Large Comet Research Articles

Nutritional Composition of Post-Catastrophic Foods.

In addition to current challenges in food production arising from climate change, soil salinization, drought, flooding, and human-caused disruption, abrupt sunlight reduction scenarios (ASRS), e.g., a nuclear winter, supervolcano eruption, or large asteroid or comet strike, are catastrophes that would severely disrupt the global food supply and decimate normal agricultural practices. In such global catastrophes, teragrams of particulate matter, such as aerosols of soot, dust, and sulfates, would be injected into the stratosphere and block sunlight for multiple years. The reduction of incident sunlight would cause a decrease in temperature and precipitation and major shifts to climate patterns leading to devastating reductions in agricultural production of traditional food crops. To survive a catastrophic ASRS or endure current and future disasters and famines, humans might need to rely on post-catastrophic foods, or those that could be foraged, grown, or produced under the new climate conditions to supplement reduced availability of traditional foods. These foods have sometimes been referred to as emergency, alternate, or resilient foods in the literature. While there is a growing body of work that summarizes potential post-catastrophic foods and their nutritional profiles based on existing data in the literature, this article documents a list of protocols to experimentally determine fundamental nutritional properties of post-catastrophic foods that can be used to assess the relative contributions of those foods to a balanced human diet that meets established nutritional requirements while avoiding toxic levels of nutrients. © 2024 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Total digestible glucans Basic Protocol 2: Apparent protein digestibility Basic Protocol 3: Vitamins B1, B3, B9, C, and D2 by HPLC Basic Protocol 4: Total antioxidant activity (DPPH-scavenging activity) Basic Protocol 5: Total phenolic compounds (Folin-Ciocalteu reagent method) Basic Protocol 6: Mineral content by ICP-OES.

Characterizing the nucleus of comet 162P/Siding Spring using ground-based photometry

ABSTRACT Comet 162P/Siding Spring is a large Jupiter-family comet with extensive archival lightcurve data. We report new r-band nucleus light curves for this comet, acquired in 2018, 2021, and 2022. With the addition of these light curves, the phase angles, at which the nucleus has been observed, range from 0.39○ to 16.33○. We absolutely calibrate the comet light curves to r-band Pan-STARRS 1 magnitudes, and use these light curves to create a convex shape model of the nucleus by convex lightcurve inversion. The best-fitting shape model for 162P has axis ratios a/b = 1.56 and b/c = 2.33, sidereal period P = 32.864 ± 0.001 h, and a rotation pole oriented towards ecliptic longitude λE = 118○ ± 26○ and latitude βE = −50○ ± 21○. We constrain the possible nucleus elongation to lie within 1.4 < a/b < 2.0 and discuss tentative evidence that 162P may have a bilobed structure. Using the shape model to correct the light curves for rotational effects, we derive a linear phase function with slope β = 0.051 ± 0.002 mag deg−1 and intercept Hr(1, 1, 0) = 13.86 ± 0.02 for 162P. We find no evidence that the nucleus exhibited an opposition surge at phase angles down to 0.39°. The challenges associated with modelling the shapes of comet nuclei from light curves are highlighted, and we comment on the extent to which we anticipate that Legacy Survey of Space and Time will alleviate these challenges in the coming decade.

Main Belt Comets and other “Interlopers” in the Solar System

According to traditional ideas about the formation of the Solar System starting from a protoplanetary disk of gas and dust, a well-defined distribution of planets and minor bodies is expected: (a) volatile-poor rocky bodies (terrestrial planets and asteroids) in the inner part of the Solar System and (b) volatile-rich objects (gaseous giant planets, comets, Centaurs, and trans-Neptunian objects) in the outer part. All these bodies are expected to orbit near a plane (the ecliptic) coinciding with that of the protoplanetary disk. However, in the modern Solar System many bodies are present that do not respect this simplistic expectation. First of all, there are the so-called Main Belt Comets, apparently asteroidal objects that show an activity similar to that of comets. In addition, there is an object (and several others very probably exist), which, despite its S-type spectrum characteristic of rocky bodies, is found on a cometary orbit. Finally, there are many asteroids on very inclined orbits with respect to the ecliptic. These very interesting groups of objects, which, according to the traditional point of view, could be collectively seen as some sort of “interlopers” of the Solar System, will be discussed in this review, which offers descriptions of their properties and their likely origin. In this respect, the possibility is discussed that many active asteroids (such as those belonging to the Taurid Complex) are the result of the fragmentation of large comets that occurred in the relatively recent past.

