies, was among the research highlighted in the conference’s session literature. The comparison between returned samples and telescopic spectra of larger populations such as the D-type and P-type objects in the outer belt and Jupiter’s Trojan swarms remains a central challenge, and new infrared surveys of these populations are helping to close the gap between the few bodies visited by spacecraft and the hundreds of thousands catalogued from Earth.

The meeting also underscored the growing role of machine learning and large-scale computation in small body science. Automated classification of light curves, neural-network detection of moving objects in survey images, and large N-body simulations of asteroid family evolution are now standard tools, and several presentations examined how these methods perform on the data volumes expected from the Vera C. Rubin Observatory’s Legacy Survey of Space and Time. Orbit determination for potentially hazardous objects, impact probability computation and the propagation of uncertainties over decadal timescales all benefit from these advances, as do efforts to link meteorite falls to asteroid families through dynamical modeling. The methodological convergence across subfields, from comet photometry to meteoroid stream modeling, was one of the practical takeaways emphasized by attendees.

Beyond the science, the Poznań meeting made a deliberate effort to broaden participation. Support from the ESA Academy, the Europlanet Society, the NASA Planetary Data System Small Bodies Node, the UAE Space Agency, the Canadian Space Agency and industry partners enabled early-career researchers and scientists from underrepresented and underprivileged groups to attend, and the UAE Space Agency sponsored the conference’s Best Poster Award. Financial backing from Poland’s Ministry of Science and Higher Education through the Wektory Nauki programme, the Wielkopolska regional government and the Initiative of Excellence – Research University programme of Adam Mickiewicz University underwrote the meeting itself. Organizers noted that the resulting exchange helped foster new collaborations across the global small body community, a stated aim of a series that has historically rotated among host countries to keep the field international.

As the 15th edition closed, the field it surveyed looked markedly different from the one that gathered three years earlier. Spacecraft are en route to metallic and primitive asteroids, survey telescopes are cataloguing small bodies at rates that outpace manual analysis, and laboratory measurements of returned samples are rewriting textbooks on Solar System formation. The questions that will define the next ACM conference, expected in 2029, are already taking shape: how efficiently kinetic impactors transfer momentum to real rubble piles, how the isotopic diversity of meteorites maps onto asteroid spectral classes, and how the smallest members of the Solar System’s debris population record the giant planets’ early history. For the community that met in Poznań, the small bodies of the Solar System have never been a bigger subject.

Subject of Research: Asteroids, comets and meteors in the Solar System

Article Title: The 15th Asteroids Comets Meteors Conference

Article References: The 15th Asteroids Comets Meteors Conference. (n.d.). https://doi.org/10.1038/s41550-026-02995-3

Image Credits: AI Generated

DOI: 10.1038/s41550-026-02995-3

Keywords: asteroids, comets, meteors, planetary defence, DART, Hera, sample return, meteorites, Kuiper belt, near-Earth asteroids, space surveys, ACM 2026

The experiment centered on two well-characterized graphite powder samples with different particle size ranges. That difference in size turned out to matter enormously. The angular dependence of the reflected light, known as the scattering phase curve, showed a non-trivial dependence on particle size, meaning that the brightness of the powder as a function of viewing and illumination geometry changed in ways that cannot be captured by a simple scaling. Most strikingly, the reflectance curves of the two samples crossed over one another at intermediate phase angles, a behavior consistent with earlier laboratory measurements of graphite powders by Kamei and Nakamura in 2002. Such crossover behavior is a reminder that the photometric fingerprints of particulate surfaces are subtle and that particle size must be treated as a first-order variable in any attempt to interpret asteroid photometry.

To produce these measurements, Deb used a goniometric setup, an instrument that rotates a light source and detector around a sample so that reflectance can be recorded at many combinations of incidence, emergence, and phase angles. The illumination was monochromatic at 535 nanometers, a wavelength in the green part of the visible spectrum. Goniometric measurements of this kind are the gold standard for building bidirectional reflectance distribution functions, the mathematical descriptions of how a surface redirects incoming light. For planetary scientists, such functions are the bridge between a telescope or spacecraft photometry and the physical properties of the surface, including grain size, porosity, and roughness.

A key strength of the study lies in the independent characterization of the samples. The graphite powders were examined with scanning electron microscopy to reveal their microscopic morphology, with X-ray diffraction to confirm their crystallographic structure, and with particle size analysis to quantify the distribution of grain dimensions. This multi-technique approach matters because light scattering depends on far more than the average particle diameter. Shape, surface texture, and internal structure all influence how photons interact with individual grains and with the packed medium as a whole. By documenting these properties alongside the optical measurements, the study produces datasets that modelers can use with confidence rather than guesswork.

