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Worldwide Help Sought For Comet Study Effort

Amateur and professional astronomers are invited to provide observations of three comets that will make close approaches to Earth over the next two years.
The three comets are 41P/Tuttle-Giacobini-Kresak, 45P/Honda-Mrkos-Pajdusakova, and 46P/Wirtanen. The comets will pass by Earth at distances ranging from 0.08 AU to 0.15 AU. AU or Astronomical Unit is the distance from the Sun to Earth. Such close approaches of three comets within two years are rare and typically occur only once every few decades.

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Avalanches, Not Internal Pressure, Cause Comet Nuclei Outbursts

Outbursts of comet nuclei are likely caused by surface avalanches rather than geyser-like eruptions from within, research by PSI Associate Research Scientist Jordan Steckloff shows.
Rapid asymmetric brightening events of comets have been observed for decades, and have long been thought to be the result of some sort of eruption of materials from deep within the interior of a comet, said Steckloff. Steckloff's abstract "Are Comet Outbursts the Result of Avalanches?" was presented today at a press conference at the joint 48th meeting of the American Astronomical Society (AAS) Division for Planetary Sciences (DPS) and 11th European Planetary Science Congress (EPSC) in Pasadena, Calif.

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Title: Observations of Comets C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3 (WISE), C/2010 S1 (LINEAR), and C/2012 K6 (McNaught) at large heliocentric distances
Author: Oleksandra Ivanova, Lubo Nesluan, Zuzana Seman Kriandova, Jan Svore, Pavlo Korsun, Viktor Afanasiev, Volodymyr Resetnyk, Maxim Andreev

Photometric and spectroscopic observations of five nearly parabolic comets with eccentricity larger than 0.99 at heliocentric distances greater than 4 AU were performed. No molecular emission was observed for any studied comet and the entire cometary activity in all cases was attributed to dust production. Upper limits of the gas production rates for the main neutral molecules in the cometary comae were calculated. The derived values of dust apparent magnitudes were used to estimate the upper limit of the geometric cross-section of cometary nuclei (upper limits of radii range from 2 km to 28 km). Due to the poor sublimation of water ice at these distances from the Sun, other mechanisms triggering activity in comets are discussed.

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Title: Comets as collisional fragments of a primordial planetesimal disk
Author: Alessandro Morbidelli, Hans Rickman

The Rosetta mission and its exquisite measurements have revived the debate on whether comets are pristine planetesimals or collisionally evolved objects. We investigate the collisional evolution experienced by the precursors of current comet nuclei during the early stages of the Solar System, in the context of the so-called "Nice Model". We consider two environments for the collisional evolution: (1) the trans-planetary planetesimal disk, from the time of gas removal until the disk was dispersed by the migration of the ice giants, and (2) the dispersing disk during the time that the scattered disk was formed. Simulations have been performed, using different methods in the two cases, to find the number of destructive collisions typically experienced by a comet nucleus of 2km radius. In the widely accepted scenario, where the dispersal of the planetesimal disk occurred at the time of the Late Heavy Bombardment about 4Gy ago, comet-sized planetesimals have a very small chance to survive against destructive collisions in the disk. On the extreme assumption that the disk was dispersed directly upon gas removal, there is a chance for a significant fraction of the planetesimals to remain intact. However, these survivors would still bear the marks of many non-destructive impacts. Thus, the Nice Model of Solar System evolution predicts that typical km-sized comet nuclei are predominantly fragments resulting from collisions experienced by larger parent bodies. An important goal for further research is to investigate, whether the observed properties of comet nuclei are compatible with such a collisional origin.

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Why Comets Are Like Deep Fried Ice Cream

Astronomers tinkering with ice and organics in the lab may have discovered why comets are encased in a hard, outer crust.
Using an icebox-like instrument nicknamed Himalaya, the researchers show that fluffy ice on the surface of a comet would crystalize and harden as the comet heads toward the sun and warms up. As the water-ice crystals form, becoming denser and more ordered, other molecules containing carbon would be expelled to the comet's surface. The result is a crunchy comet crust sprinkled with organic dust.

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Title: On measuring the size of nuclei of comets
Author: S. N. Dolya

Possibilities of measuring the size of nuclei of comets hidden by dust clouds are discussed. To this end, the dust cloud should be irradiated with a flow of rods accelerated in a linear mass accelerator to the velocity six kilometers per second. Each rod should be equipped with a transmitter with a power of one microwatt, which is destroyed in a collision with a comet's nucleus, or continues to work if the rod passes through the dust cloud without collision. Radio signals are received by three independent ground stations. At a distance of one thousand kilometers from the nucleus of the comet the power of the received signals is ten to the minus seventeenth Watt power, the receiver noise power is ten to the minus twentieth Watt power.

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Comets take pole position as water bearers

The tide of an ongoing debate about whether comets or asteroids supplied most of Earth's water has turned back to comets with the discovery that the Hartley 2 comet has a similar ratio of heavy water to ordinary water as Earth.
Paul Hartogh of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and his team measured the comet's ratio of deuterium to hydrogen using the European Space Agency's Herschel Space Observatory.

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Mystery of Comet Birth In Primordial Nebula Solved

Comets are icy bodies, yet they are made of materials formed at very high temperatures. Where do these materials come from? Researchers from the Institut UTINAM (1) (CNRS/Université de Besançon) have now provided the physical explanation behind this phenomenon.
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Title: Photophoretic transport of hot minerals in the solar nebula
Authors: A. Moudens, O. Mousis, J.-M. Petit, G. Wurm, D. Cordier and S. Charnoz

Context. Hot temperature minerals have been detected in a large number of comets and were also identified in the samples of Comet Wild 2 that were returned by the Stardust mission. Meanwhile, observations of the distribution of hot minerals in young stellar systems suggest that these materials were produced in the inner part of the primordial nebula and have been transported outward in the formation zone of comets.
Aims. We investigate the possibility that photophoresis provides a viable mechanism to transport high-temperature materials from the inner solar system to the regions in which the comets were forming.
Methods. We use a grid of time-dependent disk models of the solar nebula to quantify the distance range at which hot minerals can be transported from the inner part of the disk toward its outer regions as a function of their size (10^-5 to 10^-1 m) and density (500 and 1000 kgmV^-3). These models will also yield information on the disk properties (radius of the inner gap, initial mass, and lifetime of the disk). The particles considered here are in the form of aggregates that presumably were assembled from hot mineral individual grains ranging down to submicron sizes and formed by condensation within the hottest portion of the solar nebula. Our particle-transport model includes the photophoresis, radiation pressure, and gas drag.
Results. Depending on the postulated disk parameters and the density of particles, 10^-2 to 10^-1 m aggregates can reach heliocentric distances up to  ~35 AU in the primordial nebula over very short timescales (no more than a few hundred thousand years). 10^-3 m particles follow the same trajectory as the larger ones but their maximum migration distance does not exceed  ~26 AU and is reached at later epochs in the disks. On the other hand, 10^-5 to 10^-4 m aggregates are continuously pushed outward during the evolution of the solar nebula. Depending on the adopted disk parameters, these particles can reach the outer edge of the nebula well before its dissipation.
Conclusions. Our simulations suggest that irrespective of the employed solar nebula model, photophoresis is a mechanism that can explain the presence of hot temperature minerals in the formation region of comets. Comets probably had the time to trap the dust transported from the inner solar system either in their interior during accretion or in the form of shells surrounding their surface if they ended their growth before the particles reached their formation location.

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Comet Observing Planner



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