Comet fragmentation as a source of the zodiacal cloud

ABSTRACT Models of the zodiacal cloud’s thermal emission and sporadic meteoroids suggest Jupiter-family comets (JFCs) as the dominant source of interplanetary dust. However, comet sublimation is insufficient to sustain the quantity of dust presently in the inner Solar system, suggesting that spontaneous disruptions of JFCs may supply the zodiacal cloud. We present a model for the dust produced in comet fragmentations and its evolution. Using results from dynamical simulations, the model follows individual comets drawn from a size distribution as they evolve and undergo recurrent splitting events. The resulting dust is followed with a kinetic model which accounts for the effects of collisional evolution, Poynting–Robertson drag, and radiation pressure. This allows to model the evolution of both the size distribution and radial profile of dust, and we demonstrate the importance of including collisions (both as a source and sink of dust) in zodiacal cloud models. With physically motivated free parameters this model provides a good fit to zodiacal cloud observables, supporting comet fragmentation as the plausibly dominant dust source. The model implies that dust in the present zodiacal cloud likely originated primarily from disruptions of ∼50-km comets, since larger comets are ejected before losing all their mass. Thus much of the dust seen today was likely deposited as larger grains ∼0.1 Myr in the past. The model also finds the dust level to vary stochastically; e.g. every ∼50 Myr large (>100 km) comets with long dynamical lifetimes inside Jupiter cause dust spikes with order of magnitude increases in zodiacal light brightness lasting ∼1 Myr. If exozodiacal dust is cometary in origin, our model suggests it should be similarly variable.

Interobserver Variability of Ultrasound Features Based on American College of Radiology Thyroid Imaging Reporting and Data System Lexicon in American College of Radiology Thyroid Imaging Reporting and Data System System: A Single-Center Study With Radiologists and Radiology Residents.

The aim of this study is to evaluate the variability of selecting the ultrasound features used in American College of Radiology Thyroid Imaging Reporting and Data System (ACR-TIRADS) and in assigning the ACR-TIRADS level in a single center among radiologists and radiology residents. The study cohort consisted of 108 thyroid nodules in 102 patients who had definite cytology results after thyroid fine needle aspiration biopsy (Bethesda category II, VI) or surgery. Seven observers including 3 radiologists and 4 radiology residents evaluated the nodules according to 5 ultrasound feature categories. The evaluation process was performed after a joint meeting session, in which the "white papers" of the ACR-TIRADS committee were discussed regarding the thyroid ultrasound reporting lexicon, and final TIRADS system. Variability of ultrasound features and assigning ACR-TIRADS level was measured using Fleiss kappa statistics. Agreement for ultrasound features was "substantial" to "almost perfect" among all observers, with composition (κ = 0.86), macrocalcification (κ = 0.89) and peripheral calcification (κ = 0.92) at the highest level of agreement. The level of agreement for large comet tail artifacts and punctate echogenic foci was "moderate" in residents, whereas in radiologists, that level was "substantial." The agreement for assigning ACR-TIRADS level was moderate in resident as well as in radiologist subgroup. Agreement of thyroid ultrasound features was "substantial to almost perfect" among all observers. Although the level of agreement among resident group decreased to "moderate" level, ACR-TIRADS is a useful system in thyroid nodule management.

