Periodic Comet Research Articles

Hubble Space Telescope Observations of Comet P/Shoemaker-Levy 9 (1993e)

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between approximately 23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range approximately 2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiter's atmosphere next July will be approximately 4 x 10(30) ergs ( approximately 10(8) megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of approximately 2 x 10(27) molecules per second.

The Uppsala Comet Magnitude Database

The second part of the Comet Light Curve Catalogue/Atlas (CLICC/A) has been made available by anonymous ftp. This database contains magnitude estimates of periodic comets, reduced to a geocentric distance of 1 AU and corrected for observational effects. The database is described in some detail.

The Comet – Asteroid Transition: Recent Telescopic Observations

The phenomenon of a comet-asteroid transition presents the opportunity to study the comet reservoir in comparison to that from which asteroids are derived and to understand the evolution of comets. I discuss six significant areas which have impacted understanding of the comet-asteroid transition from recent years: new discoveries, discovery of dust and volatile comae at 2060 Chiron, observations of comet nuclei, observations of extinct-comet candidates, observations of dust trails associated with short period comets, and discovery of a dust tail on a prediscovery image of 4015 Wilson-Harrington. It is currently possible to recognize an object only as either a comet or an asteroid. There is currently insufficient information to recognize dormant or extinct cometary nuclei, though this is certainly the major goal being sought.

The infrared (3.2-3.6 m) spectrum of comet P/Swift-Tuttle: detection of methanol and other organics

We present an infrared spectrum of comet P/Swift-Tuttle which exhibits broad emission features at 3.4 and 3.52 μm with fluxes of (5.7±0.25)×10 −15 and (0.8±0.14)×10 −15 Wm −2 respectively, but has no pronounced feature near 3.28 μm. We show that the 3.52-μm feature and much of the 3.4-μm emission can be attributed to fluorescence from methanol, but that some other source is required to explain residual emission centred on 3.43 μm. The derived methanol production rate is (1.6±0.3)×10 28 molecule s −1 , which implies a ratio of methanol to water of approximately 0.05, a value more typical of dynamically new comets than of periodic comets

Disintegration Phenomena Expected During Collision of Comet Shoemaker-Levy 9 with Jupiter

In July 1994, periodic comet Shoemaker-Levy 9 is expected to collide with Jupiter. The largest fragments of the comet's original nucleus will be tidally disrupted shortly before they enter the Jovian atmosphere, and all fragments, from large rocks to small grains, will suffer ablation and disintegration as a result of interaction with the atmosphere. Even if atmospheric entry takes place on the planet's far side, secondary phenomena triggered by the terminal explosions of kilometer-sized fragments are likely to be observable from the Earth.

Estimate of the size of comet Shoemaker-Levy 9 from a tidal breakup model

THE spectacular 'string-of-pearls' appearance of periodic comet Shoemaker-Levy 9 (1993e) and its impending encounter with Jupiter's atmosphere have caused a flurry of speculation on the likely effects of the impact1. The magnitudes of the predicted effects depend critically on the masses of the fragments composing the chain, although these are currently poorly constrained. However, some limits on the comet's total size, and hence mass, can be obtained by considering its dynamical history. On its previous swing past Jupiter, the comet apparently passed within the Roche limit of the planet2,3, and it therefore seems likely that the present chain of about twenty nuclei was created when a single progenitor was split by tidal forces during closest approach. Here we describe a simple model of tidal splitting which is able to reproduce closely both the length of the observed chain and its position angle on the sky as a function of time. The length of the fragment chain is directly proportional to the size of the parent object, which we estimate to have been only about 2 km in diameter.

Detectability of water-ice clouds expected after the P/Shoemaker-Levy 9 impact with Jupiter at near-infrared molecular bands

The predicted crash of periodic comet Shoemaker-Levy 9 into Jupiter in mid-July 1994 is expected to release a considerable amount of cometary material, which is mainly composed of water, into Jupiter's atmosphere. This released water will form a water-ice cloud layer in the stratosphere. We estimated the reflected sunlight from the impact cloud at near-infrared wavelengths using a radiative transfer model. It was found that the cloud is likely to be observable when we choose some strong molecular absorption band such as methane for imaging observation, because we get higher contrast between the cometary ice cloud and background scattered light from the lower ammonia clouds at these wavelengths. This cometary cloud can be used as an atmospheric tracer of the upper troposphere and the stratosphere of Jupiter.

