Kuiper Belt Region Research Articles

Analyzing the Elements of X-ray Emission Spectrum of C/1999 S4 (LINEAR) and 9p/ Tempel 1 Comets

The study focused on the elemental abundance of two different comets. These comets were observed at an energy range of (100-10000) eV. The X-ray emission spectrum was analyzed and the results showed that comet C/1999 S4 (LINEAR) from the Oort cloud region (long-period) and comet 9p/Tempel 1 from the Kuiper Belt region (short-period) had the highest abundances of the different elements. The ds9 program used data from NASA's Chandra analysis to show the spectra of these comets and analyze their elements. The results showed that some elements were similar in all comets, such as (Carbon (C), Oxygen (O), Neon (Ne), and Silicon (Si)). While others varied from one comet to another, such as (Magnesium (Mg) and Chlorine (Cl)) elements which appeared in the C/1999 S4 (LINEAR) comet, and (Aluminum (A) and Phosphors (P)) elements which appeared in the 9P/Tempel 1 comet. This is due to these comets' areas of presence, where the elements of comets can be classified through their areas of presence.

How the formation of Neptune shapes the Kuiper belt

Hydrodynamical simulations predict the inward migration of giant planets during the gas phase of the protoplanetary disc. This phenomenon is also invoked to explain resonant and near-resonant exoplanetary system structures. The early inward migration may also have affected our Solar System and sculpted its different minor planet reservoirs. In this study we explore how the early inward migration of the giant planets shapes the Kuiper belt. We test different scenarios with only Neptune and Uranus and with all the four giant planets, also including some models with the subsequent outward planetesimal-driven migration of Neptune after the gas dispersal. We find objects populating mean motion resonances even when Neptune and Uranus do not migrate at all or only migrate inwards. When the planets are fixed, planetesimals stick only temporarily to the mean motion resonances, while inwards migration yields a new channel to populate the resonances without invoking convergent migration. However, in these cases, it is hard to populate mean motion resonances that do not cross the planetesimal disc (such as 2:1 and 5:2) and there is a lack of resonant Kuiper belt objects (KBOs) crossing Neptune’s orbit. These Neptune crossers are an unambiguous signature of the outward migration of Neptune. The starting position and the growth rate of Neptune have consequences for the contamination of the classical Kuiper belt region from neighbouring regions. The eccentricity and inclination space of the hot classical Kuiper belt objects and the scattered disc region become much more populated when all the giant planets are included. The 5:2 resonance with Neptune becomes increasingly populated with deeper inward migrations of Neptune. However, the overall inclination distribution is still narrower than suggested by observations, as is generally the case for Kuiper belt population models.

Can orbital clustering of KBOs in the ecliptic be due to the solar toroidal field generated spacetime dragging?

The Kuiper belt objects (KBOs) exhibit an orbital clustering of the outer planets lying at perihelion distances larger than Neptune and semimajor axes greater than 150 AU from the Sun. This implies a hitherto unknown dynamical mechanism to counter randomizing of the orbital elements caused by the giant solar system planets. Using the toroidal field induced frame-dragging we deduce here the observed range of the Kuiper belt region, the semi-major axis of Sedna like objects in the Kuiper belt, as well as the orbital clustering of the KBOs in the ecliptic, without assuming dynamical effects induced by trans-Neptunian-objects (TNOs). We also calculate the orbital precession rates for the inner planets and show their correspondence, within the range of observational accuracy, with recent planetary ephemerides.

