We present time-series observations of the split comet 240P/NEAT near perihelion, obtained using the Nordic Optical Telescope. The brighter component, 240P-A, has an estimated radius in the range 400 m to 600 m, and loses dust at the peak rate 130 kg/s. The ejected dust has characteristic size 50 micron, is expelled sunward at 25 m/s, with a total ejected mass in the period of observation 1.5x10^9 kg. Mass loss from the fainter component, 240P-B, peaks at 35 kg/s and the total ejected mass was 2.3x10^8 kg. The radius of 240P-B is uncertain, with a best estimate about 300 m and an absolute lower limit 50 m. 240P-A and 240P-B are currently separating at about 1 m/s, a speed that is likely accelerating as a result of differential outgassing forces, and have a separation age over 3 years. The splitting of 240P is incompatible with the action of tides, impact, and internal pressure build up. 240P fits a developing picture, in which small comets are destroyed by rotational instabilities triggered by outgassing torques, an explanation that can be tested in 240P by future observations.

We report the detection of the nucleus of interstellar object 3I/ATLAS, using a nucleus extraction technique on Hubble Space Telescope (HST) observations taken between 2025 December and 2026 January. The product of the V-band geometric albedo, pV, with the physical cross section of the nucleus is 0.22 +/- 0.07 km2, which corresponds to an effective radius of 1.3 +/- 0.2 km if assuming a comet-like albedo pV = 0.04. This size is in agreement with an independent estimate based on the reported nongravitational acceleration and activity of the interstellar object. If the measured photometric variations are solely due to the rotation of an aspherical nucleus, the axis ratio must be 2:1 or greater, and the rotation period greater than or similar to 1 hr. Leveraging the range of covered phase angles, we identified a significant opposition surge of similar to 0.2 mag with a width of 3 degrees +/- 1 degrees, which may include concurrent contributions from orbital plane crossing and tail projection, and determined a linear phase slope of 0.026 +/- 0.006 mag deg-1 for the coma dust. Compared to the preperihelion brightening trend, 3I faded more rapidly on the outbound leg, following an activity index of 4.5 +/- 0.3, not unusual in the context of solar system comets. This activity asymmetry is further corroborated by a postperihelion coma surface brightness profile that is significantly shallower than its preperihelion counterpart. From discovery statistics, we infer that multiple interstellar objects resembling 3I probably went undetected prior to the discovery of 1I/'Oumuamua, unless the overall population possesses a steep size distribution.

The rotations of cometary nuclei are known to change in response to outgassing torques. The nucleus of the Jupiter-family comet 41P/Tuttle-Giacobini-Kresak exhibited particularly dramatic rotational changes when near perihelion in 2017 April. Here, we use archival Hubble Space Telescope observations from 2017 December to study the postperihelion lightcurve of the nucleus and to assess the nucleus size. From both Hubble photometry and nongravitational acceleration measurements, we find a diminutive nucleus with effective radius rn = 500 +/- 100 m. Systematic optical variations are consistent with a two-peaked (i.e., rotationally symmetric) lightcurve with period 0.60 +/- 0.01 days, substantially different from periods measured earlier in 2017. The spin of the nucleus likely reversed between perihelion in 2017 April and December as a result of the outgassing torque. We infer a dimensionless moment arm kT = 0.013, about twice the median value in short-period comets. The lightcurve range of 0.4 mag indicates a projected nucleus axis ratio greater than or similar to 1.4:1, while the active fraction of the nucleus decreased from similar to 2.4 in 2001 (suggesting augmentation of the gas production by sublimating coma ice grains) to similar to 0.14 in 2017, a result of long-term modification of the surface. We find that the physical lifetime of this small nucleus to spin up is short compared to the reported similar to 1500 yr dynamical time spent in the current orbit. Two limiting reconciliations of this inequality are suggested. The nucleus could be in a state of unusually strong activity, leading us to overestimate the average mass-loss rate and outgassing torque and so to underestimate the physical lifetime. Alternatively, the nucleus could be the surviving remnant of a once larger body for which outgassing torques were less effective in changing the spin.

