Comet nucleus

Prialnik D. (2002) Modehng and comet nucleus interior. Application to comet C/1995 01 Hale-Bopp. Earth, Moon Planets. 89, 27-52. 

NASA s most recent comet missions are Deep Impact and Rosetta. Deep Impact was launched on January 12, 2005, with Comet Tempel 1 as its target. The spacecraft encountered Tempel 1 on July 3, 2005, at which time it released a 770-pound (350 kg) copper projectile at the comet. Cameras and spectrometers on the spacecraft photographed and collected samples of materials ejected from the comet nucleus and relayed that information to scientists on Earth. Rosetta was launched on March 2, 2004, with Comet 67P/Churyumov-Gerasimenko as its target. The spacecraft will orbit the comet and make observations for about two years as the comet approaches the Sun. It will also release a small package of instruments that make the first-ever landing on the surface of a comet. 

Not all space missions to comets have been as successful as Stardust and Deep Space 1. For example, NASA s CONTOUR (Comet Nucleus Tour), launched on July 3, 2002, was designed to rendezvous with and study Comet Encke on November 12, 2003, and Comet Schwassmann-Wachmann 3 on June 18, 2006. All contact was lost with CONTOUR on August 15, 2002, however, and the mission was canceled. 

Photodissociation is far more likely to occur than photoionization, however, because the energy needed to bring about dissociation is much less than that required for photoionization. In fact, once a water molecule escapes from the comet nucleus into the coma, the average time in which it is likely to he broken apart by solar radiation is about 25 seconds. By contrast, the average amount of time during which a water molecule is ionized by solar radiation is likely to be a few hours. Such reactions are more likely to occur when water molecules have trailed off into the comet s tail at distances of about 1 AU. Consequently, the coma of a comet can he expected to consist largely of neutral remnants of the photolysis of water molecules, such as H and OH, while ionized species, such as H+ and free electrons, are more likely to be found in the comet s tail. 

Fig.6.86. Molecular emission spectrum from Halley s comet on its 1986 return to its perihelion. Along the vertical scale the whole length of the comet tail is imaged using a long spectrometer slit. C2, C3 and CN bands are very prominent. Note that the C3 emission (the "Comet band) occurs only from a region close to the comet nucleus [6.165]...

The Philae lander carries ten scientific instruments panoramic, stereoscopic and descent camera a-p-x-ray spectrometer evolved gas analyser for elemental, molecular and isotopic composition infrared microscope comet acoustic surface and sounding experiment permittivity probe dust impact monitor multi-purpose sensor for surface and sub-surface science magnetometer plasma monitor comet nucleus sounding experiment drill and sample distribution system. 

Prior to the 1960s, comets were investigated primarily using visible-wavelength observations. The first infrared detection of a comet was achieved in the mid-1960s. Although radio astronomers had attempted to detect cometary emissions since the 1950s, the first widely accepted radio detection of a comet is that of the 18-cm OH transitions in Comet Kohoutek (1973). Radio observations of comets have been used to study all of the major components of comets nucleus, dust, neutral gas, and the plasma. 

Particles found in highly correlated orbits are called stream components and those found in random orbits are called sporadic components. It is thought that most meteor streams are formed by the decay of a comet nucleus and consequently are spread around the original orbit of the comet. When Earth s orbit intersects a meteor stream, the meteor rate is increased and a meteor shower results. Typically, a meteor shower will be active for several days. A particularly intense meteor shower... 

Reach, W.X, Rho, J. Detection of far-infrared water vapor, hydroxyl, and carbon monoxide emissions from the supernova Remnant 3C 391. Astrophys. J. Lett. 507, 93-97 (1998) Reinhard, R. Summary of the scientific results of the Giotto Mission to Halley s comet nucleus, dust and gas composition and fluxes. In Cometary and Solar Plasma Physics, p. 81 (1988)... 