Dynamical effects on the classical Kuiper belt during the excited-Neptune model

The link between the dynamical evolution of the giant planets and the Kuiper Belt orbital structure can provide clues and insight about the dynamical history of the Solar System. The classical region of the Kuiper Belt has two populations (the cold and hot populations) with completely different physical and dynamical properties. These properties have been explained in the framework of a sub-set of the simulations of the Nice Model, in which Neptune remained on a low-eccentricity orbit (Neptune’s eccentricity is never larger than 0.1) throughout the giant planet instability (Nesvorný 2015a,b). However, recent simulations (Gomes et al., 2018) have showed that the remaining Nice model simulations, in which Neptune temporarily acquires a large-eccentricity orbit (larger than 0.1), are also consistent with the preservation of the cold population (inclination smaller than 4°), if the latter formed in situ. However, the resulting a cold population showed in many of the simulations eccentricities larger than those observed for the real population. The purpose of this work is to discuss the dynamical effects on the Kuiper belt region due to an excited Neptune phase. We focus on a short period of time, of about six hundred thousand years, which is characterized by Neptune’s large eccentricity and smooth migration with a slow precession of Neptune’s perihelion. This phase was observed during a full simulation of the Nice Model (Gomes et al., 2018) just after the last jump of Neptune’s orbit due to an encounter with another planet. We show that if self-gravity is considered in the disk, the precession rate of the particles longitude of perihelion ϖ is slowed down, which in turn speeds up the cycle of ϖN−ϖ (the subscript N referring to Neptune), associated to the particles eccentricity evolution. This, combined with the effect of mutual scattering among the bodies, which spreads all orbital elements, allows some objects to return to low eccentricities. However, we show that if the cold population originally had a small total mass, this effect is negligible. Thus, we conclude that the only possibilities to keep at low eccentricity some cold-population objects during a high-eccentricity phase of Neptune are that (i) either Neptune’s precession was rapid, as suggested by Batygin et al. (2011) or (ii) Neptune’s slow precession phase was long enough to allow some particles to experience a full secular cycle of ϖ−ϖN.

Into the Extreme: US Environmental Systems and Politics beyond Earth by Valerie Olson

Reviewed by: Into the Extreme: US Environmental Systems and Politics beyond Earth by Valerie Olson Blair Bainbridge Valerie Olson, Into the Extreme: US Environmental Systems and Politics beyond Earth. Minneapolis: University of Minnesota Press, 2018. 280 pp. Since its launch in 2006, New Horizons’ journey through the solar system has been filled with historical firsts. After zooming past the outer planets in record time, this NASA space probe successfully captured the first ever up-close images of Pluto and its moons in 2015. Now, after a short slumber in the distant Kuiper Belt region of the solar system, it is back online, preparing to make history once more. In January 2019, New Horizons will fly by what a recent press release has called the “most primitive world ever observed by spacecraft” (Wall 2018). Originally called 2014 MU69, this Kuiper Belt object was the subject of a recent re-naming competition. The name settled on: Ultima Thule, (meaning, “beyond Thule”), a term used in medieval literature to refer to “the extreme limit of travel and discovery”1 and places “beyond the known world” (Wall 2018). The name was praised by New Horizons’ primary investigator, Alan Stern, who found its symbolism apposite beyond its spatial connotations. Not only would reaching this new world be an “ultimate” achievement for NASA’s space exploration program, Stern claimed; MU69 would also serve as “humanity’s next Ultima Thule” (Wall 2018). Stern’s comparison of New Horizons’ space exploration to the opening of new spatial and conceptual frontiers for humanity writ large is what anthropologist Valerie Olson would describe as a kind of “systems work” (6), and is the main subject of her new monograph, Into the Extreme. Olson defines systems work as a set of processes and techniques that “conceptually and physically formalize social, material, and spatial connections and separations, often in terms of a system–environment relationship” (6). Taking on systems as an ethnographic object rather than [End Page 1417] as an analytical tool, Olson seeks to understand how systems interlink disparate things into integrated wholes, and to what sociopolitical ends. The crux of this book lies in Olson’s argument that systems are “relational technologies” (5); in the most basic sense, its five themed chapters can be seen as ethnographic manifestations of this claim across multiple scales, sites, and objects of analysis. Moving across a wide range of speculative scientific projects, from haptic spacesuits to underwater human habitats, Olson puts forth NASA spaceflight programs as central loci of systems thinking and practice, that is, systems work, in the US from the postwar era to the present. In doing so, she reveals the integral contributions of outer space systems work to contemporary American technoscientific visions and aspirations for the future, shaped out of deep-seated settler colonial ideologies but irreducible to them. Into the Extreme investigates the unfolding of a pervasive contemporary relational imagination, an imagination which the discipline of anthropology has routinely embraced. As such, anthropologists of all stripes will want to pay attention to this book. Into the Extreme draws on Olson’s seven years of fieldwork (2005–2012) at four NASA centers, five medical centers in the US and Canada, as well as aerospace companies, conferences, and field testing sites. Olson follows projects that came about during NASA’s “Constellation” era, an initiative begun by George W. Bush in 2006 to go “back to the moon and Mars and ‘beyond’” (7), with a $230 billion USD budget to boot. These projects include NASA Extreme Environment Mission Operations’ (NEEMO) Aquarius habitat, an underwater space analog research station off the coast of South Florida (Chapter 1); biomedical research on astronaut veterans, candidates, and hopefuls, whose ecologically integrated bodies come to exemplify the human as system (Chapter 2); speculative design projects for comfortable dwelling in the extreme, including sensitive, skintight space suits and transitional and transportable habitats (Chapters 3 and 4); and near-earth object (NEO) research and political activism, involving advocacy for new off-world mining opportunities as well as new transnational disaster-preparedness strategies (Chapter 5). Olson provides an important genealogy of the systems concept in the Introduction, which is a key part of what makes Into the Extreme a worthwhile...