Rapid repurposing of pulp and paper mills, biorefineries, and breweries for lignocellulosic sugar production in global food catastrophes

Producing sugar from lignocellulosic biomass is a promising resilient food solution to counter the near-total global failure of food production due to the agricultural collapse that would likely follow an abrupt sunlight reduction catastrophe such as a nuclear winter, a supervolcanic eruption, or a large asteroid or comet impact.This study examines how quickly edible sugar production could be ramped up globally by repurposing pulp and paper mills, sugarcane biorefineries, corn biorefineries, and breweries for lignocellulosic sugar production. A sub-unit component comparison to the NREL 2017 Biochemical Sugar Model indicates that 84%, 65%, 37%, and 39% of ISBL unit components are present, respectively. Fast construction methods were studied to analyze how this and other industrial foods could be rapidly leveraged in a catastrophe.Results suggest that the world’s current sugar demand could quickly be fulfilled by repurposing pulp and paper mills for lignocellulosic sugar production, given 5 months of production ramp-up and 24/7 construction. This method could reduce construction time to an estimated 32% of the original at an increased labor cost of 1.47 times, resulting in sugar production beginning 5 months after the catastrophe at a retail cost of $0.82 USD/kg. This could not only contribute a significant share of the food requirement after the catastrophe (∼28% within the first year), but also be key to preventing global starvation between the time at which global food storages run dry and other resilient food solutions can scale up significantly.This study aims to serve as the basis for more comprehensive scenario analyses. More research is needed to characterize material and labor constraints to fast response in more depth; repurposing and fast construction pilot studies and food safety studies are recommended.

Taurid complex smoking gun: Detection of cometary activity

Using the Secular Light Curve (SLC) formalism (Ferrín, 2010), we have catalogued 88 probable members of the Taurid Complex (TC). 51 of them have useful SLCs and 34 of these (67%) exhibit cometary activity. This high percentage of active asteroids gives support to the hypothesis of a catastrophe that took place during the Upper Paleolithic (Clube and Napier, 1984), when a large short-period comet, arriving in the inner Solar System from the Kuiper Belt, experienced, starting from 20 thousand years ago, a series of fragmentations that produced the present 2P/Encke comet, together with a large number of other members of the TC. The fragmentation of the progenitor body was facilitated by its heterogeneous structure (very similar to a rubble pile) and this also explains the current coexistence in the complex of fragments of different composition and origin. We have found that (2212) Hephaistos and 169P/NEAT are active and members of the TC with their own sub-group. Other components of the complex are groups of meteoroids, that often give rise to meteor showers when they enter the terrestrial atmosphere, and very probably also the small asteroid that in 1908 exploded in the terrestrial atmosphere over Tunguska. What we see today of the TC are the remnants of a very varied and numerous complex of objects, characterized by an intense past of collisions with the Earth which may continue to represent a danger for our planet.

Using the American College of Radiology Thyroid Imaging Reporting and Data System at the Point of Care: Sonographer Performance and Interobserver Variability

The purpose of this study was to assess inter-observer variability and performance when sonographers assign features to thyroid nodules on ultrasound using the American College of Radiology (ACR) Thyroid Imaging Reporting and Data System (TI-RADS). Fifteen sonographers retrospectively evaluated 100 thyroid nodules and assigned features to each nodule according to ACR TI-RADS lexicon. Ratings were compared with one another and to a gold standard using Fleiss’ and Cohen's kappa statistics, respectively. Sonographers were also asked subjective questions regarding their comfort level assessing each feature, and opinions were compared with performance using a mixed effects model. Sonographers demonstrated only slight agreement for margin (κ = 0.18, 95% confidence interval [CI]: 0.16–0.20) and large comet tail artifact (κ = 0.08, 95% CI: 0.06–0.10) but better performance for macrocalcification (κ = 0.41, 95% CI: 0.39–0.43) and no echogenic foci (κ = 0.52, 95% CI: 0.50–0.54). Sonographer comfort level with different feature assignments did not statistically correlate with performance for a given feature. In conclusion, sonographers using ACR TI-RADS to assign thyroid nodule features on ultrasound demonstrate a range of agreement across features, with margin and large comet tail artifact showing the most variability. These results highlight potential areas of focus for sonographer education efforts as ACR TI-RADS continues to be implemented in radiology departments.

