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Origin and Evolution of the Cometary Reservoirs

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Luke Dones at Southwest Research Institute
Luke Dones
Ramon Brasser
Ramon Brasser
Ramon Brasser
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Nathan Kaib
Nathan Kaib
Nathan Kaib
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Hans Rickman
Hans Rickman
Hans Rickman
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Abstract and Figures

Comets have three known reservoirs: the roughly spherical Oort Cloud (for long-period comets), the flattened Kuiper Belt (for ecliptic comets), and, surprisingly, the asteroid belt (for main-belt comets). Comets in the Oort Cloud were thought to have formed in the region of the giant planets and then placed in quasi-stable orbits at distances of thousands or tens of thousands of AU through the gravitational effects of the planets and the Galaxy. The planets were long assumed to have formed in place. However, the giant planets may have undergone two episodes of migration. The first would have taken place in the first few million years of the Solar System, during or shortly after the formation of the giant planets, when gas was still present in the protoplanetary disk around the Sun. The Grand Tack (Walsh et al. in Nature 475:206–209, 2011) models how this stage of migration could explain the low mass of Mars and deplete, then repopulate the asteroid belt, with outer-belt asteroids originating between, and outside of, the orbits of the giant planets. The second stage of migration would have occurred later (possibly hundreds of millions of years later) due to interactions with a remnant disk of planetesimals, i.e., a massive ancestor of the Kuiper Belt. Safronov (Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets, 1969) and Fernández and Ip (Icarus 58:109–120, 1984) proposed that the giant planets would have migrated as they interacted with leftover planetesimals; Jupiter would have moved slightly inward, while Saturn and (especially) Uranus and Neptune would have moved outward from the Sun. Malhotra (Nature 365:819–821, 1993) showed that Pluto’s orbit in the 3:2 resonance with Neptune was a natural outcome if Neptune captured Pluto into resonance while it migrated outward. Building on this work, Tsiganis et al. (Nature 435:459–461, 2005) proposed the Nice model, in which the giant planets formed closer together than they are now, and underwent a dynamical instability that led to a flood of comets and asteroids throughout the Solar System (Gomes et al. in Nature 435:466–469, 2005b). In this scenario, it is somewhat a matter of luck whether an icy planetesimal ends up in the Kuiper Belt or Oort Cloud (Brasser and Morbidelli in Icarus 225:40–49, 2013), as a Trojan asteroid (Morbidelli et al. in Nature 435:462–465, 2005; Nesvorný and Vokrouhlický in Astron. J. 137:5003–5011, 2009; Nesvorný et al. in Astrophys. J. 768:45, 2013), or as a distant “irregular” satellite of a giant planet (Nesvorný et al. in Astron. J. 133:1962–1976, 2007). Comets could even have been captured into the asteroid belt (Levison et al. in Nature 460:364–366, 2009). The remarkable finding of two “inner Oort Cloud” bodies, Sedna and 2012 \(\mbox{VP}_{113}\), with perihelion distances of 76 and 81 AU, respectively (Brown et al. in Astrophys. J. 617:645–649, 2004; Trujillo and Sheppard in Nature 507:471–474, 2014), along with the discovery of other likely inner Oort Cloud bodies (Chen et al. in Astrophys. J. Lett. 775:8, 2013; Brasser and Schwamb in Mon. Not. R. Astron. Soc. 446:3788–3796, 2015), suggests that the Sun formed in a denser environment, i.e., in a star cluster (Brasser et al. in Icarus 184:59–82, 2006, 191:413–433, 2007, 217:1–19, 2012b; Kaib and Quinn in Icarus 197:221–238, 2008). The Sun may have orbited closer or further from the center of the Galaxy than it does now, with implications for the structure of the Oort Cloud (Kaib et al. in Icarus 215:491–507, 2011). We focus on the formation of cometary nuclei; the orbital properties of the cometary reservoirs; physical properties of comets; planetary migration; the formation of the Oort Cloud in various environments; the formation and evolution of the Kuiper Belt and Scattered Disk; and the populations and size distributions of the cometary reservoirs. We close with a brief discussion of cometary analogs around other stars and a summary.