Fig. 3. Comet Swift-Tuttle at 9.8 fim. Minimum contour level and level spacing are 10% of peak flux (17 mJy). The sun direction is toward the southwest. Comet Swift-Tuttle was mapped at 8.8, 9.8, 11.7, and 12.5 m (Deutsdi et al. 1992) to study the temperature, composition, spatial extent, and evolution of the dust cloud surrounding the comet nucleus. Spectral points at 8.8, 9.8, and 12.5 fim are well-flt by a Planck curve (T = 364 K). There is no evidence for a 9.8 fim amorphous silicate peak but a clear excess is seen at 11.7 fim.

Comets consist of three elements the nucleus, the coma and the tail. 

The cometary coma The coma and the nucleus form the head of the comet the streams of dust and gas released by the comet form a very large, extremely tenuous atmosphere called the coma, which can have a spread up to around 104—105 km. The coma is not developed when the comet is a long way from the sun, but when it comes closer (at around 5 AU), the ice mixture begins to sublime and is ejected as a gas stream. Dust particles are entrained at a velocity of around one kilometre per second. 

The mass of Halley s Comet is about 1014kg, and thus its mean density is only 200 kg/m3. The rate of loss of material has been estimated as 5,000 kg/s. The nucleus is loosely packed and exhibits point craters and chasms from which gas and dust escape. These emissions consist mainly of water vapour (—80% by volume) as well as 6% CO, < 3% C02, -2.5% CH4, -1.2% NH3 and < 6% N2 (Flechtig and Keller, 1987). At the point where Giotto came nearest to the comet, the estimated amount of water being ejected was close to 15,000 kg/s, while that of dust particles was between 6,000 and 10,000 kg/s. Ions derived from water were detected in the... 

Mission Deep Impact In July 2005, NASA steered a projectile, about 370 kg in weight, at the comet 9F/Tempel (dimensions 4x4x14km), in order to obtain more exact information on its structure and composition. The impact was visible from Earth the Rosetta spacecraft discussed above also sent pictures to Earth. The dust/ice ratio determined after the impact is very probably greater than unity, so that comets are probably icy dustballs rather than (as had previously been surmised) dirty snowballs . The density of the cometary nucleus, which seems to consist of porous material, is roughly equal to that of ice. The impact set free around 19 GJ of... 

Figure 6.16 Structure of the comet showing the nucleus, the coma and two tails - an ion tail and a dust tail...

The comet structure model proposed in Figure 6.16 shows clearly that the observation of molecules from Earth must be limited to those molecules present within the coma of the comet, and whilst they originate in part from the structure and composition of the nucleus the molecular observations are of the coma chemistry only. The coma observations will remain until we send a probe to land on the surface of a comet and report back the composition of the core. The Rosetta mission will do just this and we shall see the composition directly from the data it recovers, if successful. 

Heat is transported through the layers of the ice into the nucleus of the comet. The temperature and rates of heat conduction are controlled by the coefficients of thermal conductivity. 

Coma The central region around a comet immediately surrounding the nucleus. 

Grigg-Skjellerup was visited by the Giotto spacecraft in 1989. NASA s demonstration spacecraft DS1 imaged the nucleus of Comet Borrelly in 2001. 

The activity that characterizes a comet is driven by solar heating. As the comet approaches the Sun, jets of dust and gas erupt from active areas (Fig. 12.1), as they periodically rotate into the sunlight. The nucleus becomes surrounded by a spherical coma formed by the emitted gas and dust. Emitted gas becomes ionized due to interaction with solar ultraviolet radiation, and the ions are swept outward by the solar wind to form the comet s ion tail. A separate dust tail commonly has a different orientation, reflecting variations in the velocities of solid particles and ions. A popular term describing comets is dirty snowballs , although that description probably understates the proportion of rock and dust relative to ices. 

Image of comet Halley s nucleus, showing jets of dust and volatilized ices. This picture was taken by the Giotto spacecraft as it raced by in 1986. Image courtesy of ESA. 

In 2004, the NASA Stardust spacecraft passed through the dust cloud surrounding the nucleus of comet Wild2 and captured more than 10 000 particles ranging from 1 to 300 pm in size (Brownlee et al., 2006). These particles were returned to Earth for study in terrestrial laboratories in 2006. Initial results are described in the December 15, 2006, issue of Science. 

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