The Structure of the Distant Kuiper Belt in a Nice Model Scenario

Abstract This work explores the orbital distribution of minor bodies in the outer Solar System emplaced as a result of a Nice model migration from the simulations of Brasser and Morbidelli. This planetary migration scatters a planetesimal disk from between 29 and 34 au and emplaces a population of objects into the Kuiper Belt region. From the 2:1 Neptune resonance and outward, the test particles analyzed populate the outer resonances with orbital distributions consistent with trans-Neptunian object (TNO) detections in semimajor axis, inclination, and eccentricity, while capture into the closest resonances is too efficient. The relative populations of the simulated scattering objects and resonant objects in the 3:1 and 4:1 resonances are also consistent with observed populations based on debiased TNO surveys, but the 5:1 resonance is severely underpopulated compared to population estimates from survey results. Scattering emplacement results in the expected orbital distribution for the majority of the TNO populations; however, the origin of the large observed population in the 5:1 resonance remains unexplained.

THE ALBEDO-COLOR DIVERSITY OF TRANSNEPTUNIAN OBJECTS

We analyze albedo data obtained using the Herschel Space Observatory that reveal the existence of two distinct types of surface among midsized transneptunian objects. A color-albedo diagram shows two large clusters of objects, one redder and higher albedo and another darker and more neutrally colored. Crucially, all objects in our sample located in dynamically stable orbits within the classical Kuiper belt region and beyond are confined to the bright-red group, implying a compositional link. Those objects are believed to have formed further from the Sun than the dark-neutral bodies. This color-albedo separation is evidence for a compositional discontinuity in the young solar system.

Similar origin for low- and high-albedo Jovian Trojans and Hilda asteroids?

Hilda asteroids and Jupiter Trojans are two low-albedo (p$_{\rm v}$ ~ 0.07) populations for which the Nice model predicts an origin in the primordial Kuiper Belt region. However, recent surveys by WISE and the Spitzer Space Telescope (SST) have revealed that ~2% of these objects possess high albedos (p$_{\rm v}$ > 0.15), which might indicate interlopers - that is, objects not formed in the Kuiper Belt - among these two populations. Here, we report spectroscopic observations in the visible and/or near-infrared spectral ranges of twelve high-albedo (p$_{\rm v}$ > 0.15) Hilda asteroids and Jupiter Trojans. These twelve objects have spectral properties similar to those of the low- albedo population, which suggests a similar composition and hence a similar origin for low- and high-albedo Hilda asteroids and Jupiter Trojans. We therefore propose that most high albedos probably result from statistical bias or uncertainties that affect the WISE and SST measurements. However, some of the high albedos may be true and the outcome of some collision-induced resurfacing by a brighter material that could include water ice. Future work should attempt to investigate the nature of this supposedly bright material. The lack of interlopers in our sample allows us to set an upper limit of 0.4% at a confidence level of 99.7% on the abundance of interlopers with unexpected taxonomic classes (e.g., A-, S-, V-type asteroids) among these two populations.