Cumulative size distributions of several small-body populations in the Solar System from Sheppard and Trujillo (2010). The size of the bodies decreases to the right. From most to least numerous, the figure shows Kuiper Belt Objects (dashed red line), main-belt asteroids (solid blue line), Trojans of Neptune (filled circles), and Trojans of Jupiter (dotted green line). All four small populations have similar steep slopes for bodies with radii ≳50km\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\gtrsim50~\mbox{km}$\end{document}, and all have a change in their size distribution (sometimes called a knee, elbow, rollover, or flattening) at slightly smaller sizes. Sheppard and Trujillo (2010) refer to the knee around 35–50 km in radius (70–100 km in diameter) as the zone of Missing Intermediate-Sized Planetesimals (MISPs) and argue that its consistent location is related to the formation of these bodies, rather than collisional evolution. Morbidelli et al. (2009a) have reached the same conclusion from modeling of the size distribution of main-belt asteroids. New observations and modeling of the size distribution of the Kuiper Belt since 2010 have strengthened the conclusion that its size distribution flattens (see Fig. 17)
Cumulative size distributions of several small-body populations in the Solar System from Sheppard and Trujillo (2010). The size of the bodies decreases to the right. From most to least numerous, the figure shows Kuiper Belt Objects (dashed red line), main-belt asteroids (solid blue line), Trojans of Neptune (filled circles), and Trojans of Jupiter (dotted green line). All four small populations have similar steep slopes for bodies with radii ≳50km\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\gtrsim50~\mbox{km}$\end{document}, and all have a change in their size distribution (sometimes called a knee, elbow, rollover, or flattening) at slightly smaller sizes. Sheppard and Trujillo (2010) refer to the knee around 35–50 km in radius (70–100 km in diameter) as the zone of Missing Intermediate-Sized Planetesimals (MISPs) and argue that its consistent location is related to the formation of these bodies, rather than collisional evolution. Morbidelli et al. (2009a) have reached the same conclusion from modeling of the size distribution of main-belt asteroids. New observations and modeling of the size distribution of the Kuiper Belt since 2010 have strengthened the conclusion that its size distribution flattens (see Fig. 17)
… 
Left panel: The orbits of Jupiter, Saturn, Uranus, Neptune, and the planetesimal disk (i.e., the massive ancestor of the Kuiper Belt) before Jupiter and Saturn cross their 2:1 resonance. in the Nice model of Tsiganis et al. (2005). Middle: After the resonance crossing, the orbits of Uranus (light blue) and Neptune (dark blue) become eccentric. Neptune crosses much of the “Kuiper Belt,” and planetesimals are scattered throughout the Solar System. Right: The planets have now settled down into a stable configuration, and most planetesimals have been ejected from the Solar System by the planets. Also see the animation at http://www.skyandtelescope.com/sky-and-telescope-magazine/beyond-the-printed-page/chaos-in-the-early-solar-system/. Figure from https://commons.wikimedia.org/wiki/File:Lhborbits.png by Dr. Mark Booth (https://en.wikipedia.org/wiki/User:AstroMark). Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license
Left panel: The orbits of Jupiter, Saturn, Uranus, Neptune, and the planetesimal disk (i.e., the massive ancestor of the Kuiper Belt) before Jupiter and Saturn cross their 2:1 resonance. in the Nice model of Tsiganis et al. (2005). Middle: After the resonance crossing, the orbits of Uranus (light blue) and Neptune (dark blue) become eccentric. Neptune crosses much of the “Kuiper Belt,” and planetesimals are scattered throughout the Solar System. Right: The planets have now settled down into a stable configuration, and most planetesimals have been ejected from the Solar System by the planets. Also see the animation at http://www.skyandtelescope.com/sky-and-telescope-magazine/beyond-the-printed-page/chaos-in-the-early-solar-system/. Figure from https://commons.wikimedia.org/wiki/File:Lhborbits.png by Dr. Mark Booth (https://en.wikipedia.org/wiki/User:AstroMark). Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license
… 
Evolution of proper frequency g5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$g_{5}$\end{document} as a function of the ratio of the orbital periods of Saturn and Jupiter. Figure from Brasser et al. (2009)
