Halleys Comet

M. Greving, F. Praderie and R. Reinhard (Eds.), Exploration of Halleys comet (Springer-Verlag, Berlin, 1987). 

Silver-cadmium batteries have been used in a number of space applications requiring nonmagnetic properties. One such battery provided the main power for the Giotto Halley Comet intercept spacecraft. 

Alkanes have the general molecular formula C H2 +2 The srmplest one methane (CH4) rs also the most abundant Large amounts are present rn our atmosphere rn the ground and rn the oceans Methane has been found on Juprter Saturn Uranus Neptune and Pluto and even on Halley s Comet... 

Formaldehyde cyanohydrin was detected in Halley s comet by the Vega I space probe (27). 

Dale, D., Pacult, R., and Reinhard, R., The Joint NASA/ESA Cometary Mission to Comets Halley and Tempel 2, European Space Agency Report No. ESA SP-153, Paris Cedex, France, pp. 3-5, October 1979. 

Water can be found, in all three aggregate states, almost everywhere in the universe as ice in the liquid phase on the satellites of the outer solar system, including Saturn s rings and in the gaseous state in the atmospheres of Venus, Mars and Jupiter and in comets (as can be shown, for example, from the IR spectra of Halley s comet). The OH radical has been known for many years as the photodissociation product of water. 

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... 

The next most likely possibility is cometary delivery of the atmosphere but again there are some problems with the isotope ratios, this time with D/H. The cometary D/H ratios measured in methane from Halley are 31 3 x 10-5 and 29 10 x 10-5 in Hayuatake and 33 8 x 10-5 in Hale-Bopp, whereas methane measurements from Earth of the Titan atmosphere suggest a methane D/H ratio of 10 5 x 10-5, which is considerably smaller than the ratio in the comets. The methane at least in Titan s atmosphere is not exclusively from cometary sources. Degassing of the rocks from which Titan was formed could be a useful source of methane, especially as the subnebula temperature around Saturn (100 K) is somewhat cooler than that around Jupiter. This would allow volatiles to be more easily trapped on Titan and contribute to the formation of a denser atmosphere. This mechanism would, however, apply to all of Saturn s moons equally and this is not the case. 

Polymerisation of HCN species is also possible once the initial monomers have been formed by the reactions with nitrogen HCN polymers have been postulated in many places in the solar system, from the clouds of Jupiter and Saturn to the dark colour of the surface of comet Halley, not to mention its possible role in the formation of the prebiotic molecule adenine. Photolysis of HCN produces CN and then the formation of nitrile polymers ... 

Halley The comet whose passage around the Sun has been observed 30 times from 239 bc to 1986. The orbital period is 75, with aphelion outside Neptune s orbit and perihelion at 0.59 AU. 

The adjective space in the chapter title loosely means extraterrestrial and could include planetology, the study of other solid bodies in the solar system, such as Mars, Comet Halley, or asteroid Ceres. While MS is vital to all planetary exploration, these devices function much the same way as laboratory MS, except that they are remotely operated, use less power, and are considerably more expensive. But space can also have the more restricted meaning of outside the ionosphere of any planet, but inside the solar system, which will be the area discussed in this chapter. The properties and challenges of this region are very different from the lab, although the science turns out to be often the same. 

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. 

The densities of comets Halley and Tempel 2 were estimated to be 0.3 g cm 3, consistent with highly porous objects. However, comet 81 P/Wild2, imaged by the Stardust spacecraft in 2004, appears to be a more coherent object, implying that it has a higher density, -0.6 gcm 3. 

Fe/Si and Mg/Si ratios inferred from comet Halley dust spectra are significantly more scattered than analyses of comparably sized spots in the Orgueil Cl chondrite. This scattering suggests lack of well-defined minerals in cometary dust. After Lawler etal. (1989). 

Mineralogical interpretations of Halley dust analyses remain controversial. Lawler et al. (1989) saw no clustering of compositions that might suggest crystalline minerals (Fig. 12.5), whereas Fomenkova et al. (1992) identified compositions that were consistent with a number of minerals, including pyroxene, phyllosilicate, carbonate, FeNi metal, iron sulfide, and iron oxide. The characterization of minerals in returned comet dust (see below) supports the identification of some of these primary minerals but calls into question the identification of those formed by alteration. 

Formenkova, M.N., Kerridge, J. F., Marti, K. and McFadden, L. A. (1992). Compositional trends in rock-forming elements of comet Halley dust. Science, 258, 266-269. 

Lawler, M. E., Brownlee, D. E., Temple, S. and Wheelock, M. M. (1989). Iron, magnesium, and silicon in dust from comet Halley. Icarus, 80, 225-242. 

Although information on the chemical composition and reactivity of interstellar ices can be obtained only from remote observations and laboratory simulations, cometary ices and dusts are subject to direct studies, eg within Vega and Giotto (comet Halley), Stardust (comet Wild 2) and Rosetta (comet Churyumov-Gerasimenko) missions (Figure 8.12). 

Also, observations of crystalline silicates in the comets Halley (Swamy etal. 1988) and Tempel 1 (Harker et al. 2005) suggest that a large fraction of cometary dust was processed or formed in the hot inner regions of the protoplanetary disk and transported to the region where the comets formed. 

Among the goals for the Stardust mission was identifying the origin of the crystalline silicates in comets, whose presence in comets had been known from observations of comets Halley and Hale-Bopp (as reviewed in Bockelee-Morvan... 

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