We report the successful detection of the nucleus of interstellar object 3I/ATLAS, achieved by applying the nucleus extraction technique to our Hubble Space Telescope (HST) observations from December 2025 to January 2026. The product of the V-band geometric albedo, p_V, with the physical cross-section of the nucleus is 0.22 ± 0.07 km^2, which corresponds to an effective radius of 1.3 ± 0.2 km if assuming p_V = 0.04, as is typical for cometary nuclei in the solar system. This size is in agreement with our estimate derived from the reported nongravitational effect and activity of the interstellar object. If the measured photometric variations are solely due to the rotation of an aspherical nucleus, the axis ratio must be 2:1 or greater, and the rotation period ≳1 hr. Leveraging the range of covered phase angles, we identified a significant opposition surge of ∼0.2 mag with a width of 3^∘± 1^∘, which may include concurrent contributions from orbital plane crossing and tail projection, and determined a linear phase slope of 0.026 ± 0.006 mag degree^-1 for the coma dust. Compared to the preperihelion brightening trend, 3I faded more rapidly on the outbound leg, following an activity index of 4.5 ± 0.3, not unusual in the context of solar system comets. This activity asymmetry is further corroborated by a postperihelion coma surface brightness profile that is significantly shallower than its preperihelion counterpart. From the statistics, we infer that multiple interstellar objects resembling 3I likely went undetected even before the discovery of 1I/`Oumuamua.

We present optical observations of the Halley-type comet 12P/Pons–Brooks (12P) on its approach to perihelion. The comet was active even in the first observations at ∼8 au. Starting at ∼4 au, 12P exhibited an extraordinary series of outbursts, in which the brightness changed by a factor up to 100 and the coma morphology transformed under the action of radiation pressure into a distinctive “horned” appearance. Individual outburst dust masses are several ×10 ^9 kg, with kinetic energies ∼10 ^14 J, release times ∼10 ^4 s, and effective power ∼10 ^10 W. These properties are most consistent with, although do not definitively establish, an origin by the crystallization of amorphous water ice with the related release of trapped supervolatile gases. This interpretation is supported by the observation that the specific outburst energy and the specific crystallization energy are comparable (both ∼10 ^5 J kg ^−1 ).

Most Kreutz family sungrazing comets are discovered only days before perihelion, severely limiting observational opportunities to study their physical nature and decay. Kreutz sungrazer C/2024 S1 (ATLAS) was discovered a month before reaching its perihelion distance of 0.008 au, allowing physical observations from both ground- and space-based telescopes. We present observations from 0.9 to 0.4 au using the Nordic Optical Telescope showing that (1) nucleus disintegration was ongoing already at 0.7 au pre-perihelion, (2) the activity varied unpredictably with distance, and (3) the nucleus radius was <600 m (red geometric albedo 0.04 assumed). We also use coronagraphic observations from the STEREO-A spacecraft to study C/2024 S1 at heliocentric distances ≲ 0.1 au. We find that the coma scattering cross section peaked near 0.075 au and faded progressively, by a factor ∼20, toward the last observation at 0.02 au. We interpret the near-perihelion fading as a result of the sublimation of refractory coma grains, beginning at blackbody temperatures ∼1000 K, consistent with olivine composition. The comet was not detected after perihelion. We consider processes operating to destroy the nucleus when near perihelion, concluding that rotational instability and sublimation losses work together toward this end, even before entry of the comet into the Roche lobe of the Sun.

Nongravitational forces play surprising and, sometimes, centrally important roles in shaping the motions and properties of small planetary bodies. In the solar system, the morphologies of comets, the delivery of meteorites, and the shapes and dynamics of asteroids and binaries are all affected by nongravitational forces. In exoplanetary systems and debris disks, nongravitational forces affect the lifetimes of circumstellar particles and feed refractory debris to the photospheres of the central stars. Unlike the gravitational force, which is a simple function of the well-known separations and masses of bodies, the nongravitational forces are frequently functions of poorly known or even unmeasurable physical properties. Here, we present order-of-magnitude descriptions of nongravitational forces, with examples of their application.