Evolution of proper frequency g5\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$g_{5}$\end{document} as a function of the ratio of the orbital periods of Saturn and Jupiter. Figure from Brasser et al. (2009)
… 
Evolution of semi-major axis, perihelion distance, and aphelion distance for two systems of five planets (Jupiter, Saturn, and three ice giants) with a massive planetesimal disk outside their orbits, from Batygin et al. (2012). In both panels, Jupiter and Saturn begin in a 3:2 resonance. In the left panel, Saturn and the inner ice giant are in a 2:1 MMR, while both pairs of ice giants are in 5:4 MMRs. In the right panel, Saturn and the inner ice giant are in a 3:2 MMR, the inner pair of ice giants is in a 2:1 MMR, and the outer pair of ice giants is in a 4:3 MMR. In each panel, the innermost ice giant is ejected, leaving behind four planets with orbits similar to those of the outer planets in our Solar System. For the two cases shown here, the planetary system goes unstable in ≲10Myr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\lesssim}10~\mbox{Myr}$\end{document}. Morbidelli and Crida (2007), Nesvorný (2011), and Nesvorný and Morbidelli (2012) have investigated systems of five and six giant planets that are stable in isolation (without an external disk) for hundreds of Myr to >1Gyr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${>}1~\mbox{Gyr}$\end{document}. Whether the instability can be delayed for many hundreds of Myr, as in the original Nice model, for five- or six-planet systems with an external disk is not yet clear
Evolution of semi-major axis, perihelion distance, and aphelion distance for two systems of five planets (Jupiter, Saturn, and three ice giants) with a massive planetesimal disk outside their orbits, from Batygin et al. (2012). In both panels, Jupiter and Saturn begin in a 3:2 resonance. In the left panel, Saturn and the inner ice giant are in a 2:1 MMR, while both pairs of ice giants are in 5:4 MMRs. In the right panel, Saturn and the inner ice giant are in a 3:2 MMR, the inner pair of ice giants is in a 2:1 MMR, and the outer pair of ice giants is in a 4:3 MMR. In each panel, the innermost ice giant is ejected, leaving behind four planets with orbits similar to those of the outer planets in our Solar System. For the two cases shown here, the planetary system goes unstable in ≲10Myr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\lesssim}10~\mbox{Myr}$\end{document}. Morbidelli and Crida (2007), Nesvorný (2011), and Nesvorný and Morbidelli (2012) have investigated systems of five and six giant planets that are stable in isolation (without an external disk) for hundreds of Myr to >1Gyr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${>}1~\mbox{Gyr}$\end{document}. Whether the instability can be delayed for many hundreds of Myr, as in the original Nice model, for five- or six-planet systems with an external disk is not yet clear
… 
Scatter plot of osculating barycentric pericenter distance q\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$q$\end{document} vs. osculating barycentric semi-major axis a\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$a$\end{document} at six times in a simulation of Oort Cloud formation, from Dones et al. (2004). The Sun is assumed to be a single star in its current galactic environment throughout the simulation. The points are color-coded to reflect the region in which the simulated comets formed. Jupiter region comets (initial semi-major axes between 4 and 8 AU) are magenta triangles; Saturn region comets (8–15 AU) are blue triangles; Uranus region comets (15–24 AU) are green circles; Neptune region comets (24–35 AU) are red circles; Kuiper Belt comets (35–40 AU) are black circles. (a) Initial conditions for the simulation. (b) 1 Myr into the simulation. (c) 10 Myr into the simulation. (d) 100 Myr. (e) 1000 Myr. (f) Final results for the simulation, at 4000 Myr, i.e., at roughly the present time. The population of the Oort Cloud peaks about 800 Myr into the simulation. Note that in (f), there is a nearly empty gap for semi-major axes between ≈200\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\approx}200$\end{document} and 1000 AU. This region is not expected to be populated if the Sun was always a single star, but can be if the Sun were once a member of a cluster (see Sect. 5.2)
+12