A simple mapping for comets in resonance

The dynamical evolution of a low inclination, intermediate period comet in mean-motion resonance with a giant planet can be described by a simple mapping giving the comet's heliocentric energy and ecliptic longitude at successive aphelia. Provided that the energy oscillations produced by the resonance are not too large, this mapping is linear. When the amplitude of the oscillations is close to the maximum possible within the resonance, the mapping can be improved by adding non-linear terms.

Erratum "effects of Discrete-Source Outgassing on Motions of Periodic Comets and Discontinuous Orbital Anomalies"

Erratum "effects of Discrete-Source Outgassing on Motions of Periodic Comets and Discontinuous Orbital Anomalies"

Dynamical stability in the outer solar system and the delivery of short period comets

Test particle stability in the outer solar system is surveyed. Roughly 7000 test particles are numerically integrated for durations ranging from 20 to 800 million yr. The initial conditions of the test particles fall into two categories: (1) orbits initially near the Lagrange L 4 and L 5 points of the Jovian planets, and (2) circular orbits in the invariable plane with semimajor axes of 5-50 AU. Clusters of test particles near the triangular Lagrange points of Jupiter, Saturn, Uranus, and Neptune survive the full integration. However, the stable regions near Saturn's L 4 and L 5 points exclude the actual Lagrange points. Nearly all particles between the outer planets are removed by close encounters with the planets during the course of these integrations

The infrared light curve of Periodic Comet Halley 1986 III and its relationship to the visual light curve, C2, and water production rates

The near-IR light curve of Periodic Comet Halley 1986 III is analyzed and compared with C2 production, water production, and the visual light curve. This is the most complete IR light curve compiled to date for any comet. The scattering phase function at small sun-comet-earth angles is shown to affect the slope of near-IR light curve significantly. P/Halley's dust production, as inferred from the IR light curve showed an increased production rate near perihelion which appears to be correlated with the onset of significant jet activity. The near-IR light curve, visual light curve, C2, and water production rates displayed different heliocentric variations, suggesting that one parameter cannot be accurately estimated from another. This is particularly true of the early preperihelion visual light curve. A peak of 0.3-0.5 magnitude in the visual magnitude, representing the integrated brightness of the comet's visible coma, lagged the other parameters by about a day. The near-IR color, J-H, was less red during periods of strong dust activity.

Short Communication: Optical probing of dust in comet Grigg-Skjellerup from the Giotto spacecraft

The optical probe experiment (OPE) has provided the first in situ high-resolution optical probing of the very inner coma of a short period comet, during the 10 July 1992 encounter of the Giotto spacecraft with Grigg-Skjellerup. Preliminary results of the encounter are presented below. Indications of entering the dust coma occurred around 17,000 km before closest approach. From the rapid brightening of the signals detected under the observational geometry, there is good evidence that the distance of Giotto's closest approach to the nucleus was below 200 km, and that Giotto passed the nucleus on the anti-sunward side. The scanning of the dust coma at about 70° from the line of motion clearly shows the transition from the coma on the dark side to the coma on the sunny side, where changes in the optical properties of grains occur. The evolution of the polarization degree at about 90° phase angle demonstrates that the optical properties of dust grains differ in the very inner coma, in the dust jets and in the outer coma.

Effects of discrete-source outgassing on motions of periodic comets and discontinuous orbital anomalies

Isoline maps of the Style II nongravitational parameters A1 and A2 of periodic comets are presented for a baseline model as functions of the parameters of the inertially fixed spin vector and the source's location-dependent thrust angle. The parameters A1 and A2 for a collection of sources are equal to the sum of their values for the individual contributors, thus depending on the source distribution. The model is consistent with even the most extreme among detected perturbations, including instances of rapid temporal changes in A2 and the discontinuous orbital anomalies, which are interpreted as products of a sudden redistribution of the transferred momentum due to episodic activation of short-lived sources. It is argued that the lifetimes of active regions vary considerably from case to case, and forced precession of the nucleus strongly influences the comet's activity pattern.

Dynamical Mechanisms for Transferring Comets In and Out of the Oort Cloud

AbstractWe study scenarios where comets are not original members of the Solar System but have been acquired from the surrounding medium through dynamical evolution. This leads to estimates of the present day number density of comets in the interstellar medium which are not in contradiction with observational upper limits. We also consider the dynamical transfer of Oort Cloud comets into short period comets. The process is very sensitive to the inclination of the comet orbit which leads to strong bias in favour of low inclinations in short comet orbits.

