No abstract

We compute masses and densities for ten periodic comets with known sizes: 1P/Halley, 2P/Encke, 6P/d'Arrest, 9P/Tempel 1, 10P/Tempel 2, 19P/Borrelly, 22P/Kopff, 46P/Wirtanen, 67P/Churyumov-Gerasimenko and 81P/Wild 2. The method follows the one developed by Rickman and colleagues (Rickman 1986, 1989; Rickman et al. 1987), which is based on the gas production curve and on the change in the orbital period due to the non-gravitational force. The gas production curve is inferred from the visual lightcurve. We found that the computed masses cover more than three orders of magnitude: ~(0.3 - 400)*10^12 kg. The computed densities are in all cases very low (<= 0.8 g cm^-3), with an average value of 0.4 g cm^-3, in agreement with previous results and models of the cometary nucleus depicting it as a very porous object. The computed comet densities turn out to be the lowest among the different populations of solar system minor bodies, in particular as compared to those of near-Earth asteroids (NEAs). We conclude that the model applied in this work, in spite of its simplicity (as compared to more sophisticated thermophysical models applied to very few comets), is useful for a statistical approach to the mean density of the cometary nuclei. However, we cannot assess from this simple model if there is a real dispersion among the bulk densities of comets that could tell us about differences in physical structure (porosity) and/or chemical composition.Comment: 26 pages, 16 figures. Accepted for publication in MNRA

We analyse the population of near‐Earth long‐period comets (LPCs; perihelion distances q < 1.3 au and orbital periods P > 103 yr). We have considered the sample of LPCs discovered during the period 1900–2009 and their estimated absolute total visual magnitudes H. For the period 1900–1970 we have relied upon historical estimates of absolute total magnitudes, while for the more recent period 1970–2009 we have made our own estimates of H based on Green’s photometric data base and IAU Circulars. We have also used historical records for the sample of brightest comets (H < 4.5) covering the period: 1500–1899, based mainly on the Vsekhsvyatskii, Hasegawa and Kronk catalogues. We find that the cumulative distribution of H can be represented by a three‐modal law of the form log10N< H=C+αH, where the C values are constants for the different legs, and α≃ 0.28 ± 0.10 for H < 4.0, α≃ 0.56 ± 0.10 for 4.0 ≤H < 5.8, and α≃ 0.20 ± 0.02 for 5.8 ≤H < 8.6. The large increase of the slope of the second leg of the H‐distribution might be at least partially attributed to splitting of comet nuclei, leading to the creation of two or more daughter comets. The cumulative H‐distribution tends to flatten for comets fainter than H≃ 8.6. LPCs fainter than H≃ 12 (or diameters D≲ 0.5 km) are extremely rare, despite several sky surveys of near‐Earth objects implemented during the last couple of decades, suggesting a minimum size for an LPC to remain active. We also find that about 30 per cent of all LPCs with q < 1.3 au are new (original bound energies 0 < Eor < 10−4 au−1), and that among the new comets about half come from the outer Oort cloud (energies 0 ≲Eor≲ 0.3 × 10−4 au−1), and the other half from the inner Oort cloud (energies 0.3 × 10−4≲Eor≲ 10−4 au−1).

We analyze a sample of 139 near-Earth asteroids (NEAs), defined as those that reach perihelion distances q < 1.3 au, and that also fulfill the conditions of approaching or crossing Jupiter's orbit (aphelion distances Q > 4.8 au), having Tisserand parameters 2 < T < 3 and orbital periods P < 20 yr. In order to compare the dynamics, we also analyze a sample of 42 Jupiter family comets (JFCs) in near-Earth orbits, i.e. with q < 1.3 au. We integrated the orbits of these two samples for 10 4 yr in the past and in the future. We find that the great majority of the NEAs move on stable orbits during the considered period, and that a large proportion of them are in one of the main mean motion resonances with Jupiter, in particular the 2:1. We find a strong coupling between the perihelion distance and the inclination in the motion of most NEAs, due to Kozai mechanism, that generates many sungrazers. On the other hand, most JFCs are found to move on very unstable orbits, showing large variations in their perihelion distances in the last few 10 2 − 10 3 yr, which suggests a rather recent capture in their current near-Earth orbits. Even though most NEAs of our sample move in typical 'asteroidal' orbits, we detect a small group of NEAs whose orbits are highly unstable, resembling those of the JFCs.

We estimate masses for a selected sample of long‐period comets (LPCs) (with orbital periods P > 1000 yr and perihelion distances q < 2 au), with good photometric visual light curves and known non‐gravitational parameters. We follow a procedure similar to that developed by Szutowicz et al. to estimate the masses of comets C/1995 O1 (Hale–Bopp) and C/1996 B2 (Hyakutake). The method also requires the knowledge of the water production rates Q, for which we find a new correlation between Q and the visual total heliocentric magnitude mh of LPCs, which can be expressed as log10Q= 30.53 − 0.234mh. The computed masses are in the range [0.5, 10]× 1012 kg. From these masses, and by assuming a cometary bulk density ρ of 0.4 g cm−3, we estimate the effective nuclear radii RN for the studied LPCs to be in the range [0.7, 1.8] km. From the computed values of RN, we find that the active surface areas are (in most cases) greater than the geometric ones (fractions f ranging from 0.8 to 2.4). We argue that this could be understood as a state of hyperactivity, which can be explained if a significant fraction of the sublimated water molecules come from the sublimation of icy grains in the coma. Hyperactivity would make LPC nuclei smaller (i.e. less massive) than expected from their coma brightness. Other effects, like extremely low densities or overestimated non‐gravitational parameters, might also contribute to the rather high computed value of f. For comet C/1995 O1 (Hale–Bopp), we find a seeming inconsistency between some size estimates and some computed non‐gravitational parameters. We analyse some alternatives to this apparent paradox. We also compute absolute total magnitudes and photometric indices for the studied sample of LPCs and find a new correlation between masses and absolute total visual magnitudes H. By assuming ρ= 0.4 g cm−3, we can convert masses to diameters D, leading to a relation log10D(km) = 1.5 − 0.13H.