Scatter plot of osculating barycentric pericenter distance q\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$q$\end{document} vs. osculating barycentric semi-major axis a\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$a$\end{document} at six times in a simulation of Oort Cloud formation, from Dones et al. (2004). The Sun is assumed to be a single star in its current galactic environment throughout the simulation. The points are color-coded to reflect the region in which the simulated comets formed. Jupiter region comets (initial semi-major axes between 4 and 8 AU) are magenta triangles; Saturn region comets (8–15 AU) are blue triangles; Uranus region comets (15–24 AU) are green circles; Neptune region comets (24–35 AU) are red circles; Kuiper Belt comets (35–40 AU) are black circles. (a) Initial conditions for the simulation. (b) 1 Myr into the simulation. (c) 10 Myr into the simulation. (d) 100 Myr. (e) 1000 Myr. (f) Final results for the simulation, at 4000 Myr, i.e., at roughly the present time. The population of the Oort Cloud peaks about 800 Myr into the simulation. Note that in (f), there is a nearly empty gap for semi-major axes between ≈200\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}${\approx}200$\end{document} and 1000 AU. This region is not expected to be populated if the Sun was always a single star, but can be if the Sun were once a member of a cluster (see Sect. 5.2)
… 
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Space Sci Rev (2015) 197:191–269
DOI 10.1007/s11214-015-0223-2
Origin and Evolution of the Cometary Reservoirs
Luke Dones1·Ramon Brasser2·Nathan Kaib3·
Hans Rickman4,5
Received: 13 February 2015 / Accepted: 5 October 2015 / Published online: 24 November 2015
© Springer Science+Business Media Dordrecht 2015
Abstract Comets have three known reservoirs: the roughly spherical Oort Cloud (for long-
period comets), the flattened Kuiper Belt (for ecliptic comets), and, surprisingly, the asteroid
belt (for main-belt comets). Comets in the Oort Cloud were thought to have formed in the
region of the giant planets and then placed in quasi-stable orbits at distances of thousands or
tens of thousands of AU through the gravitational effects of the planets and the Galaxy. The
planets were long assumed to have formed in place. However, the giant planets may have un-
dergone two episodes of migration. The first would have taken place in the first few million
years of the Solar System, during or shortly after the formation of the giant planets, when gas
was still present in the protoplanetary disk around the Sun. The Grand Tack (Walsh et al.
in Nature 475:206–209, 2011) models how this stage of migration could explain the low
mass of Mars and deplete, then repopulate the asteroid belt, with outer-belt asteroids origi-
nating between, and outside of, the orbits of the giant planets. The second stage of migration
would have occurred later (possibly hundreds of millions of years later) due to interactions
with a remnant disk of planetesimals, i.e., a massive ancestor of the Kuiper Belt. Safronov
(Evolution of the Protoplanetary Cloud and Formation of the Earth and the Planets, 1969)
and Fernández and Ip (Icarus 58:109–120, 1984) proposed that the giant planets would have
migrated as they interacted with leftover planetesimals; Jupiter would have moved slightly
inward, while Saturn and (especially) Uranus and Neptune would have moved outward from
the Sun. Malhotra (Nature 365:819–821, 1993) showed that Pluto’s orbit in the 3:2 reso-
nance with Neptune was a natural outcome if Neptune captured Pluto into resonance while
We thank the NASA Cassini Data Analysis Program for support of some of the work described here.
BL. Dones
luke@boulder.swri.edu
1Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302-5142, USA
2Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-IE-1 Ookayama, Meguro-ku,
Tokyo, 152-8550, Japan
3H.L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks St.,
Norman, OK 73019, USA
4Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
5PAS Space Research Center, Bartyckya 18A, 00716 Warszawa, Poland
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... The Centaur population is composed of small, likely icy bodies on dynamically unstable orbits transitioning from the trans-Neptunian region to the inner solar system. Most Centaurs originate in the scattered disk, with a small fraction coming from other trans-Neptunian object (TNO) populations and the Oort Cloud (e.g., Gomes et al. 2008;Dones et al. 2015;Di Sisto & Rossignoli 2020;Kaib & Volk 2022). Centaurs represent an interesting stage in the dynamical evolution of primordial small bodies scattered by planetary perturbations from the "deep freeze" of the TNO reservoir, where they have been stored for billions of years. ...