INCLINATION MIXING IN THE CLASSICAL KUIPER BELT

We investigate the long-term evolution of the inclinations of the known classical and resonant Kuiper belt objects (KBOs). This is partially motivated by the observed bimodal inclination distribution and by the putative physical differences between the low- and high-inclination populations. We find that some classical KBOs undergo large changes in inclination over gigayear timescales, which means that a current member of the low-inclination population may have been in the high-inclination population in the past, and vice versa. The dynamical mechanisms responsible for the time-variability of inclinations are predominantly distant encounters with Neptune and chaotic diffusion near the boundaries of mean motion resonances. We reassess the correlations between inclination and physical properties including inclination time-variability. We find that the size-inclination and color-inclination correlations are less statistically significant than previously reported (mostly due to the increased size of the data set since previous works with some contribution from inclination variability). The time-variability of inclinations does not change the previous finding that binary classical KBOs have lower inclinations than non-binary objects. Our study of resonant objects in the classical Kuiper belt region includes objects in the 3:2, 7:4, 2:1, and eight higher-order mean motion resonances. We find that these objects (some of which were previously classified as non-resonant) undergo larger changes in inclination compared to the non-resonant population, indicating that their current inclinations are not generally representative of their original inclinations. They are also less stable on gigayear timescales.

The Scattered Disk as the Source of the Jupiter Family Comets

The short-period Jupiter family comets (JFCs) are thought to originate in the Kuiper Belt; specifically, a dynamical subclass of the Kuiper Belt known as the is argued to be the dominant source of JFCs. However, the best estimates from observational surveys indicate that this source may fall short by more than 2 orders of magnitude of the estimates obtained from theoretical models of the dynamical evolution of Kuiper Belt objects into JFCs. We reexamine the scattered disk as a source of the JFCs and make a rigorous estimate of the discrepancy. We find that the uncertainties in the dynamical models combined with a change in the size distribution function of the scattered disk at faint magnitudes (small sizes) beyond the current observational limit offer a possible but problematic resolution to the discrepancy. We discuss several other possibilities: that the present population of JFCs is a large fluctuation above their long-term average, that larger scattered disk objects tidally break up into multiple fragments during close planetary encounters as their orbits evolve from the trans-Neptune zone to near Jupiter, or that there are alternative source populations that contribute significantly to the JFCs. Well-characterized observational investigations of the Centaurs, objects that are transitioning between the trans-Neptune Kuiper Belt region and the inner solar system, can test the predictions of the non-steady state and the tidal breakup hypotheses. The classical and resonant classes of the Kuiper Belt are worth reconsideration as significant additional or alternate sources of the JFCs.

EF Chamaeleontis: Warm Dust Orbiting a Nearby 10 Myr Old Star

Most Vega-like stars have far-infrared excess (60 μm or longward in IRAS, ISO, or Spitzer MIPS bands) and contain cold dust (150 K) analogous to the Sun's Kuiper Belt region. However, dust in a region more akin to our asteroid belt and thus relevant to the terrestrial planet building process is warm and produces excess emission in mid-infrared wavelengths. By cross-correlating Hipparcos dwarfs with the MSX catalog, we found that EF Cha, a member of the recently identified, ~10 Myr old, Cha-Near moving group, possesses prominent mid-infrared excess. N-band spectroscopy reveals a strong emission feature characterized by a mixture of small, warm, amorphous, and possibly crystalline silicate grains. Survival time of warm dust grains around this A9 star is 105 yr, much less than the age of the star. Thus, grains in this extrasolar terrestrial planetary zone must be of a and not a remnant of primordial dust and are suggestive of substantial planet formation activity. Such second generation warm excess occurs around ~13% of the early-type stars in nearby young stellar associations.