The theoretical framework most widely used to interpret such measurements is the Hapke bidirectional reflectance model, which describes the reflectance of a semi-infinite particulate surface in terms of the single-particle scattering albedo, the single-particle phase function, and terms accounting for multiple scattering and the opposition effect. The opposition effect, a sharp surge in brightness when the Sun, the surface, and the observer are nearly aligned at phase angles below about ten degrees, arises largely from shadow hiding, the mutual masking of grains that disappears as the viewing geometry approaches zero phase. The new graphite measurements, spanning a wide range of phase angles, offer a demanding test for such models on a strongly absorbing material where multiple scattering is weak and the physics of single-particle scattering dominates.

Graphite is a particularly instructive analog because of its optical properties. It is one of the most strongly absorbing common materials in the visible, which means that photons penetrating a graphite powder surface are absorbed quickly and rarely survive to be scattered multiple times. This makes the reflectance of graphite powders sensitive to the outermost layer of grains and to their individual scattering phase functions. The material also has direct planetary relevance: graphite grains and graphitic carbon have been identified in samples returned from carbonaceous asteroids, including a graphite spherule reported in material from asteroid Ryugu, and carbonaceous chondrite meteorites carry abundant dark, absorbing phases. Understanding how such material scatters light in powdered form therefore speaks directly to the interpretation of dark asteroid surfaces.

The study also addresses a practical problem that often plagues laboratory work with powders: converting between different descriptions of the size distribution. Commercial sieves sort powders by mesh size, which yields a distribution expressed in weight percentage per size class. But scattering theory typically requires a number-based distribution, counting how many particles of each size are present. The paper includes a detailed appendix showing how to convert a weight-based sieve distribution into a normalized number frequency distribution, using the fact that particle mass scales with the cube of the diameter. This seemingly technical step is essential, because a small number of large grains can dominate a weight distribution while contributing little to the scattering behavior of the surface.

The broader context of this work is a renaissance in small-body exploration. Missions to asteroids Bennu and Ryugu returned samples that revealed unexpectedly complex surfaces, and remote sensing of these bodies depends on photometric models calibrated against laboratory analogs. Recent years have seen a wave of goniometric measurements on regolith simulants, Apollo samples, meteorites, and artificial agglomerates, all aimed at constraining how porosity, roughness, grain shape, and absorption shape the observed brightness. The graphite datasets add a strongly absorbing end-member to this library, complementing measurements on brighter silicate materials and helping to span the range of albedos encountered on real asteroids.

Beyond planetary science, the results have relevance for remote sensing in general, for aerosol physics, and for optical characterization of materials. Strongly absorbing particulate coatings appear in contexts from soot and black carbon in the atmosphere to graphite components in high-temperature blackbody radiators used for metrology. The crossover behavior observed at intermediate phase angles suggests that photometric inversions that ignore particle size could misestimate surface properties, and the openly available datasets, released for further theoretical study, invite computational groups working with discrete dipole approximations, T-matrix methods, and geometric optics codes to test their models against real measurements on irregular, absorbing grains.

What makes this study notable is its economy: a single carefully constructed instrument, two well-documented samples, and a systematic sweep of scattering geometry have produced results that speak to one of the oldest problems in planetary photometry. Every interpretation of asteroid phase curves, every estimate of regolith grain size from spacecraft data, ultimately rests on laboratory measurements of exactly this kind. As sample-return missions continue to deliver material from the early solar system, the demand for precise, well-characterized scattering measurements of relevant analog materials will only grow. This work demonstrates that even a familiar substance like pencil lead, ground into powder and measured with patience, can sharpen our view of worlds hundreds of millions of kilometers away.

Subject of Research: Experimental measurement of angular light scattering from graphite powder analogs of asteroid regolith

Article Title: Light scattering from graphite powders – an experimental investigation

Article References: Deb, D. (2026). Light scattering from graphite powders – an experimental investigation. Astrophysics and Space Science, 371(10), Article 114. https://doi.org/10.1007/s10509-026-04646-5

Image Credits: AI Generated

DOI: 10.1007/s10509-026-04646-5

Keywords: graphite powder, light scattering, regolith, asteroids, bidirectional reflectance, Hapke model, goniometry, planetary science, remote sensing, particle size, opposition effect, carbonaceous asteroids