We describe preperihelion optical observations of interstellar comet 3I/ATLAS taken during 2025 July–September using the Nordic Optical Telescope. Fixed aperture photometry of the comet is well described by a power-law function of heliocentric distance, r H , with the exponent (“index”) n = 3.8 ± 0.3 across the 4.6–1.8 au distance range (phase function 0.04 ± 0.02 mag degree −1 assumed). This indicates that the dust production rates vary in proportion to r H − 1.8 ± 0.3 . An r H − 2 variation is expected of a strongly volatile material, and consistent with independent spectroscopic observations showing that carbon dioxide is the primary driver of activity. The measured heliocentric index is unremarkable in the context of solar system comets, for which n is widely dispersed, and provides no basis on which to describe 3I as either dynamically old (thermally processed) or new (pristine). The morphology of the comet changes from a Sun-facing dust fan in the early 2025 July observations, to one dominated by an antisolar dust tail at later dates. We attribute the delayed emergence of the tail to the large size (effective radius 100 μ m) and slow ejection (5 m s −1 ) of the optically dominant dust particles, and their consequently sluggish response to solar radiation pressure. Small (micron-sized) particles may be present but not in numbers sufficient to dominate the scattering cross section. Their relative depletion possibly reflects interparticle cohesion, which binds small particles more effectively than large ones. A similar preponderance of 100 μ m grains was reported in 2I/Borisov. However, 2I differed from 3I in having a much smaller (asteroid-like) heliocentric index, n = 1.9 ± 0.1. Dust production rates in 3I are ∼180 kg s −1 at 2 au, compared with ∼70 kg s −1 in 2I/Borisov at the same distance.

We combine ground- and space-based observations of long-period comet C/2021 O3 (Panstarrs; perihelion distance 0.287 au) in order to investigate its reported near-perihelion destruction. Preperihelion photometric observations show a remarkably small heliocentric dependence of the scattered light, with s = 2.59 +/- 0.21, distinct from values reported in other long-period comets, for which s = 4 is the canonical standard. The index is smaller than expected of coma production by equilibrium sublimation of either supervolatiles (for which s similar to 4 is expected) or water ice (s similar to 6-8) across the similar to 4 to 2 au range. The absolute magnitude deduced from the preperihelion data is H = 13.0 +/- 0.3 (coma scattering cross section similar to 225 km(2) for an assumed geometric albedo 0.04), while after perihelion the cross section fades by a factor of 25 to H = 16.5 (similar to 9 km(2)). STEREO spacecraft observations near perihelion show a long debris trail whose properties are consistent with forward scattering from radius similar to 7 mu m particles. The data show that the nucleus of C/2021 O3 was not destroyed at perihelion. Although the light curve from 3.9 au inbound to 0.8 au outbound is not uniquely determined, a simple explanation is provided by seasonal dimming on a nucleus having high obliquity and an asymmetric distribution of near-surface volatiles. The survival of the nucleus against rotational disruption suggests a preperihelion nucleus radius r(n) greater than or similar to 1.0 km, while the photometric limit to the radius of the nucleus after perihelion is r(n) < 1.7 km (geometric albedo 0.04 assumed).

We present high-angular-resolution observations of the third known interstellar interloper, 3I/ATLAS, from the Hubble Space Telescope. The object is clearly active at 3.8 au preperihelion, showing dust emitted from the hot, Sun-facing side of the nucleus and a weak, radiation-pressure-swept tail away from the Sun. We apply a simple model to estimate the mass loss rate in dust as dM/dt similar to 12 a mu 1/2 kg s-1, where a mu is the mean particle size in microns. With 1 <= a mu <= 100, we infer dM/dt similar to 12-120 kg s-1. A fit to the surface brightness distribution of the inner coma limits the effective radius of the nucleus to rn <= 2.8 km, assuming red geometric albedo 0.04. Conversely, the nucleus cannot be smaller than similar to 0.22 km in radius if its coma is supplied by sublimation of carbon monoxide and must be larger if a less volatile molecule drives the mass loss.