The dynamical stability of a Kuiper Belt-like region

The dynamics of the Kuiper Belt region between 33 and 63 au is investigated just taking into account the gravitational influence of Neptune. Indeed the aim is to analyse the information which can be drawn from the actual exoplanetary systems, where typically physical and orbital data of just one or two planets are available. Under this perspective we start our investigation using the simplest three-body model (with Sun and Neptune as primaries), adding at a later stage the eccentricity of Neptune and the inclinations of the orbital planes to evaluate their effects on the Kuiper Belt dynamics. Afterwards we remove the assumption that the orbit of Neptune is Keplerian by adding the effect of Uranus through the Lagrange-Laplace solution or through a suitable resonant normal form. Finally, different values of the mass ratios of the primary to the host star are considered in order to perform a preliminary analysis of the behaviour of exoplanetary systems. In all cases, the stability is investigated by means of classical tools borrowed from dynamical system theory, like Poincare mappings and Lyapunov exponents.

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to- para abundance ratios (OPRs) of cometary ammonia were determined from the NH 2 emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28 ± 0.07 (nuclear spin temperature: T spin = 24 ± 2 K ) and 1.26 ± 0.08 ( T spin = 25 ± 2 K ), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH 2 emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH 2 on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 ( T spin > 17 K ) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.

NASA Returns Rocks from a Comet

Cometary particles returned by the Stardust Discovery Mission are primarily silicate materials of solar system origin. Some of the grains were formed at high temperatures close to the Sun, but then transported far out to the Kuiper belt region of the solar system before being incorporated in the comet.

Nuclear spin temperature of ammonia in Comet 9P/Tempel 1 before and after the Deep Impact event

The Deep Impact mission succeeded in excavating inner materials from the nucleus of Comet 9P/Tempel 1 on 2005 July 04 (at 05:52 UT). Comet 9P/Tempel 1 is one of Jupiter family short period comets, which might originate in the Kuiper belt region in the solar nebula. In order to characterize the comet and to support the mission from the ground-based observatory, optical high-dispersion spectroscopic observations were carried out with the echelle spectrograph (UVES) mounted on the 8-m telescope VLT (UT2) before and after the Deep Impact event. Ortho-to- para abundance ratios (OPRs) of cometary ammonia were determined from the NH 2 emission spectra. The OPRs of ammonia on July 3.996 UT and 4.997 UT were derived to be 1.28 ± 0.07 (nuclear spin temperature: T spin = 24 ± 2 K ) and 1.26 ± 0.08 ( T spin = 25 ± 2 K ), respectively. There is no significant change between before and after the impact. Actually, most materials ejected from the impact site could have moved away from the nucleus on July 4.997 UT, about 17 h after the impact. However, a small fraction of the ejected materials might remain in the slit of UVES instrument at that time because an excess of about 20% in the NH 2 emission flux is observed above the normal activity level was found [Manfroid, J., Hutsemékers, D., Jehin, E., Cochran, A.L., Arpigny, C., Jackson, W.M., Meech, K.J., Schulz, R., Zucconi, J.-M., 2007. Icarus. This issue]. If the excess of NH 2 on July 04.997 UT was produced from icy materials excavated by the Deep Impact, then an upper-limit of the ammonia OPR would be 1.75 ( T spin > 17 K ) for those materials. On the other hand, the OPR of ammonia produced from the quiescent sources was similar to that of the Oort cloud comets observed so far. This fact may imply that physical conditions where cometary ices formed were similar between Comet 9P/Tempel 1 and the Oort cloud comets.