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  • Nov 2023
The motion of planetesimals initially located in the feeding zone of the planet Proxima Centauri c, at distances of 500 AU from the star to the star’s Hill sphere radius of 1200 AU was considered. In the analyzed non-gaseous model, the primary ejection of planetesimals from most of the feeding zone of an almost formed planet c to distances greater than 500 AU from the star occurred during the first 10 million years. Only for planetesimals originally located at the edges of the planet’s feeding zone, the fraction of planetesimals that first reached 500 AU over the time greater than 10 million years was more than half. Some planetesimals could reach the outer part of the star’s Hill sphere over hundreds of millions of years. Approximately 90% of the planetesimals that first reached 500 AU from Proxima Centauri first reached 1200 AU from the star in less than 1 million years, given the current mass of the planet c. No more than 2% of planetesimals with aphelion orbital distances between 500 and 1200 AU followed such orbits for more than 10 million years (but less than a few tens of millions of years). With a planet mass equal to half the mass of the planet c, approximately 70–80% of planetesimals increased their maximum distances from the star from 500 to 1200 AU in less than 1 million years. For planetesimals that first reached 500 AU from the star under the current mass of the planet c, the fraction of planetesimals with orbital eccentricities greater than 1 was 0.05 and 0.1 for the initial eccentricities of their orbits eo = 0.02 and eo = 0.15, respectively. Among the planetesimals that first reached 1200 AU from the star, this fraction was approximately 0.3 for both eo values. The minimum eccentricity values for planetesimals that have reached 500 and 1200 AU from the star were 0.992 and 0.995, respectively. In the considered model, the disk of planetesimals in the outer part of the star’s Hill sphere was rather flat. Inclinations i of the orbits for more than 80% of the planetesimals that first reached 500 or 1200 AU from the star did not exceed 10°. With the current mass of the planet c, the percentage of such planetesimals with i 20° did not exceed 1% in all calculation variants. The results may be of interest for understanding the motion of bodies in other exoplanetary systems, especially those with a single dominant planet. They can be used to provide the initial data for models of the evolution of the disk of bodies in the outer part of Proxima Centauri’s Hill sphere, which take into account gravitational interactions and collisions between bodies, as well as the influence of other stars. The strongly inclined orbits of bodies in the outer part of Proxima Centauri’s Hill sphere can primarily result from bodies that entered the Hill sphere from outside. The radius of Proxima Centauri’s Hill sphere is an order of magnitude smaller than the radius of the outer boundary of the Hills cloud in the Solar System and two orders of magnitude smaller than the radius of the Sun’s Hill sphere. Therefore, it is difficult to expect the existence of a similarly massive cloud around this star as the Oort cloud around the Sun.
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The Comet Interceptor Mission
Article
Full-text available
  • Jan 2024
  • SPACE SCI REV
Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum Δ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\varDelta $\end{document}V capability of 600 ms−1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$600\text{ ms}^{-1}$\end{document}. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule.
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Survivability of amorphous ice in comets depends on the latent heat of crystallization of impure water ice
Article
  • Jan 2024
Comets are believed to have amorphous rather than crystalline ice at the epoch of their accretion. Cometary ice contains some impurities that govern the latent heat of ice crystallization, Lcry. However, it is still controversial whether the crystallization process is exothermic or endothermic. In this study, we perform one-dimensional simulations of the thermal evolution of kilometer-sized comets and investigate the effect of the latent heat. We find that the depth at which amorphous ice can survive significantly depends on the latent heat of ice crystallization. Assuming the cometary radius of 2 km, the depth of the amorphous ice mantle is approximately 100 m when the latent heat is positive (i.e., the exothermic case with Lcry = +9 × 104 J kg−1). In contrast, when we consider the impure ice representing the endothermic case with Lcry = −9 × 104 J kg−1, the depth of the amorphous ice mantle could exceed 1 km. Although our numerical results indicate that these depths depend on the size and the accretion age of comets, the depth in a comet with the negative latent heat is a few to several times larger than in the positive case for a given comet size. This work suggests that the spatial distribution of the ice crystallinity in a comet nucleus depends on the latent heat, which can be different from the previous estimates assuming pure water ice.
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