Herschel map of Saturn’s stratospheric water, delivered by the plumes of Enceladus

Context. The origin of water in the stratospheres of giant planets has been an outstanding question ever since its first detection by the Infrared Space Observatory some 20 years ago. Water can originate from interplanetary dust particles, icy rings and satellites, and large comet impacts. Analyses of Herschel Space Observatory observations have proven that the bulk of Jupiter’s stratospheric water was delivered by the Shoemaker-Levy 9 impacts in 1994. In 2006, the Cassini mission detected water plumes at the South Pole of Enceladus, which made the moon a serious candidate for Saturn’s stratospheric water. Further evidence was found in 2011 when Herschel demonstrated the presence of a water torus at the orbital distance of Enceladus that was fed by the moon’s plumes. Finally, water falling from the rings onto Saturn’s uppermost atmospheric layers at low latitudes was detected during the final orbits of Cassini’s end-of-mission plunge into the atmosphere. Aims. In this paper, we use Herschel mapping observations of water in Saturn’s stratosphere to identify its source. Methods. We tested several empirical models against the Herschel-HIFI and -PACS observations, which were collected on December 30, 2010, and January 2, 2011, respectively. Results. We demonstrate that Saturn’s stratospheric water is not uniformly mixed as a function of latitude, but peaks at the equator and decreases poleward with a Gaussian distribution. We obtain our best fit with an equatorial mole fraction 1.1 ppb and a half width at half maximum of 25°, when accounting for a temperature increase in the two warm stratospheric vortices produced by Saturn’s Great Storm of 2010–2011. Conclusions. This work demonstrates that Enceladus is the main source of Saturn’s stratospheric water.

The hazard from fragmenting comets

Abstract Comet disintegration proceeds both through sublimation and discrete splitting events. The cross-sectional area of material ejected by a comet may, within days, become many times greater than that of the Earth, making encounters with such debris much more likely than collisions with the nucleus itself. The hierarchic fragmentation and sublimation of a large comet in a short period orbit may yield many hundreds of such short-lived clusters. We model this evolution with a view to assessing the probability of an encounter which might have significant terrestrial effects, through atmospheric dusting or multiple impacts. Such an encounter may have contributed to the large animal extinctions and sudden climatic cooling of 12,900 years ago, and the near-simultaneous collapse of civilisations around 2350 BC.

The orbit and size-frequency distribution of long period comets observed by Pan-STARRS1

We introduce a new technique to estimate the comet nuclear size frequency distribution (SFD) that combines a cometary activity model with a survey simulation and apply it to 150 long period comets (LPC) detected by the Pan-STARRS1 near-Earth object survey. The debiased LPC size-frequency distribution is in agreement with previous estimates for large comets with nuclear diameter ≳1 km but we measure a significant drop in the SFD slope for small objects with diameters <1 km and approaching only 100 m diameter. Large objects have a slope αbig = 0.72 ± 0.09(stat.) ± 0.15(sys.) while small objects behave as αsmall = 0.07 ± 0.03(stat.) ± 0.09(sys.) where the SFD is ∝10αHN and HN represents the cometary nuclear absolute magnitude. The total number of LPCs that are >1 km diameter and have perihelia q < 10 au is 0.46 ± 0.15 × 109 while there are only 2.4 ± 0.5(stat.) ± 2(sys.) × 109 objects with diameters >100 m due to the shallow slope of the SFD for diameters <1 km. We estimate that the total number of ‘potentially active’ objects with diameters ≥1 km in the Oort cloud, objects that would be defined as LPCs if their perihelia evolved to <10 au, is (1.5 ± 1) × 1012 with a combined mass of 1.3 ± 0.9 M⊕. The debiased LPC orbit distribution is broadly in agreement with expectations from contemporary dynamical models but there are discrepancies that could point towards a future ability to disentangle the relative importance of stellar perturbations and galactic tides in producing the LPC population.