Dynamical structure and origin of the Trans-Neptunian population

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.

The Kuiper Belt and the Solar System's Comet Disk

Our planetary system is embedded in a small-body disk of asteroids and comets, vestigial remnants of the original planetesimal population that formed the planets. Once formed, those planets dispersed most of the remaining small bodies. Outside of Neptune, this process has left our Kuiper belt and built the Oort cloud, as well as emplacing comets into several other identifiable structures. The orbits in these structures indicate that our outer solar system's comet disk was shaped by a variety of different physical processes, which teach us about how the giant planets formed. Recent work has shown that the scattered disk is the most likely source of short-period comets. Moreover, a growing body of evidence indicates that the sculpting of the Kuiper belt region may have involved large-scale planetary migration, the presence of other rogue planetary objects in the disk, and/or the close passage of other stars in the Sun's birth cluster.

Evidence for a Collisional Mechanism Affecting Kuiper Belt Object Colors

The Kuiper belt region of the solar system, stretching from 30 to at least 50 AU, contains Pluto-Charon, some ~105 Kuiper belt objects (KBOs) larger than 100 km in diameter, and myriad smaller bodies produced by collisions among KBOs (Farinella, Davis, & Stern, published in 2000). The most notable physical attribute of the KBOs discovered to date is their reflectance color diversity, ranging from gray (i.e., neutral) to very red (Tegler & Romanishin, published in 2000). I report here evidence for a statistical correlation between the colors of KBOs and the mean random impact speed that these objects experience, lending evidence to suggestions some years ago by modelers (Jewitt & Luu, published in 1996) that a competition between collisional resurfacing and radiation reddening may contribute to the color diversity of KBOs.

The Humps of KBO’s Size Distribution

AbstractWe study the possible humps or deviation from the single-power law for the size distribution of the Kuiper Belt Objects (KBOs). Both the current observational data and theoretical simulations show evidence of such humps. We conclude that this is an imprint of the depletion of the outer Solar System in the Kuiper Belt region.

Collision Rates in the Present-Day Kuiper Belt and Centaur Regions: Applications to Surface Activation and Modification on Comets, Kuiper Belt Objects, Centaurs, and Pluto–Charon

We present results from our model of collision rates in the present-day Edgeworth–Kuiper Belt and Centaur region. We have updated previous results to allow for new estimates of the total disk population in order to examine surface activation and modification time scales due to cratering impacts. We extend previous results showing that the surfaces of Edgeworth–Kuiper Belt objects are not primordial and have been moderately to heavily reworked by collisions. Objects smaller than about r=2.5 km have collisional disruption lifetimes less than 3.5 Gyr in the present-day collisional environment and have probably been heavily damaged in their interiors by large collisions. In the 30- to 50-AU region, impacts of 1-km-radius comets onto individual 100-km-radius objects occur on 7×10 7–4×10 8-year time scales, cratering the surfaces of the larger objects with ∼8–54 craters 6 km in diameter over a 3.5-Gyr period. Collision time scales for impacts of 4-m-radius projectiles onto 1-km-radius comets range from 3×10 7, to 5×10 7 years. The cumulative fraction of the surface area of 1- and 100-km-radius objects cratered by projectiles with radii larger than 4 m ranges from a few to a few tens percent over 3.5 Gyr. The flux of Edgeworth–Kuiper Belt projectiles onto Pluto and Charon is also calculated and is found to be ∼3–5 times that of previous estimates. Our impact model is also applied to Centaur objects in the 5- to 30-AU region. We find that during their dynamical lifetimes within the Centaur region, objects undergo very little collisional evolution. Therefore, the collisional/cratering histories of Centaurs are dominated by the time spent in the Edgeworth–Kuiper Belt rather than the time spent on planet-crossing orbits. Further, we find that the predominant surface activity of Centaur objects like Chiron is most likely not impact-induced.