The Nucleus of Comet Hyakutake (C/1996 B2) (original) (raw)

Physical Properties of the Nucleus of Comet 2P/Encke

Icarus, 2000

We report a new study of the nucleus of Comet 2P/Encke, which the CONTOUR spacecraft is scheduled to encounter in November 2003. During the comet's close approach to Earth in July 1997, we measured the mid-infrared thermal and optical scattered continua with data from the TIMMI instrument (imaging) at the ESO 3.6-m telescope (wavelength λ from 8 to 12 µm), the ISOPHOT instrument (photometry) aboard ISO (3.6 µm ≤ λ ≤ 100 µm), and the STIS instrument (imaging) aboard HST (5500Å ≤ λ ≤ 11000Å ). The optical images show the nucleus with very little coma contamination, and the ISO photometry allowed us to separate the comatic and nuclear contributions to the ESO images. We used the Standard Thermal Model for slow rotators to calculate an effective nuclear radius of 2.4 km ± 0.3 km. The comet's mid-IR light curve implies a nuclear rotation period of 15.2 h ± 0.3 h, although some subharmonics of this also satisfy the data. If we assume that the nucleus is a triaxial ellipsoid in principal short axis rotation with the axis direction in 1985 as derived by Sekanina (1988, Astron. J. 95, 911), then by combining our data with light curves from the 1980s we find that the nucleus' angular momentum vector migrates, making a would-be circle in less than 81 years, and that one axial ratio is at least 2.6. The nucleus' optical linear phase coefficient is 0.06 mag/degree, making it one of the most phasedarkened objects known. The surface is also rougher than that of 146 FERNÁNDEZ ET AL. most asteroids. The visual geometric albedo is 0.05 ± 0.02, within the range found for other cometary nuclei. c 2000 Academic Press Key Words: comets; infrared observations. 12 µm. The images have 64 2 pixels and cover (21.8 ) 2 . Each pixel width covered 65 to 87 km at the comet during the observing run. The plate scale was measured using the known relative positions of α Cen A and B . The pointspread function's (PSF) full width at half-maximum (FWHM) varied from 0.7 to 1.0 arcsec. Chopping of the secondary mirror northward and nodding of the telescope westward, with typical throws of 30 arcsec, were employed. An array flat field was created by measuring the relative photometry of a bright star at 23 different locations on the array and then interpolating a surface with a minimum of curvature. We observed the comet at three wavelengths, but only at λ = 10.7 µm was the comet bright enough to let us build a well-sampled time series of data. Absolute flux calibration was done using α Cen A, whose 10.7-µm magnitude is −1.56 ± 0.05, interpolating from photometric data given by . The magnitude scale zero point is 35.7 Jy. Color corrections were done and were at most

Physical characteristics of Comet Nucleus C/2001 OG108 (LONEOS)

Icarus, 2005

A detailed description of the Halley-type Comet C/2001 OG 108 (LONEOS) has been derived from visible, near-infrared, and mid-infrared observations obtained in October and November 2001. These data represent the first high-quality ground-based observations of a bare Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series photometry suggests that the nucleus has a rotation period of 57.2 ± 0.5 h with a minimum nuclear axial ratio of 1.3, a phase-darkening slope parameter G of −0.01 ± 0.10, and an estimated H = 13.05 ± 0.10. The rotation period of C/2001 OG 108 is one of the longest observed among comet nuclei. The V -R color index for this object is measured to be 0.46 ± 0.02, which is virtually identical to that of other cometary nuclei and other possible extinct comet candidates. Measurements of the comet's thermal emission constrain the projected elliptical nuclear radii to be 9.6 ± 1.0 km and 7.4 ± 1.0 km, which makes C/2001 OG 108 one of the larger cometary nuclei known. The derived geometric albedo in V -band of 0.040 ± 0.010 is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were combined to derive the nucleus's reflectance spectrum over a 0.4 to 2.5 µm wavelength range. These measurements represent one of the few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0 µm range such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001 OG 108 . The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry all indicate that C/2001 OG 108 has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured albedo and diameter of C/2001 OG 108 with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus, a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids. C/2001 OG 108 probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet, * Corresponding author. Fax: +1 281 483 5276.