Implications of the small spin changes measured for large Jupiter-family comet nuclei

Rotational spin-up due to outgassing of comet nuclei has been identified as a possible mechanism for considerable mass-loss and splitting. We report a search for spin changes for three large Jupiter-family comets (JFCs): 14P/Wolf, 143P/Kowal-Mrkos, and 162P/Siding Spring. None of the three comets has detectable period changes, and we set conservative upper limits of 4.2 (14P), 6.6 (143P) and 25 (162P) minutes per orbit. Comparing these results with all eight other JFCs with measured rotational changes, we deduce that none of the observed large JFCs experiences significant spin changes. This suggests that large comet nuclei are less likely to undergo rotationally-driven splitting, and therefore more likely to survive more perihelion passages than smaller nuclei. We find supporting evidence for this hypothesis in the cumulative size distributions of JFCs and dormant comets, as well as in recent numerical studies of cometary orbital dynamics. We added 143P to the sample of 13 other JFCs with known albedos and phase-function slopes. This sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos. Partly based on findings from recent space missions to JFCs, we hypothesise that this correlation corresponds to an evolutionary trend for JFCs. We propose that newly activated JFCs have larger albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. If confirmed, this could be used to analyse surface erosion from ground and to distinguish between dormant comets and asteroids.

Impact-related microspherules in Late Pleistocene Alaskan and Yukon \u201cmuck\u201d deposits signify recurrent episodes of catastrophic emplacement

Large quantities of impact-related microspherules have been found in fine-grained sediments retained within seven out of nine, radiocarbon-dated, Late Pleistocene mammoth (Mammuthus primigenius) and bison (Bison priscus) skull fragments. The well-preserved fossils were recovered from frozen “muck” deposits (organic-rich silt) exposed within the Fairbanks and Klondike mining districts of Alaska, USA, and the Yukon Territory, Canada. In addition, elevated platinum abundances were found in sediment analysed from three out of four fossil skulls. In view of this new evidence, the mucks and their well-preserved but highly disrupted and damaged vertebrate and botanical remains are reinterpreted in part as blast deposits that resulted from several episodes of airbursts and ground/ice impacts within the northern hemisphere during Late Pleistocene time (~46–11 ka B.P.). Such a scenario might be explained by encounters with cometary debris in Earth-crossing orbits (Taurid Complex) that was generated by fragmentation of a large short-period comet within the inner Solar System.

Formation and past evolution of the showers of 96P/Machholz complex

In this work we model the dynamical evolution of meteoroid streams of comet 96P/Machholz, and the largest member of the Marsden sunskirters, comet P/1999 J6. We simultaneously fit the characteristics of eight meteor showers which have been proposed to be linked to the complex, using observations from a range of techniques - visual, video, TV and radar. The aim is to obtain a self-consistent scenario of past capture of a large comet into a short-period orbit, and its subsequent fragmentation history. Moreover, we also aim to constrain the dominant parent of these showers.The fit of our simulated shower characteristics to observations is consistent with the scenario of a capture of a proto-comet 96P/Machholz by Jupiter circa 20000 BCE, and a subsequent major breakup around 100–950 CE which resulted in the formation of the Marsden group of comets. We find that the Marsden group of comets are not the immediate parents of the daytime Arietids and Northern and Southern δ-Aquariids, as previously suggested. In fact, the hypothesis that the Northern δ-Aquariids are related to the Marsden group of comets is not supported by this study.The bulk of the observational characteristics of all eight showers can be explained by meteoroid ejection primarily from comet 96P/Machholz between 10000 BCE and 20000 BCE. Assuming the Marsden group of comets originated between 100 CE–950 CE, we conclude that sunskirting comets contribute mainly to the meteoroid stream near the time of the peak of the daytime Arietids, Southern δ-Aquariids, κ-Velids. Finally, we find that the meteor showers identified by Babadzhanov and Obrubov (1992) as the α-Cetids, the Ursids and Carinids correspond to the daytime λ-Taurids, the November ι-Draconids or December α-Draconids and the θ-Carinids.

Evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects: Avoiding a trans-Plutonian planet or just plain bias?

Abstract It is a well-known fact that the presence of a massive perturber interacting with a population of minor bodies following very eccentric orbits can strongly affect the distribution of their nodal distances. The details of this process have been explored numerically and its outcome confirmed observationally in the case of Jupiter, where a bimodal distribution of nodal distances of comets has been found. Here, we show evidence for a possible bimodal distribution of the nodal distances of the extreme trans-Neptunian objects (ETNOs) in the form of a previously unnoticed correlation between nodal distance and orbital inclination. This proposed correlation is unlikely to be the result of observational bias as data for both large semimajor axis Centaurs and comets fit well into the pattern found for the ETNOs, and all these populations are subjected to similar background perturbations when moving well away from the influence of the giant planets. The correlation found is better understood if these objects tend to avoid a putative planet with semimajor axis in the range of 300–400 au.

Global warming and ocean stratification: A potential result of large extraterrestrial impacts

AbstractThe prevailing paradigm for the climatic effects of large asteroid or comet impacts is a reduction in sunlight and significant short‐term cooling caused by atmospheric aerosol loading. Here we show, using global climate model experiments, that the large increases in stratospheric water vapor that can occur upon impact with the ocean cause radiative forcings of over +20 W m−2 in the case of 10 km sized bolides. The result of such a positive forcing is rapid climatic warming, increased upper ocean stratification, and potentially disruption of upper ocean ecosystems. Since two thirds of the world's surface is ocean, we suggest that some bolide impacts may actually warm climate overall. For impacts producing both stratospheric water vapor and aerosol loading, radiative forcing by water vapor can reduce or even cancel out aerosol‐induced cooling, potentially causing 1–2 decades of increased temperatures in both the upper ocean and on the land surface. Such a response, which depends on the ratio of aerosol to water vapor radiative forcing, is distinct from many previous scenarios for the climatic effects of large bolide impacts, which mostly account for cooling from aerosol loading. Finally, we discuss how water vapor forcing from bolide impacts may have contributed to two well‐known phenomena: extinction across the Cretaceous/Paleogene boundary and the deglaciation of the Neoproterozoic snowball Earth.

The variability of meteoroid falling

We analysed a historical catalogue of meteoroid falling during the last 400years. We report here for the first time the synchronization between observed meteors and solar barycentric parameters in 19.6 and 13.2years periodicities using a new multiple cross wavelet. The group of moderated number of meteors is distributed around the positive phase of the solar barycentric periodicity of 13.2years. While the group of severe number of meteors are distributed on the positive phase of the solar barycentric periodicity of 19.6years. These periodicities could be associated with Jupiter periodicities. So understanding the modulation of meteoroid falling is important for determining the falling patterns of these objects and for knowing when it is more likely to expect the entry of one of these objects into the Earth's atmosphere, because bodies falling onto the Earth can cause damage from minor impacts to mass-extinctions events. One of the most extreme events was the formation of the Chicxulub impact crater 65,000,000years ago that caused one of the five major mass extinctions in the last 500,000,000years. During the 20th and 21st centuries, a series of events demonstrated the importance of collisions between planets and small bodies (comets and asteroids), which included our own planet. In the case of the Earth, we can cite three examples: Tunguska, Curuça and Chelyabinsk. These events invite us to think that perhaps the occurrence of this phenomenon might be more common than we realize, but the lack of communication or people in the area where they happened prevents us from having a complete record. Modern man has not witnessed the impact of large asteroids or comets on our planet, but it has been observed on other planetary bodies. The most spectacular of these events was the collision of fragments of the comet Shoemaker–Levy 9 with Jupiter in 1994. The total energy of the 21 impacts on Jupiter's atmosphere was estimated as the equivalent of tens of millions of megatons of TNT.

