Comet sizes and masses

The idea that comets had planetary size could not be sustained for too long. In 1805 Laplace could place an upper limit to the comet mass by noting that the close encounter of comet Lexell with the Earth in 1770 did not produce any perceptible perturbation in Earth's motion. He found that the mass of the comet could not be greater than 1 5000 of the Earth's mass. It was then becoming clear that comet heads and tails, despite their large volumes, were of extremely low density. This fact was...

N sin iUx

Where a RG ap, and Ux is the radial component of U, which is given by Ux2 2 1 A A(1 e2), (9.16) We note that eq. (9.15) will not be valid for i & 0 . Yet (Opik (1951) notes that secular variations will act in such a way that the average sin i will be always nonnegligible. The estimated mean collision probability for a JF comet with the Earth is pJF 1.3 x 10-9 per year, while for Jupiter is 9 x 10-7 (Olsson-Steel 1987, Shoemaker et al. 1994). If the average lifetime of a JF comet is 104 yr...

Direct numerical integration

The simplified picture presented before is enough to illustrate the main dynamical features of Oort cloud comets under the action of the galactic potential. A more accurate treatment would require the direct integration of the equations of motion (e.g. Torbett 1986, Fouchard 2004). For the sake of completeness we will describe it in this section. Let us first define a fixed reference system (x,y,z) in which x points toward the galactic center, z is normal to the galactic plane and points to the...

Binary TNOs

Another interesting feature is the discovery of a few percent of binaries among the TNOs, which show wide separations of about 100 to 1,000 times their radii (for instance, the Moon is at about 60 Earth's radii), and mass ratios of order unity (Fig. 8.10). Pluto and Charon may be taken as the first example of binary TNOs of comparable mass, though their separation is only 17 times Pluto's radius. The determination of the orbital period of binary systems will permit to derive their total mass...

Dynamics of close encounters

As mentioned, most aphelia of JF comets lie very close to Jupiter's orbit, so their dynamics is dominated by close encounters with Jupiter. Some JF comets may fall temporarily in resonances that keep them safe from suffering close encounters with Jupiter, but only for very short time spans. It is also possible that nongravitational forces decouple some JF comets from Jupiter's influence zone. The paradigmatic case is 2P Encke whose aphelia Q 4.1 AU is well inside Jupiter's orbit. Fern ndez et...

The deuterium problem

The answer to the question of how much water and organic material on the Earth comes from cometary bodies of the Uranus-Neptune accretion zone or the trans-neptunian belt is still uncertain. Yet, the study of the composition of the deuterium hydrogen (D H) isotopic ratio of water in comets, and how it compares with that of the Earth's oceans, may help to shed light on this issue. For the oceans the ratio is found to be D H 1.56 x 104, while for the three comets in which it was so far measured...

Early Ideas About Comets

Colophon Comet

The ancient peoples paid special attention to whatever occurred in the heavens, noting on one side the regularity of several celestial phenomena, such as the rise and the setting of the Sun, Moon and stars, and the phases of the Moon, and on the other side the irruption of unexpected transient events, like eclipses, comets, novae and meteors, that broke such a regularity. Since the heavenly bodies were associated to divinities with influence on terrestrial affairs, the unexpected events caused...

The physical structure of icy planetesimals

What the models of accretion processes in the protoplanetary disk tell us about the physical structure of the formed aggregates, and how comets - if they are indeed the fossil records of that early epoch - match such models We have already presented some ideas about sticking mechanisms and grain assemblage in Section 10.4. We will continue now delving into this problem. Donn (1963) developed a model of formation of comet nuclei by accumulation of grains in which the drag by the nebular gas...

Cyanogen Comet Spectra

987 6 5 4 3 2 1 2 3 4 5 6 789 Heliocentric distance (AU) Figure 3.5. Observed CO and water outgassing rates of comet C 1995 O1 (Hale-Bopp) versus heliocentric distance (left pre-perihelion right post-perihelion). Gray circles are for CO, and black squares for H2O. Open triangles are upper limits for OH (or water production rate). These results were compiled from different observers (Biver 2001). The first spectra of comets were obtained by the Florentine astronomer Giovanni Battista Donati...

And Centaurs

There is already a good wealth of photometric data in the visible and near-infrared of several TNOs and Centaurs. The observations have been carried out mainly in the broad band filters BVRIJHK and have permitted a preliminary broad spectral characterization of these populations. Near-infrared reflection spectra of 1996 TO66 (Brown et al. 1999), 20000 Varuna (Licandro et al. 2001), and 1999 DE9 (Jewitt and Luu 2001) show absorption features corresponding to water ice. Yet other TNOs like 1993...

Collision with the

The recent discovery of a large number of sungrazing comets by the coronograph on board of the ESA NASA Solar and Heliospheric Observatory SOHO spacecraft has renewed the interest in this class of comets. Before the SOHO discoveries, there were 23 sungrazers discovered visually, and by the SOLWIND and Solar Maximum Mission satellites. The SOHO discoveries has raised the number to nearly 500 in a short time span of six years 1996-2002 Fig. 9.9 . About 84 of the sungrazers have orbital elements...

Early phases of star formation

Interstellar molecular clouds provide the placental material out of which new stars form. Most of the molecular mass is in the form of giant molecular clouds GMCs with typical masses 105-6 M , diameters 50 pc, average densities 102 H2 molecules cm-3, and an average temperature of 10 K Blitz 1993 . The distribution of molecular gas in the Galaxy can be traced through molecules with strong transition lines, like CO at 2.6 mm wavelength. Heyer and Terebey 1998 have shown that the CO emission in...

Dynamics Of Lp Comets Entering The Inner Planetary Region

A comet entering or approaching the planetary region will experience the action of planetary perturbations, so it will slowly depart from a Keplerian motion. The comet's orbit can in principle be determined from three accurate astrometric positions in practice, several more spanning through several days. First a parabolic solution is tried to fit the observations. Once good astrometric positions covering a period of several weeks are available, an elliptic or sometimes hyperbolic solution is...

Statistics Of Cometary Orbits And Magnitudes

As we showed in the previous chapter, the earlier references of cometary apparitions are usually so vague that they do not allow to estimate fairly good orbits. Since about 1700 the record has steadily improved both in quantity and quality, adding to the naked-eye discoveries those made by telescopic means Fig. 2.1 . Ancient comet apparitions have been compiled in several catalogs, among the first ones we can mention the seventeenth-century tract Historia Cometarum by Stanislaus de Lubienietz,...

Info

The Holetschek effect is the best known one e.g. Everhart 1967a,b, Kresak 1975 . It was found by the Vienna astronomer Johann Holetschek more than a century ago, and it is associated with the fact that comets reaching perihelion on the opposite side of the Sun, as seen from the Earth, are less likely to be discovered. Figure 2.8 shows that indeed many more of the discovered LP comets reached perihelion near conjunction with the Sun difference of longitudes of the comet at perihelion...

Color albedo and phase function

Colors have already been determined for several comet nuclei, mainly in the V, R bands. Most of them belong to the Jupiter family. The mean value is found to be lt V R 0.42 Meech et al. 2004, Lamy et al. 2005 , which is somewhat redder than the solar color V R 0.35, but substantially less red than the average color index of trans-neptunian objects lt V R gt tno 0.61. We note however that there is a high dispersion in the observed colors of JF comets, going from V R 0.02 0.22 for 14P Wolf to...