Probing the internal structure of the nuclei of comets

Planetary and Space Science, 2009

There is no direct evidence about the internal structure of cometary nuclei, which are mostly hidden by their gas and dust comae, and have not yet been orbited by any spacecraft. Their densities are low, typically of about 400 kg m À3 for 9P/Tempel 1 (that was impacted by the Deep Impact probe) and 67P/Churyumov-Gerasimenko (that is the target of the Rosetta mission). Such low densities are in favour of a high macro-porosity, or a high micro-porosity, or both. Observations of disruption or splitting of nuclei indeed suggest that some huge sub-nuclei or some meter-sized fragments could be the building blocks of comets. Analysis, from in-situ measurements and from remote light scattering observations, of the structure of the dust particles, which significantly consist of fluffy aggregates of submicron-sized grains, could be in favour of a fractal structure. However, the presence of huge icy grains in the innermost coma, and of flat layers on the surface of 9P/Tempel 1, are clues to the complexity of these objects, which have suffered drastic erosion phenomena on their elongated orbits. It is expected that the Rosetta mission will provide a fair understanding of the structure of the deep interior of the nucleus of 67P/Churyumov-Gerasimenko, thanks to the on-board CONSERT experiment. r

In Situ Observations of Cometary Nuclei

Comets II, 2004

It is only through close spacecraft encounters that cometary nuclei can be resolved and their properties determined with complete confidence. At the time of writing, only two nuclei (those of Comets 1P/Halley and 19P/Borrelly) have been observed, both by rapid flyby missions. The camera systems onboard these missions have revealed single, solid, dark, lumpy, and elongated nuclei. The infrared systems gave surface temperatures well above the free sublimation temperature of water ice and close to blackbody temperatures. The observed nuclei were much more similar than they were different. In both cases, significant topography was evident, possibly reflecting the objects' sublimation histories. Dust emission was restricted to active regions and jets in the inner comae were prevalent. Active regions may have been slightly brighter than inert areas but the reflectance was still very low. No activity from the nightside was found. In this chapter, the observations are presented and comparisons are made between Comets Halley and Borrelly. A paradigm for the structure of cometary nuclei is also described that implies that the nonvolatile component defines the characteristics of nuclei and that high porosity, largescale inhomogeneity, and moderate tensile strength are common features.

Rotation and activity of comets

Advances in Space Research, 2007

We explore how nuclear rotation and activity can be used as effective probes of the gross nuclear structure and therefore of the interior of comets. We present a model of nuclear activity and discuss that in the context of how activity and rotation can control the present day size distribution of active short period comets. We argue that there is a real paucity of sub-km comets when compared with what one expects based on the size distribution of the known Kuiper Belt Objects.

The Nature of Comets [and Discussion]

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 1987

The vast scientific campaign associated with the 1986 return of Halley’s Comet has greatly improved and expanded our knowledge of comets. An overview of the first results is presented here with emphasis on the large-scale structure, the chemistry, and the nucleus. Biermann and Alfven’s basic large-scale picture involving the interaction with the solar wind was confirmed. The interaction extends over very large distances and involves the draping of magnetic field lines from the solar wind around the head region. The near-nuclear region is essentially free of magnetic field. The cometary environment is a rich plasma physics laboratory as well as the site of spectacular disconnection events. As Whipple proposed, the chemical composition of the nucleus is largely water, and the breakup of the water molecule produces the large hydrogen-cloud surrounding the comet. Minor constituents with high molecular mass have been observed in the comet. The composition of the dust generally resembles ...

Spitzer Space Telescope observations of the nucleus of comet 67P/Churyumov-Gerasimenko

Astronomy and Astrophysics, 2008

We have used the Spitzer Space Telescope InfraRed Spectrograph (IRS) 22-μm peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA's Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet's antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0:95 AE 0:20 indicates a small effective radius for the nucleus of 0:57 AE 0:08 km and low geometric albedo 0:028 AE 0:009 (1σ). With this nucleus size and a water production rate of 3 × 10 28 molecules s À1 at perihelion, we estimate that ∼100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ∼5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact's first target, comet 9P/Tempel 1, Hartley 2's nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (∼700 yr) at its current rate of mass loss.

The Thermal, Mechanical, Structural, and Dielectric Properties of Cometary Nuclei After Rosetta

Space Science Reviews, 2019

The physical properties of cometary nuclei observed today relate to their complex history and help to constrain their formation and evolution. In this article, we review some of the main physical properties of cometary nuclei and focus in particular on the thermal, mechanical, structural and dielectric properties, emphasising the progress made during the Rosetta mission. Comets have a low density of 480 ± 220 kg m −3 and a low permittivity of 1.9-2.0, consistent with a high porosity of 70-80%, are weak with a very low global tensile strength < 100 Pa, and have a low bulk thermal inertia of 0-60 J K −1 m −2 s −1/2 that allowed them to preserve highly volatiles species (e.g. CO, CO 2 , CH 4 , N 2) into their interior

A Portrait of the Nucleus of Comet 67P/Churyumov-Gerasimenko

Space Science Reviews, 2007

In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two regions for the direction of the rotational axis of the nucleus: RA = 220°+50° −30° and Dec = −70° ± 10° (retrograde rotation) or RA = 40°+50° -30° and Dec = +70°± 10° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function, although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel 1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m−3, lead to a mass of ~ 8 × 1012 kg which should ease the landing of Philae and insure the overall success of the Rosetta mission.