KIC 8462852: TRANSIT OF A LARGE COMET FAMILY

ABSTRACT We investigate the plausibility of a cometary source of the unusual transits observed in the KIC 8462852 light curve. A single comet of similar size to those in our solar system produces a transit depth of the order of 10−3 lasting less than a day which is much smaller and shorter than the largest dip observed ( &sim; 20 &percnt; ?> for ∼3 days), but a large, closely traveling cluster of comets can fit the observed depths and durations. We find that a series of large comet swarms, with all except one on the same orbit, provides a good fit for the KIC 8462852 data during Quarters 16 and 17, but does not explain the large dip observed during Quarter 8. However, the transit dips only loosely constrain the orbits and can be fit by swarms with periastrons differing by a factor of 10. To reach a transit depth of ∼0.2, the comets need to be in a close group of ∼30, if they are ∼100 km in radius or in a group of ∼300 if they are ∼10 km in radius. The total number of comets required to fit all of the dips is ∼70 ∼ 100 km or ∼700 ∼ 10 km comets. A single comet family from a tidally disrupted Ceres-sized progenitor or the start of a Late Heavy Bombardment period explains the last ∼60 days of the unusual KIC 8462852 light curve.

Size and albedo distributions of asteroids in cometary orbits using WISE data

We study the distributions of effective diameter ($D$), beaming parameter ($\eta$), and visible geometric albedo ($p_V$) of asteroids in cometry orbits (ACOs) populations, derived from NASA's Wide-field Infrared Explorer (WISE) observations, and compare these with the same, independently determined properties of the comets. The near-Earth asteroid thermal model (NEATM) is used to compute the $D$, $p_V$ and $\eta$. We obtained $D$ and $p_V$ for 49 ACOs in Jupiter family cometary orbits (JF-ACOs) and 16 ACOs in Halley-type orbits (Damocloids). We also obtained $\eta$ for 45 of them. All but three JF-ACOs (95% of the sample) present a low albedo compatible with a cometary origin. The $p_V$ and $\eta$ distributions of both ACO populations are very similar. For the entire sample of ACOs, the mean geometric albedo is $\bar{p_V} = 0.05 \pm 0.02$, ($\bar{p_V} = 0.05 \pm 0.01$ and $\bar{p_V} =0.05 \pm 0.02$ for JF-ACOs and Damocloids, respectively) compatible with a narrow albedo distribution similar to that of the Jupiter family comets (JFCs), with a $\bar{p_V} \sim 0.04$. The $\bar{\eta} =1.0 \pm 0.2$. We find no correlations between $D$, $p_V$ , or $\eta$. We compare the cumulative size distribution (CSD) of ACOs, Centaurs, and JFCs. Although the Centaur sample contains larger objects, the linear parts in their log-log plot of the CSDs presents a similar cumulative exponent ($\beta = 1.85 \pm 0.30$ and $1.76 \pm 0.35$, respectively). The CSD for Damocloids presents a much shallower exponent $\beta = 0.89 \pm 0.17$. The CSD for JF-ACOs is shallower and shifted towards larger diameters with respect to the CSD of active JFCs, which suggests that the mantling process has a size dependency whereby large comets tend to reach an inactive stage faster than small ones. Finally, the population of JF-ACOs is comparable in number that of JFCs, although there are more tens-km JF-ACOs than JFCs.

The dilemma of nuclear energy in space

Nuclear energy, used in weapons as well as for electricity generation, has the potential to destroy life on Earth. But it also has the potential to save life as we know it. Currently, nuclear explosives are the only technology with the capability to deflect, on relatively short notice, a large asteroid fragment or comet headed for a collision with Earth. However, a number of international treaties prohibit the use of nuclear explosives in space. The peaceful and critically effective use of nuclear energy to prevent a civilization-threatening collision is at odds with its potential for inducing a catastrophic thermonuclear war on Earth. The author uses the Fermi paradox—if there are extraterrestrial civilizations in our galaxy, why haven’t they communicated with us?—to examine the dichotomy of whether nuclear explosives are a barrier or a path to long-term human survival.