We observed the split comet 157P/Tritton in 2022 October-November and 2024 January with the Nordic Optical Telescope. Our observations show that the splitting continued during the entire observing campaign. Fragmentation was associated with outbursts, consistent with the action of outgassing torques that spun up the nucleus and its fragments to the point of rotational instability. The outburst-fragmentation events can lead to a runaway process where the increasing spin rate, driven by outgassing torques, results in repeated mass loss until the sublimating body completely disintegrates.

We present Hubble Space Telescope observations of interstellar comet 2I/Borisov on five occasions between UT 2019 October 12 and 2020 January 29. Our high-resolution images show persistent asymmetry in the dust coma (Figure 1), best explained by a thermal lag on the rotating nucleus, with peak mass loss occurring in the comet nucleus afternoon (Figure 2). In this interpretation, the nucleus rotates with an obliquity of 30 deg (pole direction R.A. = 205 deg and decl. = 52 deg). The subsolar latitude varied from -35 deg (southern solstice) at the time of discovery to 0 deg (equinox) in 2020 January, suggesting that long-term variations in the coma brightness and activity level may be influenced by seasonal effects (Figure 3). Our model shows that newly reported photometric outbursts (Drahus et al. 2020) and the release of a fragment (Jewitt et al. 2020) could result from a seasonal effect, as the northern hemisphere is illuminated for the first time.Reference: Kim, Y., et al. 2020, ApJL, 895, L34. Figure 1: HST images of 2I/Borisov marked with UT dates of observation. Isophotal contours and extended antisolar and negative velocity vectors (dashed lines) are overlaid to highlight an asymmetry in the coma. Figure 2: Best-fit solutions for the model jet direction (red circles and dashed line), showing a systematic drift with time from 2019 October to 2020 January. The solutions follow the changing projected direction of the Sun (black squares and solid line), but are offset from it by ~20 deg. The existence of this offset suggests a thermal lag on the rotating nucleus. Figure 3: Subsolar (red) and sub-Earth (blue) latitude of 2I/Borisov as a function of time, together with the heliocentric distance (black solid line) on the right axis. We assumed a rotation pole orientation of R.A. = 205 deg and decl. = 52 deg.

The practical distinctions between asteroids and comets, viewed as products of accretion on either side of the snow line, are less clear-cut than previously understood. In this chapter, we discuss the numerous solar system populations which have physical and dynamical properties that conflict with any simple diagnosis of their nature and origin. Studies of these so-called"continuum"or"transition objects", which include many of the most intriguing bodies in the solar system, have implications for a broad range of scientific topics from the demise of comets and the activation of asteroids to the production of interplanetary debris and the origin of the terrestrial planet volatiles. We present an overview of the current state of knowledge concerning the asteroid-comet continuum and discuss the numerous physical processes behind the activity shown by small bodies in the solar system.

We discuss the development of activity in the extraordinary, distant long-period comet C/2017 K2 over the heliocentric distance range 9 < rH < 16 AU.  C/2017 K2 is an incoming long-period comet with a period so long (~ 3 Myr) that no heat from the previous perihelion can be retained; we can be sure that the observed mass-loss is driven by the current insolation and not by a thermal lag.  The comet is characterized by a steady-state coma of sub-millimeter and larger particles ejected at low (4 m/s) velocity, filling a roughly spheroidal coma with a characteristic scale of 80,000 km.  In a fixed, co-moving volume around the nucleus we find that the scattering cross-section of the coma, C, is related to the heliocentric distance by a power law, C ~ rH-s, with heliocentric index s = 1.14+/-0.05. This dependence is significantly weaker than the rH-2, variation of the insolation as a result of two effects.  These are, first, the heliocentric dependence of the dust velocity and, second, a lag effect due to very slow-moving particles ejected long before the observations were taken.   A Monte Carlo model of the photometry shows that dust production beginning at rH ~ 35 AU is needed to match the measured heliocentric index, with only a slight dependence on the particle size distribution.  Dust mass loss rates at 10 AU are of order dM/dt ~ 103 a1 kg/s, where 0.1 < a1 < 1 is the effective particle radius expressed in millimeters.The expulsion of submillimeter and larger grains, beginning at Kuiper belt distances, is likely the result of the sublimation of near-surface supervolatile ice (probably CO, as suggested by the recent detection of this molecule at 6.7 AU; Yang et al. Ap. J. Letters, in press). Water ice is involatile over the observed distance range and even the energy and gas release triggered by the crystallization of amorphous ice, if present, cannot produce activity at 35 AU.  Comet C/2017 K2 will reach perihelion near Mars' orbit in December 2022.   This work is described in D. Jewitt, Y. Kim. M. Mutchler, J. Agarwal, J. Li and H. Weaver (2021).  Astronomical Journal, 161:188 (11pp)