Orbital evolution of dust particles from comets and asteroids

In a computer simulation, dust grains of radius 10, 30, and 100 μm were released at perihelion passage from each of 35 different celestial bodies: 15 main belt asteroids, 15 short period comets with perihelion greater than 1 AU, and 5 short period comets with perihelion less than 1 AU. The evolving orbit of each of the 105 released dust grains was then continuosly computed with the Everhart numerical integrator until the orbit aphelion passed inside of 0.387 AU, or the dust grain had been ejected from the Solar System. The forces due to the gravity of the Sun and the planets as well as radiation pressure, Poynting-Robertson drag, and solar wind drag were all included in these numerical simulations. It is found that when dust grains evolve to intersection with the Earth's orbit, they nearly always retain orbital characteristics indicative of their origins; grains from main belt asteroids differ significantly in orbital characteristics, especially in orbital eccentricity, from grains that evolve from comets. Average intersect velocities with the Earth, before the Earth's gravitationa acceleration is taken into account, are about 5 km/sec for asteroidal and in excess of 12 km/ sec for cometary-derived grains. Average orbital eccentricities are about 0.1 for asteroidal grains and usually in excess of 0.4 for cometary grains when their orbits can intersect the Earth's orbit. These results mean that accurate trajectory measurements of meteoroids collected with a near-Earth space platform would make it possible to distinguish asteroidal grains from cometary grains.

The Solar Motion, the Galaxy and the Aphelia Distribution of Long Period Comets

We analyze the incoming direction of 180 long period comets, taken into account the gravitational influence of the planets and the Galaxy as a whole. We have made an improvement of some previous works on the matter, by inclusion of comets observed up to 1989. An asymmetry of aphelia is detected that cannot be explained as due to observational selection. We conclude that the so called Dynamical Friction induced by the solar motion through the interstellar medium could be the proper answer to this observed fact.

Splitting of Periodic Comets

Among the comets that were observed to break in two or more fragments, only a few of them are periodic. So far the dynamic study of the relative motion of a secondary nucleus with respect to the primary has supposed that a cometary fragment is subject to a small and continuousradialnongravitational force after separation at rest. This force acts against the solar attraction and varies according to an inverse square law. A small impulse at break up may also be invoked in some case. A different approach is followed in this paper when dealing with a fragment of a periodic comet: after separation the motion of a secondary nucleus is characterized by nongravitational forces which vary according to the sameg(r) law currently used for the primary.Results of the study of comets P/Biela and P/du Toit-Hartley show that the motion of their fragments after separation is characterized by nongravitational parameters which are larger than those of the parent bodies. Both fragments lasted for about 2 full revolutions and three returns.

Origin of short period comets

Reasons for interest in the origin of short-period comets and the difficulties of computing their long-term dynamcal evolution are reviewed. The relative advantages of a source region in an extended ‘inner core’ of the Oort cloud or a compact ‘comet belt’ just beyond the planetary system are finely balanced, and it is premature to consider the problem solved. A complication is that some comets belonging to the Jupiter family may be part of a time-dependent system, possibly the remains of a giant comet such as Chiron which could have been part of the system ≈ 104 yr ago. The origin of short-period comets plays a pivotal role in many areas of solar system science: planet formation, the source of water (possibly life) on the terrestrial planets, the cratering record on the terrestrial planets and satellites of the outer planets, and the environmental impact posed by massive bodies and their decay products in the Earth's near-space environment.

Dynamics of Comets

The gravitational processes affecting the dynamics of comets are reviewed. At great distances from the Sun the motion of comets is primarily affected by the vertical component of the galactic field, as well as by encounters with stars and giant molecular clouds. When comets move in the region of the planets, encounters with these can strongly affect their motion. A good fraction of all periodic comets spend some time in temporary libration about mean motion resonances with Jupiter; some comets can be captured by this planet as temporary satellites. Finally, there is a small number of objects with orbital characteristics quite different from those of all other short-period comets.

Cometary habitats for primitive life

Comet Halley studies indicate most of the nucleus is covered by an insulating crust, presumed of pyrolysed organic material. The subcrust is warmed and percolated by gases within 2AU, so provides one habitat for primitive replicating organisms. Cracks and crevices within contaminated ice in the craters provides a habitat for photosynthesising organisms. Subsurface lakes on the Europa model, though insulated by some metres of ice, would require a trigger (perhaps meteorite impact) and energy source (chemical or metabolic energy) to initiate and maintain a suitable habitat on short period comets. Constraints on transfer between comets and other planetary bodies implies that radiation-resistant species with lengthy hibernation potential would be expected.