Ice naturally forms in the disordered or ``amorphous'' state when accreted from vapor at temperatures and pressures found in the interstellar medium and in the frigid, low density outer regions of the Sun's protoplanetary disk. It is therefore the expected form of ice in comets and other primitive bodies that have escaped substantial heating since formation. Despite expectations, however, the observational evidence for amorphous ice in comets remains largely indirect. This is both because the spectral features of amorphous ice are subtle and because the solar system objects for which we possess high quality data are mostly too close to the Sun and too hot for amorphous ice to survive near the surface, where it can be detected. This chapter reviews the properties of amorphous ice, the evidence for its existence and its consequences for the behavior of comets.

1I/`Oumuamua and 2I/Borisov are the first macroscopic interstellar objects to be detected in the solar system. Their discovery has triggered a tsunami of scientific interest regarding the physical properties, dynamics and origin of the so-called interstellar interloper population. While it is clear that a deep understanding of these issues cannot be reached from a sample of just two bodies, the emergence of this new field of astronomical study is particularly fascinating, with ramifications from planetary science to galactic dynamics.

Planetary Systems Now, pp. 259-283 (2023) No AccessChapter 10: Planetary Astrophysics of Small BodiesDavid JewittDavid JewittDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, USAhttps://doi.org/10.1142/9781800613140_0010Cited by:0 PreviousNext AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail Abstract: This is an intentionally brief overview of Solar System science as seen from the perspective of an observational planetary astronomer. Instead of following the format used in my lectures for the school, I thought it would be more fun to focus on a set of topics that illustrate current problems and areas of activity in planetary science. FiguresReferencesRelatedDetails Planetary Systems NowMetrics History PDF download

Kuiper Belt objects exhibit a wider color range than any other solar system population. The origin of this color diversity is unknown, but likely the result of the prolonged irradiation of organic materials by galactic cosmic rays (GCRs). Here, we combine ultrahigh-vacuum irradiation experiments with comprehensive spectroscopic analyses to examine the color evolution during GCR processing methane and acetylene under Kuiper Belt conditions. This study replicates the colors of a population of Kuiper Belt objects such as Makemake, Orcus, and Salacia. Aromatic structural units carrying up to three rings as in phenanthrene (C 14 H 10 ), phenalene (C 9 H 10 ), and acenaphthylene (C 12 H 8 ), of which some carry structural motives of DNA and RNA connected via unsaturated linkers, were found to play a key role in producing the reddish colors. These studies demonstrate the level of molecular complexity synthesized of GCR processing hydrocarbon and hint at the role played by irradiated ice in the early production of biological precursor molecules.

Short-period comet 108P/Ciffreo is known for its peculiar double morphology, in which the nucleus is accompanied by a co-moving, detached, diffuse 'blob'. We report new observations of 108P/Ciffreo taken with the Hubble Space Telescope and the Nordic Optical Telescope and use them to determine the cause of this unusual morphology. The separation and the longevity of the blob across several orbits together rule out the possibility of a single, slow-moving secondary object near the primary nucleus. We use a model of coma particle dynamics under the action of solar gravity and radiation pressure to show that the blob is an artifact of the turn-around of particles ejected sunward and repelled by sunlight. Numerical experiments limit the range of directions which can reproduce the morphology and explain why the co-moving blob appearance is rare.